
I'm Scott Aaronson, quantum computing/computational complexity researcher. AMA - ScottAaronson
Hey HN,<p>We recently recorded a podcast (https:&#x2F;&#x2F;blog.ycombinator.com&#x2F;scott-aaronson-on-computational-complexity-theory-and-quantum-computers&#x2F;) where I discussed my research, AI, and advice for nerds in general or people who want careers in science.<p>We covered many but not all of the questions submitted over the internet so AMA!
======
DC-3
Hi Scott,

Shtetl-Optimized's tagline is famously "Quantum computers would not solve hard
search problems instantaneously by simply trying all the possible solutions at
once". What phrase do you think should replace 'trying all the possible
solutions at once' in the public conciousness as a succinct description of the
mechanisms of a quantum computer? Or is this topic simply too complex to be
distilled into a neat synopsis while retaining accuracy?

~~~
ScottAaronson
A quantum computer is a device that exploits constructive and destructive
interference among exponentially many _amplitudes_ , which are numbers that
are closely related to probabilities but can be positive, negative, or even
complex.

If you feel that sentence wasn't clear enough, and it would take at least a
few more paragraphs to flesh it out ... well, duh, what did you expect? :-D

For a SLIGHTLY longer account, see my attempt to explain quantum computing in
35 seconds or fewer, which Maclean's magazine challenged me and others to do
in response to Justin Trudeau's quantum computing explanation:
[https://www.scottaaronson.com/blog/?p=2694](https://www.scottaaronson.com/blog/?p=2694)

When I did a piece for the New York Times, I managed to get an explanation
that I was reasonably happy with into ~6 paragraphs:
[https://www.nytimes.com/2011/12/06/science/scott-aaronson-
qu...](https://www.nytimes.com/2011/12/06/science/scott-aaronson-quantum-
computing-promises-new-insights.html)

Given that quantum mechanics is, famously, one of the most counterintuitive
things that humanity ever discovered, I don't think it's that big of an ask
for people to read 6 paragraphs about QC before they decide they basically
know what it's about. :-)

~~~
stcredzero
Here's my attempt at a two sentence over-over-over simplification that at
least gets people away from the "magic bit-sting that contains your answer."
(It also harkens back to an old Einstein quote, so may be attractive to
science writers.)

Quantum computing is a technique that lets you sample a problem's answer-space
using "loaded dice," such that the problem's correct answers correspond with
probability spikes in your dice throws. Right now, we only know how to
usefully "load" those dice for certain problems, and it's pretty hard to do.

~~~
bariswheel
Do you mean all the possible solutions to a problem when you say 'answer
space' ?

~~~
tadhgds
That's how I would interpret it, yes

------
rsln-s
Hi Scott,

Do you think there is a significant chance that quantum will never take off
(i.e. there are non-obvious limitations that will prevent quantum
architectures like superconducting qubits / trapped ions / quantum dots /...
from ever outperforming classical supercomputers)? Related, what in your
opinion is the best indicator (or would be the best indicator if demonstrated)
of the potential of quantum devices?

~~~
ScottAaronson
Yes, I do think there's a significant chance of that. If it happens, my main
interest would be to understand WHY. What _are_ the non-obvious limitations
that you mention? What is true about the world that makes it seem to have this
exponential explosion of amplitudes, yet makes it impossible or infeasible to
harness them for computation?

The depressing possibility, of course, is that we never succeed in building
useful QCs, but we also never learn anything deep about why we failed: it was
just too complicated, too messy, and then at some point the funding ran out.

But I like to proceed on the assumption that the world is ultimately
comprehensible (what other choice does one have in science? :-) ). On that
assumption, if QC can _never_ work, then there must be a deep reason that's
not articulated in any of the existing physics books: either a breakdown of
quantum mechanics itself, or else some new principle on top of QM that
"screens off" or "censors" QC. Needless to say, the discovery of that
principle would itself be a revolution in science -- indeed, I'd personally be
_more_ excited about it than a "mere success" in building scalable QC! (But my
own bet is on the "boring, conservative" possibility, that QC can ultimately
work.)

If we see the milestone of "quantum supremacy" achieved in the next few years
-- i.e., a 50-70 qubit quantum computer used to solve some artificial sampling
task many orders of magnitude faster than we know how to solve it classically
-- that will obviously be one strong indicator that the potential of QC can be
realized. An even better indicator would be the use of a quantum error-
correcting code, like the Kitaev surface code, to keep encoded qubits alive
for longer than the underlying physical qubits are staying alive for (or
better still, to perform 1- and 2-qubit gates on them).

~~~
OscarCunningham
There's a famous study somewhere showing that people interpret phrases
describing probabilities differently.

By "significant chance" do you mean something like 10% or something like 70%?

~~~
dreamcompiler
He means (-40 + 17i)%.

Sorry, couldn't resist :-)

~~~
ScottAaronson
Yes, that's PRECISELY what I meant.

------
SRasch
You were said to be a skeptic of quantum computing company d wave. Then you
started believing and then went back to skepticism. What is your current
status, do you think it works? What would you like to see from them?

Also, what is your take on Max Tegmark's quantum suicide experiment. Would it
work? If yes would that imply that each of us should expect to live a really
long time subjectively?

~~~
ScottAaronson
My position on the technical fundamentals never changed much: namely, D-Wave
is building devices that could be interesting from various engineering
perspectives, but that as far as most of us can tell, are not getting speedups
over existing computers that are clearly attributable to quantum computation
(as opposed to building special-purpose hardware that's, essentially, very
fast at simulating itself). If you want quantum computing speedups, I think
you're going to need qubits of much higher quality, and ultimately error
correction or at least error mitigation. In principle, D-Wave could do that,
and I applaud any steps they take in that direction. However, I'm personally
much more excited right now about the experimental efforts in superconducting
quantum computing that are happening at Google, IBM, Intel, and Rigetti -- all
of which use qubits with orders-of-magnitude better coherence times than
D-Wave's qubits. In some sense, D-Wave optimized for being able to say that
they had 2000 qubits as quickly as possible, rather than for the qubits
actually doing what we want.

On a more sociological level, D-Wave earned a lot of bad blood with the
academic QC community by making false, inflated, and overhyped claims (with a
primary offender being its founder, Geordie Rose, who's since left the
company). And I certainly took them to task for those sorts of things on my
blog. Then the D-Wave folks met with me, John Preskill, and other academics,
and pledged to improve in how they communicated, so I was nicer to them for a
while. Then they went back to egregious hype about speedups that weren't real,
so I criticized them again. Nothing more to it than that. :-)

Regarding quantum suicide: no, I do NOT recommend killing yourself any time
anything happens in your life that makes you unhappy, on the theory that other
versions of you will survive, in other branches of the quantum-mechanical
wavefunction where the bad event didn't happen. This is partly because, even
assuming you accept the Many-Worlds Interpretation, "your" moral concern and
responsibility presumably extend only to those branches that are in "your"
future -- you have no contact with the other branches! And partly it's because
I take it as almost an axiom of rationality that, if a metaphysical belief
leads you to do "obviously insane" things with your life, then it's probably
time to look for a better metaphysical belief. :-) (I wouldn't say the same
about scientific or mathematical beliefs.)

~~~
martin1975
Am I hearing this right, you think the whole multiverse concept is... meta-
physics at best?

~~~
AgentME
I think he was saying about whether you should morally care about the other
branches counted as meta-physics.

~~~
ScottAaronson
Yup.

------
pc
This is slightly off-topic (I'm going to be that "this is sorta more a comment
than a question..." guy for a second), but I just want to say that Scott's
blog is one of my favorite blogs on the whole internet. If only there were
more like it!

~~~
ScottAaronson
Thanks!!!

~~~
lisper
I know that me-too type posts are frowned upon here on HN, but in this case I
think an exception is warranted. Too many scientists ensconce themselves in
the ivory tower and treat the rest of the world with attitudes ranging from
indifference to outright disdain. I also want to thank you for not following
that model.

On a totally unrelated note, I've been trying to wrap my brain around coherent
states and the photon-number/phase uncertainty relationship (e.g.
[http://hitoshi.berkeley.edu/221a/coherentstate.pdf](http://hitoshi.berkeley.edu/221a/coherentstate.pdf)).
Do you know of any simple intuitive stories one can tell about that like one
can with position-momentum uncertainty? I know this isn't really in your
wheelhouse, but people who both understand this stuff and are willing to field
questions like this are exceedingly rare (see above paragraph).

(FWIW, and for the benefit of lurkers, this question was prompted by the
discussion on this blog post: [http://blog.rongarret.info/2018/05/a-quantum-
mechanics-puzzl...](http://blog.rongarret.info/2018/05/a-quantum-mechanics-
puzzle-part-drei.html). Also FWIW, that's my blog.)

------
danharaj
A lot of machines that purport to solve an NP-hard problem in a short period
of time have some trick that makes them impractical, like requiring the
ability to reduce the noise floor in a signal exponentially or pump an
exponential amount of energy into a system. Has it been shown that such
physical resources are more-or-less interchangeable with running time for the
purposes of complexity theory?

~~~
ScottAaronson
No, I think this is more a question for physics than for complexity theory --
complexity theory basically just takes the computational model and the
resources you care about as inputs, then uses math to study how much of the
resources are inherently required to solve a given problem.

Absent an ultimate theory of fundamental physics, we're unlikely to have a
full answer to your question -- e.g., to be able definitively to rule out the
possibility of "hypercomputers" solving NP-hard problems in polynomial time.
What we can do is

(1) to explain the failure (often, the forehead-bangingly obvious,
don't-point-to-the-exponential-elephant-in-the-room failure) of all EXISTING
proposals along these lines, and

(2) to point to deep discoveries in fundamental physics -- most notably, the
Bekenstein bound
[https://en.wikipedia.org/wiki/Bekenstein_bound](https://en.wikipedia.org/wiki/Bekenstein_bound)
\-- which seem to constrain any future quantum theory of gravity to have a
form that would rule out these sorts of hypercomputers (for example, by
limiting the amount of energy that can be pumped into a finite region, without
causing the region to collapse to a black hole, and by likewise ruling out
computer components that are smaller than 1 Planck length across or that do
more than 1 step per Planck time).

I've often speculated that ultimately, the hardness of NP-complete problems in
the physical world might come to be seen as analogous to the impossibility of
faster-than-light signalling or perpetual motion machines---i.e., something
that we simply take as primitive and then use to explain _other_ phenomena in
physics. But while the hardness of NP-complete problems sometimes gets used in
that way already, I also think we have a lot more work to do before the
situations are truly parallel. (For starters, we could prove P!=NP. :-) )

------
beefman
What good is oracle separation? The recent separation between BQP and PH, for
example, doesn't seem to mean much. Of course such results may spur the
discovery of new proof techniques, or new ways of thinking about a problem,
but is the result itself useful? I must be missing something, like a way of
stringing together oracle separations to produce a real separation... or is it
just the case that oracle separations are considered some kind of suggestive
evidence that a real separation exists?

Edit: This post[1] says two classes are equal with no oracle if they are equal
with respect to every oracle, but apparently only because "every oracle"
includes the null oracle...

[1]
[https://www.scottaaronson.com/blog/?p=451](https://www.scottaaronson.com/blog/?p=451)

~~~
jkabrg
I think oracle separations are currently only useful as barrier results,
showing that certain proof techniques can't solve the full problem.

------
ahelwer
Hey, Scott! Love your book, Quantum Computing Since Democritus. There is one
section which confused me, in the Quantum chapter
([https://www.scottaaronson.com/democritus/lec9.html](https://www.scottaaronson.com/democritus/lec9.html)).
I have two questions:

1) You say applying the unitary operation is the quantum analogue of "taking a
coin and flipping it" \- what do you mean by this? Could we think of, for
example, a pair of slits as applying this transformation on a photon qbit
being sent through them, so it's now in superposition with regard to which
slit it went through?

2) What does it mean when you say there are two paths to |1>? I understand
this section is getting at the physical underpinnings of this mathematical
model, but I can't quite wrap my head around how there are two "paths". My
mind is stuck thinking of the Bloch sphere as a state machine you can
deterministically hop around by applying unitary transformations.

P.S. I did a talk called Quantum Computing for Computer Science which covers
everything up to the one-bit Deutsch Oracle problem in 1.5 hours; I found that
presenting quantum algorithms as running on a "unit circle state machine"
(basically 2d Bloch sphere by restricting states to real numbers) was a very
effective way to explain the subject to software engineers!
([https://www.youtube.com/watch?v=F_Riqjdh2oM](https://www.youtube.com/watch?v=F_Riqjdh2oM))

~~~
ScottAaronson
1) Yes, a unitary transformation like the Hadamard gate maps the state |0> to
|0>+|1>, while mapping |1> to |0>-|1>. In either case, if you then just
measured immediately in the {|0>,|1>} basis without doing anything else, you'd
see |0> or |1> with equal probabilities, so it would have the effect of a coin
flip. But of course, in other cases---e.g., if you measured in a different
basis, or if you applied the Hadamard to a state that wasn't just a |0> or |1>
basis state---you could see that Hadamard is _not_ just a coin-flipping
transformation, because it's able to produce interference.

2) When we talk about the different "paths" that contribute to a given
amplitude, it's just a fancy way of saying that we can organize the matrix
multiplications in such a way that the amplitude we want is a giant sum. So
for example, suppose we apply Hadamard twice in sequence to the initial state
|0>. The first Hadamard maps |0> to (|0>+|1>)/sqrt(2). The second Hadamard
maps |0> to (|0>+|1>)/sqrt(2) and |1> to (|0>-|1>)/sqrt(2). So by linearity,
it maps (|0>+|1>)/sqrt(2) to

((|0>+|1>)/sqrt(2) + (|0>-|1>)/sqrt(2))/sqrt(2) = (1/2+1/2)|0> \+
(1/2-1/2)|1>.

So in this case, we could say that there are "two paths leading to |0>," both
of which contribute 1/2 to its final amplitude (so that the amplitude is 1).
There are also "two paths leading to |1>," but one contributes 1/2 to its
amplitude and the other contributes -1/2, so the two contributions interfere
destructively and the final amplitude of |1> is 0.

This is sometimes called the "Feynman" or "sum-over-paths" picture of quantum
mechanics. As you can see, though, it's just a different way of looking at
exactly the same math, namely multiplication of matrices and vectors.

So then why use the sum-over-paths picture at all? Well, a few reasons:
physicists like it because it often gives them more insight into what's going
on, into what are the more and less important contributions to a given
process, and it can also make calculations easier. Meanwhile, computer
scientists like the picture because it lets us simulate a quantum computer by
a classical computer, still using exponential time but now using only a linear
amount of memory, rather than the exponential amount of memory we'd need if we
tried to store all 2^n amplitudes at once.

~~~
i-blis
The cleanest explanation I read of information / state linearity properties of
quantum operators. Thanks! (Working through Brian Hall's Quantum Theory for
Mathematicians and Frederic Schuller's course at the moment, both highly
recommended).

------
code_coyote
Hello,

As for "careers in science", what can someone later in life (I just turned 60,
can't believe it) still do? I have a CS degree from ages ago, and have a
scientific mindset (runs in my family, e.g., my sister was a research
scientist [mathematician] before retirement). I don't want to retire anytime
soon. But I'm unlikely to get into grad school. I can take MOOCs of course,
and do. My interest in science tends toward neurology and embodiment.

~~~
ScottAaronson
I admire your determination to start a scientific career at age sixty! I once
faced the "converse" problem---how to start doing research at age 14 or 15,
before I'd found any mentor to help me---and it was one of the great
revelations of my life that, at least in theoretical fields, the barriers
standing in the way are more internal than external. So long as you have the
time, and enough income to live off, you can start reading textbooks and arXiv
papers. You can email the authors your questions. You can go to conferences,
with or sometimes without registering (just don't tell anyone I said so ;-) ).
You can sit in on classes at a nearby university (ask the professor; most are
fine with it). You can talk to the professors. You can start working on a
problem that interests you. If you get somewhere, you can write it up and
submit it to a conference or journal. You can offer your services as a
research assistant. The gates are open.

Of course there's a chicken-and-egg problem here, where the stronger your
research record, the more busy researchers will want to talk to you, and the
more they'll talk to you, the stronger you'll be able to make your record.
Just like with dating or anything else, I guess, it does take persistence to
break this cycle. :-)

And of course, you'll need to be considerate of people's time, and you'll need
to decline to take it personally if some people are too busy to answer you.
Just plow ahead and ask others.

Lastly, let me strongly suggest finding some other people who started their
research careers later in life, and asking _their_ advice. They'll surely be
able to think of things I didn't!

------
erdevs
I'd just first like to say that I love reading your work. I'm always delighted
when I see an update on Shtetl-Optimized and I admire how you are both
simultaneously rigorous and funny in your papers and posts. Your PnP survey
stands out in my mind as a truly fun and insightful read in particular.

Two questions:

1\. Any meaningful updates you'd make to the PnP survey today?

2\. As a total aside, I'm curious for your thoughts on blockchains. Not
specifically proof of work as a BFT system per se, but more broadly curious to
what degree you think "trustless" transactions and data processing might or
might not be transformative.

Apologies in advance if you've recently provided thoughts on either of these
topics and I've sadly missed them!

~~~
ScottAaronson
1\. The P vs. NP survey is only 2 years old, so all the edits I'd make to it
now would be rather minor ones: for example, including some more recent
circuit lower bounds of Ryan Williams and others, some more no-go results for
Geometric Complexity Theory, and Raz and Tal's BQP vs. PH breakthrough (which
required a new circuit lower bound, though not of a kind that can evade the
natural proofs barrier).

2\. See here for my answer to someone who asked me for my thoughts on
blockchain just a couple weeks ago:
[https://www.scottaaronson.com/blog/?p=3861#comment-1768247](https://www.scottaaronson.com/blog/?p=3861#comment-1768247)

~~~
erdevs
Thanks for the reply! And apologies I didn't realize the autocorrect on my
phone turned "PvNP" to "PnP". I'll read up on the recent works you mentioned
here. Appreciate the link to your comment on Blockchain technology as well.

If you still have time to answer questions.. is there any recent work in the
cryptographic or blockchain space you see as standout? I haven't referred
recently to any citations or updates, but found Ben-Sasson, Bentov, Horesh and
Riabzev's recent work
([https://eprint.iacr.org/2018/046](https://eprint.iacr.org/2018/046)) on ZK-
STARKs intriguing.

------
blr246
As a programmer and practitioner, I'm curious about what kinds of skills and
training it takes to program quantum computers.

Can you shed some insight into what's really different about the tools and
task of programming a quantum computer versus using classical programming
languages and tools? Do you think quantum computer programming will rapidly
become standard training for CS undergrads, or do you expect it to remain a
niche skillset like FPGAs, etc, since it will only supplement and not replace
classical computers.

Also, nice to meet you. Your essays have been inspiring over the years.

~~~
adrianN
Check out some lecture notes, e.g.
[https://cs.uwaterloo.ca/~watrous/LectureNotes/CPSC519.Winter...](https://cs.uwaterloo.ca/~watrous/LectureNotes/CPSC519.Winter2006/all.pdf)

~~~
mrybczyn
Also another course:
[http://www.math.uwaterloo.ca/~jyard/qic710/](http://www.math.uwaterloo.ca/~jyard/qic710/)

------
bruth
Hi Scott. I did not listen to the podcast (yet), so sorry if you answered this
already, but what, in your opinion, could be the greatest impact quantum
computing has in society? I know its a broad question, but curious of a few
bullet points.

~~~
ScottAaronson
Short answer: I think the greatest impacts might be in simulating quantum
physics and chemistry themselves, and thereby giving a new window into nature
that could have applications to drug design, materials science, batteries,
high-temperature superconductors, and more. But I could be as badly wrong as
(e.g.) someone speculating about the impacts of classical computers in the
1940s, who could see applications to weather prediction and other physical
simulation problems but would've totally missed the creation of Hacker News.

Long answer: See my recent blog post
[https://www.scottaaronson.com/blog/?p=3848](https://www.scottaaronson.com/blog/?p=3848)
about exactly this!

------
smikhanov
What is your opinion on how advances in QC may advance our understanding of
human consciousness? I remember that in your QCSD book you make few remarks
about Penrose's microtubules theory, the one that tries addressing two
questions about human mind:

1\. Are mind processes Turing-computable?

2\. If yes, at which level in the brain does the consciousness emerge?

Penrose's answers to that (as far as I know) are not generally accepted by the
scientific community. What is your intuition about these two questions?

~~~
ttctciyf
I don't want to prejudice an actual answer here, but it's worth noting Mr
Aaronson has produced a fantastic paper, almost uniquely substantive in its
field, commenting on several aspects of possible relations between quantum
physics and the mind, see
[https://www.scottaaronson.com/blog/?p=1438](https://www.scottaaronson.com/blog/?p=1438)

See also his blog posts on IIT, etc., such as:
[https://www.scottaaronson.com/blog/?p=1799](https://www.scottaaronson.com/blog/?p=1799)

------
erostrate
Hi Scott

There are non-quantum physical systems that exhibits positive and negative
amplitudes and interference.

Can I factor large numbers by throwing rocks in a lake and measuring the water
height at the right place? Why not?

~~~
mindcrime
Not Scott, and this has nothing to do with interference, but it _is_ possible
to "calculate" certain things using physical phenomenon like water levels.
It's basically a variation of "analog computing".

[https://en.wikipedia.org/wiki/Analog_computer](https://en.wikipedia.org/wiki/Analog_computer)

 _An analog computer or analogue computer is a form of computer that uses the
continuously changeable aspects of physical phenomena such as electrical,
mechanical, or hydraulic quantities to model the problem being solved_

I don't think it has much to do with QC, but analog computing is interesting
in its own right.

~~~
erostrate
Thanks! This is indeed related and part of my inspiration for this question.

Some other interesting references:

\- The waterfall argument in paragraph 6 of the mind-boggling "Why
Philosophers Should Care About Computational Complexity".
[https://www.scottaaronson.com/papers/philos.pdf](https://www.scottaaronson.com/papers/philos.pdf)

\- "NP-complete Problems and Physical Reality"
[https://www.scottaaronson.com/papers/npcomplete.pdf](https://www.scottaaronson.com/papers/npcomplete.pdf)

------
mpodlasin
What would be your advice for older (25+) people who want to get into science?
Is it even possible? Or should I just accept that the train has left and focus
on something else? Can you develop your math/logic/critical thinking skills at
that point?

How about if you never excelled at these topics in school? Is hard work
enough, or do you think some people are born with these talents?

~~~
WhompingWindows
The brain is able to change well into adulthood ("neuroplasticity"), and that
includes mathematical/scientific/abstraction centers. There are plenty of
folks who didn't get a great start in STEM, but through hard work and
dedication, they pushed through the inherent frustration in learning STEM.

While some people might be born with a proclivity for these activities, I
wouldn't say any individual could not get into science. For the truly
uninitiated, check out Planet Earth, Blue Planet, or Cosmos. For the novices,
check out your local astronomy club, ask scientists you know to explain their
work to you, and don't be afraid to ask follow-up questions. Get into reading
science articles in the popular press, and use those to find links to the
_real_ research articles, which will be VERY hard to read for beginners. Feel
free to skim those, look at graphs/charts/evidence, and read the
abstract/conclusions rather than the intro/methods and the whole thing.
Finally, check out some MOOCs or local community classes/continuing education.

~~~
code_coyote
Good uplifting answer. But it is for what can lead to a hobby, rather than a
career. Even certificates from MOOCs won't lead to a job in science,
researcher or not.

~~~
mindcrime
_Even certificates from MOOCs won 't lead to a job in science, researcher or
not._

That depends on how tightly you define "science". If you mean "science in
traditional academia, working for/at a major research university or research
consortium" then you are almost certainly correct. But if you expand the
definition to include the corporate world, and roles that maybe aren't _pure_
research, then I would argue that you can get a job doing science with less
"paper credentials" than one might expect.

Whether or not that would/could apply to anything related to QC, I'm not sure,
as I don't work in (or even really near) the QC domain. But to pick one
example: in terms of machine learning / AI, I've definitely seen it. But maybe
AI/ML is an exception to everything else just because it is (at the present)
such an empirical / observation / experiment based domain.

Outside of all of that is the notion of "create your own job". If you want to
be a researcher in Field X, start a company related to Field X and hire
yourself. And, no, I don't intend that to be a glib answer, and I certainly
acknowledge that it A. isn't easy, and B. is probably harder / easier in some
domains than in others.

------
da-bacon
Does the recent result on BQP not being in PH relative to an oracle do
anything to your priors about the power of quantum computers relative to
classical computers? If you had to distill why that oracle separation works,
what would you say is the main trick from the perspective of "where does the
power of quantum computers come from"?

~~~
ScottAaronson
Honestly, almost all of the technical innovation in this breakthrough had to
do with classical circuit complexity—-once you know my Forrelation problem,
there’s almost no further input you need about quantum computation. (Well, a
slight amount, since Raz and Tal had to modify Forrelation a bit to get their
proof to go through.)

For an attempt at a popular summary of what the circuit lower bound
innovations consisted of, see my blog post:

[https://www.scottaaronson.com/blog/?p=3827](https://www.scottaaronson.com/blog/?p=3827)

or, of course, their paper.

No, this doesn’t much change my priors about the power of quantum
computation—for one thing, because we all (or at least I :-) ) were already
pretty damn confident that Forrelation was not in PH. On the other hand, I was
not expecting that the separation could be proved right now—certainly, not
without first proving some weaker separations like BQP vs. AM.

------
thetoejam
What do you think would be the best route into Quantum Computing? Through
physics, maths or computer science?

~~~
ScottAaronson
That depends on you and your interests! From the very beginning and up through
the present, the majority of people in quantum computing have goten there via
physics. But my own background is in theoretical computer science---I've only
gradually picked up little bits of physics through osmosis (what's a boson,
what's a Hamiltonian, etc. :-) ) and there are still embarrassing gaps in my
knowledge. And computer scientists and mathematicians, such as Peter Shor,
Gilles Brassard, Umesh Vazirani, and Dorit Aharonov, have clearly made crucial
contributions to the field, and we plan to continue doing so!

By now, there are large interdisciplinary programs in quantum information
science (at Waterloo, Caltech, MIT, Maryland, Berkeley, CWI Amsterdam,
Singapore, Oxford, and elsewhere), as well as smaller programs like the one
we've been building at UT Austin -- where in some sense, the work of blending
math, CS, and physics into the smoothie of quantum information science has
already been done. So one obvious option for a student interested in this
field would be to seek out one of those programs -- they typically have
courses and research opportunities even at the undergraduate level.

------
rw
Hi Scott, thank you for writing your blog all these years. Your Busy Beaver
essay ignited my passion for computer science, especially in algorithm
analysis, logic, undecidability, and probability theory. I used to be someone
who only thought in code; thanks to you, I now also think in math.

~~~
ScottAaronson
Thanks; that made my day!!!

------
tomlemon
In your mind what is the solution to the "Dice Room" paradox you describe in
"PHYS771 Lecture 17: Fun With the Anthropic Principle"
([https://www.scottaaronson.com/democritus/lec17.html](https://www.scottaaronson.com/democritus/lec17.html))?

I take it you don't believe in the SIA? Is this paradox irreducible? Is the
world going to end soon?

~~~
ScottAaronson
There are at least two plausible solutions to the Dice Room, depending on
whether you adopt SSA or SIA. I wish I knew something more insightful to say
about it than that, but I still don’t know the right way to think about
indexical probabilities. Do you?

------
andrepd
How do you feel about the state of higher education? Specifically, do you
think expensive degrees, underpaid PhD/Postdoc positions, and shortage of
tenured jobs may discourage students from joining academia and push them into
industry instead? It feels to me our best brains are at Facebook and Google
doing cutting-edge research on how best to manipulate our mammal brains into
buying ever more crap...

~~~
ScottAaronson
Yes, I'm (not surprisingly :-) ) a fan of increasing federal funding for
academic research. Equally important, enough of the funding needs to be in
unrestricted, competitively awarded grants for basic science, rather than tied
up in huge projects that are someone's hobbyhorse. And dramatically cutting
NSF graduate fellowships---something that the NSF will likely do if Trump gets
his way with slashing the NSF budget---would obviously be a huge step in the
wrong direction.

In quantum computing in the US, we have no right to complain right now about
the level of federal funding, since almost everyone else has it worse! For us,
the limiting factor has instead tended to be tenure-track faculty positions in
CS and physics departments. All the funding and brilliant students in the
world wouldn't matter if, outside of a few elite universities, the CS
departments all said "this is really physics," the physics departments all
said "this is really CS," and both treated it as a passing fad that might
disappear in a couple years. Fortunately, though, the situation has steadily
improved and I expect that to continue.

On the whole, I think the fact that Google, Microsoft, and other companies
hire so many CS PhDs is good, not bad. That's a big part of what lets us admit
lots of talented PhD students, and honestly tell them that their career
prospects afterward are good. On the other hand, I think it's also true, as
you say, that society could better capture the value of a lot of these people
by trying to keep them in the open academic world.

------
hirundo
Hi Scott, I'm astounded by the accomplishment of AlphaZero in quickly becoming
a chess master without chess specific programming. Could a program of the same
kind be adapted to infer or deduce the rules of chess from a large set of
valid games? Or is that a different kind of problem?

If so, could it be adapted to learn the rules when we're not clear on them
either, like those for the games of love or politics?

~~~
dreamcompiler
Most of the rules of chess are trivial and thus should be deducible from
observing less than one complete game. Rare things like castling might take a
couple of games.

Love and politics have rulesets that are many orders of magnitude more
complex; so complex that we don't even know how to write them all down.

~~~
YeGoblynQueenne
>> Most of the rules of chess are trivial and thus should be deducible from
observing less than one complete game. Rare things like castling might take a
couple of games.

You'd think so. Try googling "chess rules induction".

------
montrose
Do you think the state of the art in quantum computing is already more
advanced than we realize, in the same way that the state of the art in
cryptography was when R, S, and A thought they had discovered RSA?

~~~
ScottAaronson
If I knew the answer, I couldn't tell you. :-)

More seriously: people have mooted this possibility for as long as I've been
in this field (~20 years). But keep in mind that, when Cocks and Williamson at
GCHQ discovered what would later become known as RSA and Diffie-Hellman key
exchange---so, 3-4 years ahead of the open world---cryptography essentially
didn't yet exist as an academic subject. Almost _all_ the action was still
closely tied to the intelligence community. So, no surprise that a not-yet-
existing discipline had fallen behind!

By contrast, quantum computing has been openly studied for decades and has
thousands of people working on it all over the world. The central thing that
causes me to be skeptical of the "million-qubit quantum computer sitting in
the NSA's basement" hypothesis, is that we pretty much _know_ who the best
people are, and we haven't noticed any effort to vacuum them all up analogous
to the Manhattan Project.

Like, it's no secret that the NSA and DoD, and other military and intelligence
agencies around the world, are interested in this field and fund a good deal
of work on it. In fact my main grant right now (the Vannevar Bush Faculty
Fellowship) comes from the Office of Naval Research. But if the secret world
is light-years ahead of the open world, then they'd _also_ need to be
executing a giant cover operation of _pretending_ to care about what we in
academia are doing! :) So at what point does it become an unfalsifiable
conspiracy theory?

------
gb22
Hi Scott

1\. What do you think is the potential for emergence of a third computing
model, apart from classical and quantum, which is sufficiently different in
its fundamental computing paradigm from these? Do you believe biological
substrates offer opportunity for computing which classical/QC cannot
replicate?

2\. If we achieve quantum supremacy, who do you think will be the biggest
winners and losers, in terms of professional skill set and in terms of
business models for private companies? Will physicists become much more
influential if their ability to model and deliver new evidence (and thereby
theory) and convert them into commercial products is advanced?

3\. Would it be fair to say that d-wave machine is more an analog computer
than a quantum computer; And do you think there is possibly a class of
computing problems that might be better/more efficiently solved by analog
computers than digital, and without requiring QC?

~~~
ScottAaronson
1\. Quantum computing is the most powerful model of computation we have based
on _currently known physics_ \---in the sense that anything more powerful
would need to be based on new physics. From the standpoint of theoretical
computer science, a biological computer is "just" a different way to implement
classical computation, typically with very slow speed but very enormous
parallelism. It _might_ someday have practical advantages, but unless we're
totally mistaken about biology, it's not going to solve any problems
efficiently that are outside BPP (i.e., classical probabilistic polynomial
time). For that you need a quantum computer.

2\. Yes, if quantum computing is successful, I imagine that might be good for
the careers of many of the people involved in quantum computing.

3\. Regarding D-Wave, there's this weird tendency to get fixated on words and
definitions (but is it "really" QC or "really" analog?) even after you've
explained the reality of the situation. See
[https://news.ycombinator.com/item?id=17426023](https://news.ycombinator.com/item?id=17426023)
for my current summary.

Again, I'm skeptical that analog computers will ever be able to solve any
problems outside BPP---for that I think you need a QC. (I.e., if not for
quantum mechanics, I _would have_ believed in the Extended Church-Turing
Thesis. :-) ) Whether analog computers will ever again compete with digital
ones on the constant factors, leaving aside the asymptotics, for problems that
people actually care about, is a harder and more complicated question to which
I don't know the answer.

------
NoKnowledge
Hi Scott,

Do you think we will see a Quantum Winter (or even several), similar to how
we've had several AI Winters?

We see tremendous amounts of funding in academia (and also industry) to build
ambitious projects that still have significant issues to overcome (both
theoretical and engineering) before being any close to being implemented. At
the same time there (still) seems to be much misunderstanding, such as Vivek
Wadhwa's recent post[0] or various reports about an unhackable internet based
on quantum information. I am afraid that this may lead to people having
unrealistic expectations on what can be built and how fast it can be built,
with the consequence that we will see less and less funding and investments in
the nearby future.

[0]:
[https://www.scottaaronson.com/blog/?p=3645](https://www.scottaaronson.com/blog/?p=3645)

~~~
ScottAaronson
Yes, when you look at the history of AI, a "quantum winter" is an obvious
worry---i.e., a situation where the hype about QC becomes _so_ unmoored from
the reality as finally to cause the popular narrative to switch its polarity.
Picture a pointy-haired boss who's now pouring money into QC, for no better
reason than that it's new and faster and it's the future and people are
talking about it and it could speed up his company's data mining by trying all
possible answers in parallel. If something spooked that boss, one could
imagine him joining a stampede for the exits with no greater understanding,
canceling good research along with bad. If it happened to AI, then why not to
us?

The fear of a quantum winter is one reason---if any reasons were needed
besides the truth!---why I've spent so much effort on my blog over the past
decade trying to counter irresponsible QC hype. Like, if anyone ever comes to
me and says "you lied to me! it turns out that scalable QCs are really hard to
build, and a lot more basic science needs to be done, and even if you did
manage to build them, as far as anyone knows they'd only give you exponential
speedups for a few special problems, not for most of the stuff my company
cares about," I'll have a pretty enormous record that I can point to when I
reply, "I WAS SCREAMING THAT AT THE TOP OF MY LUNGS AND YOU DIDN'T WANT TO
HEAR IT!" For all the good it will do. :-)

Then again, maybe the worry is overblown. Some people claim that we've now
passed the point where there will never again be an AI winter, any more than
there will be an "electricity winter." The train just has too much momentum.
Likewise, so long as the pressure continues to get more and more computing
power and stave off the end of Moore's Law, it could be that QC will continue
to entice people, regardless of the naysayers and regardless of how hard the
engineering problems turn out to be.

I honestly don't know. I feel like I have a hard enough time understanding and
communicating the truth about where this field stands in the present---and
sometimes, trying to advance it by a small increment---without _also_
prognosticating its future. :-) The latter involves all sorts of questions of
economics, politics, and psychology that I have no special expertise about.

------
mudil
Hi Scott,

David Deutsch, in his book "The Fabric of Reality", gives example of a problem
that is presumably easily solvable by the quantum computer. On the other hand,
he explains that the total number of particles in the universe is smaller than
number of computations that will take place. The question that he poses is
where the problem was actually solved? His answer is that there parallel
universes, and the computation is performed in these universes. He also
stipulates that the "weird" quantum behavior of particles, such as spooky
action at a distance, all could be explained by the multiverse. What is your
opinion about the multiverse and potential of quantum computers to solve
quantum physics problems?

~~~
mudil
I found the relevant passage.

"Logically, the possibility of complex quantum computations adds nothing to a
case [for the Many Worlds Interpretation] that is already unanswerable. But it
does add psychological impact. With Shor’s algorithm, the argument has been
writ very large. To those who still cling to a single-universe world-view, I
issue this challenge: explain how Shor’s algorithm works. I do not merely mean
predict that it will work, which is merely a matter of solving a few
uncontroversial equations. I mean provide an explanation. When Shor’s
algorithm has factorized a number, using 10^500 or so times the computational
resources that can be seen to be present, where was the number factorized?
There are only about 10^80 atoms in the entire visible universe, an utterly
minuscule number compared with 10^500. So if the visible universe were the
extent of physical reality, physical reality would not even remotely contain
the resources required to factorize such a large number. Who did factorize it,
then? How, and where, was the computation performed?"

------
thecupisblue
Hi!

So, what do you do when you're not flipping qubits around?

Got any cool stories you wanna share?

~~~
ScottAaronson
When I'm not flipping qubits around, I eat, sleep, blog, answer emails, play
with my two kids, get depressed reading the news about US politics, and then
get even more depressed reading people saying mean things about me and my
nerdy friends on Twitter and Reddit.

~~~
thecupisblue
Awwww man! That sucks, but screw those naysayers. They ain't the ones running
point on quantum computing.

(I don't really have much to ask you since anything I could comprehend is
easily googlable and I don't wanna waste ur time. Just wanna say keep up the
good work and thanks!)

~~~
ScottAaronson
Thanks!!!

------
taserian
Hiya, Scott! I recently heard the first episode of Rationally Speaking podcast
episode where you were the guest (yes, I'm catching up with the RS Archives
during my long commute). I've also started reading QCSD (hey, if it's going to
be our decade's GEB, we may as well give it an acronym. :: grin:: )

I've already corrected my very mistaken understanding of how QC works, as in
the tagline of your blog. Are there any other concepts that we need to shake
off that would make explaining these concepts easier for the layman? Do you
have any words of wisdom for the masses?

~~~
ScottAaronson
Sure, you could shake off the idea (if you haven't already...) that quantum
entanglement means communication faster than light. This is, interestingly,
exactly the same kind of error as the one that says that a quantum computer is
just like a classical computer but with exponential parallelism. Namely, you
look at the resources that would be needed to _simulate_ a quantum system
using a classical system (faster-than-light communication in the one case,
exponential parallelism in the other). You then confuse those with the
resources that the quantum system itself provides you.

In reality, quantum mechanics is carving out a third profile of abilities,
which is neither as weak as the classical profile, nor as strong as the thing
that people mistakenly overcorrect to once you tell them that the classical
profile is inadequate. E.g., you can violate the Bell inequality but NOT send
instantaneous signals; you can solve factoring in polynomial time but probably
NOT NP-complete problems. As I like to say (someone already quoted it
elsewhere), it's a sufficiently strange state of affairs that no science-
fiction writer would have had the imagination to invent it.

------
chrispeel
Quantum information theorists such as Patrick Hayden and Leonard Susskind
suggest that information can be neither created or destroyed. Do you ever
think about such things? If so, how does it enter into your work?

~~~
ScottAaronson
Yes, a fundamental property of _quantum information undergoing unitary
evolution_ (meaning, no measurements, no discarding stuff into the
environment, etc.) is that it can be neither created nor destroyed. This fact
enters into my work, or the work of anyone else in quantum information, sort
of like how the number '2' enters into our work---i.e., so thoroughly that it
would be hard to pick it out as a separate ingredient!

------
OscarCunningham
How do you feel about the Axiom of Choice?

~~~
ScottAaronson
It feels really uncomfortable that, if you have infinitely many people wearing
red or blue hats that they can't see, then they can all guess their own hat
color with only finitely many of them being wrong. Not to mention Banach-
Tarski and a hundred other strange phenomena. This all militates toward
rejecting AC.

But then, if we reject AC, we can have infinite sets that are incomparable
(i.e., they're not isomorphic and neither is larger than the other). So pick
your poison!

Thinking about such things for too long makes me feel grateful that I spend
most of my time in the finite world (or, let's say, the world of continuous
parameters that we only ever measure to finite precision, so that the
statements we care about can ultimately be phrased arithmetically). In this
world, because of e.g. the Shoenfield absoluteness theorem, anything that can
be proved with the help of AC can also be proved without it.

~~~
phs
As a follow-up, what is your position on intuitionist/constructivist logic?

~~~
ScottAaronson
I don't begrudge others their sincerely held faiths! But I confess that I
haven't yet seen the need for nonstandard logics for anything I've personally
been interested in.

------
elizabeth8890
Are you collaborating with the National Labs at all, i.e.,
[http://quantum.lanl.gov/q_computing.shtml](http://quantum.lanl.gov/q_computing.shtml)

~~~
ScottAaronson
I had close colleagues at LANL, but many of them (Leonid Gurvits, Howard
Barnum, Manny Knill) have since left. I'll probably visit Argonne and Sandia
sometime in the next few years to give talks, and to learn about what they're
doing in quantum computing. I don't think I've written papers yet with anyone
from those labs, but, uhh ... "there are no strangers, just coauthors you
haven't coauthored with yet!" :)

------
enriquto
You wrote this:

> For example, breaking almost any cryptographic code can be phrased as an NP
> problem. So if P=NP—and if, moreover, the algorithm that proved it was
> “practical” (meaning, not n^1000 time or anything silly like that)—then all
> cryptographic codes that depend on the adversary having limited computing
> power would be broken.

Can you explain this reasoning more precisely? The class P contains difficult
problems that require O(n^googolplex) algorithms, so are not solvable in
practice. The fact that P=NP would not make these problems any easier.

~~~
Recursing
He explicitly says «not n^1000 time or anything silly like that» in the
sentence you quote, n^googolplex would be way more silly

~~~
enriquto
my point exactly. The class P contains this silly stuff.

~~~
svat
He's already covered that; not sure what's left to explain: he's said that if
P=NP _and_ if we get there with a practical running time algorithm (e.g. one
that solves 3SAT in O(n^4) or something, and with reasonable constants too),
_then_ such-and-such consequences. So what are you asking?

~~~
enriquto
The second part of the claim seems much, much stronger than the first part,
but he makes it sound like it's a minor detail. I am confused as to why.

~~~
teraflop
The claim was that "all cryptographic codes that depend on the adversary
having limited computing power would be broken."

Here's a (very slightly) more rigorous justification:

If P=NP, then any NP problem is in P with at most a polynomial _slowdown_.
That is, if there's an algorithm taking T steps on a non-deterministic Turing
machine, we can solve it on a deterministic Turing machine in f(T) steps,
where f is a polynomial. Presumably, a "practical" algorithm would be one for
which f has a low degree.

The kinds of algorithms we're concerned about in cryptography (and plenty of
other fields) already have low time complexity. For example, generating or
verifying an HMAC is O(n) in the length of the input. So if we had a way to
solve NP problems with a low-degree polynomial slowdown, we could break HMACs
in low-degree polynomial time.

It doesn't matter that there are O(n^1000) problems out there that would still
be realistically unsolvable, because those problems don't have practical
applications in the first place.

------
denimalpaca
I took a grad-level quantum computing class which I didn't quite have the
physics background for, and the lecture that lost me, about 3 weeks in, was on
the different kinds of physical gates quantum computers use.

My recollection is that there were 3 necessary gates for a quantum
functionally complete set: One gate was classically functionally complete, the
other two are where I got lost. Can you explain like I'm 5 (or explain like
I'm an undergrad with minimal QM knowledge) what these other gates are doing?

Thanks!

~~~
ScottAaronson
Actually, if you already have a classically functionally complete gate (say,
the Fredkin or Toffoli gates), then you only need _one_ other gate to get a
functionally complete set for quantum computing. This one other gate could be
the Hadamard, which I discussed in other answers. The Hadamard gate is needed
to put your machine into a superposition of states, and then also to create
interference between the different branches of the superposition. Of course,
if you have no superposition and no interference, then it isn't much of a
quantum computer!

If your question was instead to explain the concepts of superposition and
interference themselves, then unfortunately that would take more time.
However, you could try some essays that I wrote a while ago

[https://www.scottaaronson.com/writings/highschool.html](https://www.scottaaronson.com/writings/highschool.html)

[https://www.scottaaronson.com/blog/?p=208](https://www.scottaaronson.com/blog/?p=208)

in addition to the resources that have been linked to elsewhere on this
thread.

------
nqureshi
What advice would you give for productivity/getting things done?

~~~
ScottAaronson
I'm like the worst person on earth to be giving anyone else advice about
that!! Do you have _any idea_ how much time I waste obsessively reading the
news, or worrying about people saying mean things about me on social media,
rather than doing research or anything else useful for the world? I suppose my
advice would be: don't do what I do. As my former PhD adviser, Umesh Vazirani,
likes to tell people, "concentrate on the high-order bits."

~~~
VladimirGolovin
> how much time I waste obsessively reading the news

This is oddly comforting. As Tim Ferris said in Tools of Titans, every
successful person is dysfunctional in some way. I guess the trick is to work
around your own personal deficiensies, and that's something everyone must
figure out on their own.

------
matrixmatty
Hi Scott,

Is there any chance advances in quantum computing will affect crypto
currencies. I.e. it might be possible to shortcut mining or mess with the
quorum of the distributed ledger?

I'm curious as crypto currency is so hyped, it would dismay me that advances
in quantum computing might disrupt the current landscape, or even better
improve it in some way. Entanglement is already deployed and in use for data
transit, in curious how annealing or other physics might be a game changer

~~~
ScottAaronson
See this recent survey article:
[https://arxiv.org/abs/1710.10377](https://arxiv.org/abs/1710.10377)

Briefly, a fully fault-tolerant quantum computer could give a square-root
speedup for Bitcoin's proof-of-work, and could completely break its elliptic-
curve-based signature scheme. Both issues could in principle be fixed by
migrating Bitcoin to quantumly harder problems, though in practice doing so
could open up security holes of its own. This hasn't been done, but I think
maybe there are other cryptocurrencies trying to use quantum-secure crypto
from the outset? Googling just now, I found something called "Quantum
Resistant Ledger" ([https://theqrl.org/](https://theqrl.org/)) -- does anyone
here know more about what's out there?

Let me stress that none of this is a concern with the QCs _of the near future_
, which will have at most a few hundred decent qubits and no error correction,
and which will not be able to threaten Bitcoin or any other cryptography.

------
algon33
Hi Scott,

Do you think that chaotic systems would be better analysed by quantum
computing over classical computing? More generally, is BQP powerful enough to
deal with chaotic systems in the same way P is for linear systems?

Oh, and I think your review of "Enlightenment Now" was a bit too rosy. When he
analysed the data he's superb, but he seems to lambast people he heavily
disagrees with. Its a tad disheartening.

~~~
ScottAaronson
No, I don't think that QCs will help much for simulating chaotic systems,
except insofar as those systems are also quantum-mechanical. For (e.g.)
predicting the weather, there may be small quantum speedups that you can get
here and there, from Grover's algorithm and faster gradient computation and so
forth, but at a fundamental level, as far as anyone knows, you still need to
just iterate the partial differential equation from one time step to the next,
same as a classical computer does.

Stepping back, note that "quantum" and "nonlinear" are two completely
different concepts -- in fact, at the level of amplitudes (i.e., the
Schrodinger equation), quantum mechanics is the one example we have in physics
of a perfectly LINEAR theory! Alan Sokal had a lot of fun with that point in
his "Social Text" parody article:
[http://www.physics.nyu.edu/sokal/transgress_v2/transgress_v2...](http://www.physics.nyu.edu/sokal/transgress_v2/transgress_v2_singlefile.html)

On the other hand, this is perfectly compatible with quantum systems having
chaotic phenomena at the level of observables (like the positions and momenta
of particles), and in fact there's a whole field called "quantum chaos" that
studies such situations.

------
kuwze
Why is P=NP/P!=NP so difficult to prove?

I just wanted to say that I think your writing helped me appreciate Michael
Cohen and learn what an amazing person he was. The more I read about him, the
more I want to be like him. What qualities do you think helped him contribute
and be such a great person? What do you think a lackluster programmer could do
to be more like him?

~~~
jderick
If you have not seen it, you may be interested in his 116-page survey of the
problem.

[https://www.scottaaronson.com/blog/?p=3095](https://www.scottaaronson.com/blog/?p=3095)

~~~
kuwze
Thank you for posting that! That is a great overview and exactly what I was
looking for.

------
rain1
If I use a quantum random number to pick my actions does that mean there's a
universe where I took each option?

This terrifies me

~~~
ScottAaronson
Relax, my friend. If you accept the Many-Worlds Interpretation at all, then
you're _constantly_ splitting into parallel copies, millions of times per
second, with every little decoherence event that takes place in your brain.
Whether or not you use a quantum random number generator is totally beside the
point. :-)

(But if it makes you feel less terrified, you could also choose one of the
interpretations according to which "only our branch is real." As far as anyone
can tell today, there are no consequences for any experiment we'll ever be
able to do.)

------
r4um
How far are we from "emergence" in terms of AI ecosystem ? Will quantum
computing pave the way for it ?

~~~
ScottAaronson
Sorry, I don't know what "emergence" means in this context. If you mean AGI, I
think (hope?) that we're still quite some ways away from it. Yes, quantum
computing could help with AI -- for example through Grover's algorithm, which
lets you solve many search, optimization, and planning problems in roughly the
square root of the number of steps you would need classically. But it's a
complicated story: many of the problems we care about will still be
asymptotically hard even for quantum computers; conversely, as the spectacular
recent achievements of deep learning and reinforcement learning (not to
mention our own brains? :-) ) remind us, there's a great deal that can be done
even with classical computers---often, outside the regime where we understand
theoretically why the methods work. If you check back in 5 years or so, I'm
optimistic that we'll know more about the applications of quantum computing to
AI than we do right now.

~~~
BenoitEssiambre
"not to mention our own brains"

Is this a known known that our brain doesn't use QC?

~~~
ScottAaronson
No, it's an unknown known. :-)

------
Yajirobe
Hi Scott. I've read your 'Why Philosophers Should Care About Computational
Complexity' \- I found it to be a very nice read. Is your book 'Quantum
Computing Since Democritus' worth getting my hands on? Is it not going to be
too repetitive given that I've read your article?

~~~
ScottAaronson
Thanks!

Given your particular situation, it sounds to me like you should not only buy
QCSD, but buy 200 copies of it to give all your friends and family. :-)

More seriously, the overlap between QCSD and WPSCACC is actually relatively
small---e.g., WPSCACC just has one short section about quantum mechanics,
whereas that's at least half of QCSD. QCSD is also written in a much chattier
style (but at the same time, it has more space to exposit basic material). I
can't guarantee that you'll like QCSD, but I hope that gives you a sense for
the diff.

------
akeck
I think you've written that QM is in part probability math using complex
numbers. I've also read that human decision making doesn't map well to
classical probability. Do you know if QM/"complex probability" has been used
to build better models of human decision making?

~~~
ScottAaronson
Every few months there's another paper on the arXiv trying to do exactly that.
Pretty much without exception, I've found the papers to be terrible -- leaping
immediately to QM without first considering more prosaic stories for whatever
human behavior they're trying to explain. It's like quantum mechanics is a
hammer, and explaining human decision-making is a steak, and all these people
want to use the hammer to slice the steak for some weird reason, their
strongest argument being that the problem of slicing the steak doesn't map
well to their bare fingers. It's amazing what some people think they can get
others to swallow just by using the word "quantum"! :-)

------
vslira
What would be the best way to understand the relation between amplitudes and
probabilities in the “non-quantum” world? In your NYT piece you say that it
would be weird to give a “sq(-1) probability of rain tomorrow”, but are there
relatable uses or cases for amplitudes outside of quantum physics?

~~~
ScottAaronson
It's not obvious why you'd want to talk about complex amplitudes, whose
squared absolute values are probabilities, outside the context of quantum
mechanics. Maybe a better way to say it is: once you're talking about such
amplitudes, essentially by definition you ARE talking about quantum mechanics.
:-)

Having said that, in classical probability theory, it's sometimes useful to
look at the (positive) square roots of probabilities, for example to get a
distance measure between probability distributions. See here:

[https://en.wikipedia.org/wiki/Bhattacharyya_distance](https://en.wikipedia.org/wiki/Bhattacharyya_distance)

Also, we've often been able to use quantum tools to prove new results even
about classical theoretical computer science. See here for a beautiful survey,
though one that's already a decade out of date:

[https://arxiv.org/abs/0910.3376](https://arxiv.org/abs/0910.3376)

In this way, the mathematical tools that we've developed in quantum
information can "pay rent" in classical CS, even if we counterfactually
imagined that our world wasn't quantum-mechanical at all. It would take some
time to go through an example of this, but (e.g.) it was shown that if a
certain classical error-correcting code existed (called a 2-query locally
decodable code of subexponential size), then an even better quantum error-
correcting code would also exist, but the latter was something that people
already knew how to rule out.

In these situations, some people would argue that we're not "really" using
quantum physics or amplitudes; we're just taking MATH that was developed for
that purpose, and repurposing it for something else. But it would be weird to
say in a talk, "I'm now going to introduce a unit vector of complex numbers
that's just an _ordinary_ vector, nothing physical at all about it, then apply
the following tensor product of 2x2 and 4x4 unitary matrices to it...," when
everyone knows full well that all your intuition about this came from quantum
states. Indeed, I confess that even when I'm doing linear algebra that has
nothing to do with QM---i.e., my vectors _really are_ just vectors of real or
complex numbers, not amplitudes---I sometimes slip up and use the physicists'
notation for quantum states (called the Dirac ket notation).

------
n0mi1k
If advanced quantum computing were available in smartphones and other small
devices today, what applications would be possible that are not currently? (Or
which would be greatly improved)

Would it affect the things many people do most on their phones like messaging,
news and social media

~~~
ScottAaronson
As far as I know, it would have zero effect on any of that ... unless you need
to simulate quantum physics or chemistry, factor large integers, calculate
discrete logarithms, or possibly solve some large optimization problems while
doing your messaging, news, and social media. :-)

------
api
Do you believe there is now a quantum Moore's Law in place? I've seen graphs
showing quantum chips from Google, IBM, and Intel plotted on a log scale that
are suggestive. (These exclude adiabatic quantum computers which are different
beasts.)

If there is do you think we're perhaps less than 10 years from QC capable of
breaking common number theory based asymmetric cryptographic algorithms like
RSA or elliptic curve for at least lower key strengths? That's what these
graphs suggest.

(I know breaking crypto is not by any stretch the only or the most valuable
thing you can do with QC but it's the one that gets the most press and it's
relevant to my current work.)

~~~
ScottAaronson
I think it's too early in the field, and there's too much basic research still
to be done, to talk usefully about a "Moore's Law." For godsakes, we're not
even sure yet whether superconducting qubits or trapped ions or something else
(or a hybrid) will be the way forward!

Yes, you can make plots of the number of qubits, coherence times, etc. as a
function of year -- and if you listen to talks by John Martinis, Chris Monroe,
or the other leading experimentalists, you'll often see such plots. But at the
very least, you need to look at both dimensions (qubits and coherence time) --
not just at "number of qubits," which will be severely misleading! And even if
you do, there are very few data points to use for extrapolation, since it's
really only within the last ~6-7 years that people have even gotten qubits to
work well in isolation, let alone scaling them up. So it's really hard to
extrapolate.

Like, I'm hopeful that within the next decade, we'll have systems with a few
hundred qubits that will be good enough to do _some_ useful tasks that are
classically intractable (such as quantum simulation), though they certainly
won't be threatening public-key crypto yet. But I'm not sure even about that.
And I'd prefer to see what happens with this before speculating about the
timescale for the next step, of building a full universal QC (the kind that
_would_ break our existing public-key cryptosystems)!

------
godzillabrennus
Do you have any thoughts on the IEEE Quantum Computing Nomenclature Working
Group?

[http://standards.ieee.org/develop/wg/QCN-
WG.html](http://standards.ieee.org/develop/wg/QCN-WG.html)

~~~
ScottAaronson
No, I hadn't heard of it before your comment! Please no one tell the physicist
David Mermin about this, or he'll picket the group with his long-running
campaign to change the spelling of qubit to "Qbit." :-)

~~~
Y_Y
Is it pronounced kew-bit or kuh-bit or kwu-bit?

~~~
ScottAaronson
" _cue_ -bit." I.e., the name of the letter Q, followed by "bit."

------
bloone
Hey there Scott!

I just wanted to say I wrote a student-paper on quantum computational
complexity last year and I felt like your papers made up almost 90% of my
references. Thank you for making my learning about the subject possible!

On to my question: I'm working on a capstone project right now that's using
quantum computing to create a small video-game. I'm using the 5-qubit quantum
experience from IBM and I was wondering if you had any cool ideas/suggestions
for small game experiences that could use actual quantum computing resources
to teach people about the properties of quantum mechanics?

Thanks again for all your work in understanding the quantum world, Scott.

~~~
ScottAaronson
That's a tough one! So far, I've only seen ONE game meant to teach quantum
mechanics that I thought actually worked, in terms of being (1) actually about
QM rather than some vague analogy, and (2) fun to play. It's this one:

[http://quantumgame.io/](http://quantumgame.io/)

Notably, this game doesn't even try to teach about entanglement (which, no
surprise, is hard to keep track of in your head!). Tt deals only with a single
photon passing through a network of beamsplitters and phaseshifters: a
situation that has one foot in quantum physics and one foot in classical
physics (the macroscopic state of a laser beam obeys exactly the same math).
But the puzzles are really clever!

If you're just trying to create an educational game, why bother using the
Quantum Experience? Won't it just introduce enormous errors and delays (~30
seconds per run when I tried it), complicating and obfuscating whatever you're
trying to teach? Why not just make some puzzles that force people to reason
about small, idealized quantum systems, along the lines of the successful
example above?

~~~
bloone
That game is really quite cool, and certainly in the same vein as what I was
thinking of doing - thanks so much for the suggestion (I've already spent too
much time solving puzzles)!

I am looking at using something like the Quantum Experience for two reasons:
1) Because giving people real quantum computing results is really cool, and 2)
Because it's my capstone project and the work has to have a certain threshold
of technical exploration. That being said, I'm not well-versed in alternatives
that could maybe just simulate quantum properties without having to go through
an actual quantum computer.

I have been looking into creating a small quantum neural net (with either real
or simulated qubits) to show how that differs from a neural network running on
classical computing. I've been trying to work that into a game idea but I'd be
lying if I said I've come up with anything compelling yet. If you got any cool
game mechanics/ideas you think quantum computing resources would lend
themselves to then I'm all ears!

------
jd007
Hi Scott,

This may be an ill-formed question, but it's something I've been thinking
about for a long time:

Do you think the human mind is equivalent to Turing machines, or somehow above
it? Assuming we have an infinite tape/memory and time.

~~~
ScottAaronson
There are really two questions here.

The first one is, can the human brain be _simulated_ by a Turing machine in
its input-output behavior (to a suitable precision, given appropriate initial
data, yada yada)? Note that, even though you specified "infinite tape/memory
and time," from the outset I'm going to outlaw "simulations" that simply cache
what the human would do in response to every possible stimulus in a gargantuan
lookup table. For that kind of simulation _trivially_ exists, and its
existence tells us nothing interesting. I'll insist instead that the
simulation be "reasonable"\---so, at a minimum, that it simulate the brain
without an exponential blowup in size.

I don't know for certain, but as a believer in the physical Church-Turing
Thesis, my guess is going to be yes, this is possible. I.e., I guess that the
laws of physics are fully computable---that there's nothing in them like what
Roger Penrose wants---and I see no evidence that the brain can violate the
laws of physics that govern everything else in the universe.

(Even here, though, there remains the extremely interesting question of
whether, even in principle, one could scan a _specific_ person's brain well
enough to "bring additional copies of the person into being," without simply
killing the person if one tried. My friends in the futurist and singularity
movements expect that the answer is yes, but if one needed to scan all the way
down to the molecular level, then the No-Cloning Theorem of quantum mechanics
would certainly present an obstacle. For my thoughts and speculations about
this question, see my "Ghost in the Quantum Turing Machine" essay, which was
referenced elsewhere on this thread:
[https://www.scottaaronson.com/papers/giqtm3.pdf](https://www.scottaaronson.com/papers/giqtm3.pdf)
)

Anyway, the second question is whether, even if we agree that a human brain
can be _simulated_ by an appropriate Turing machine, there's some special
sauce of consciousness that makes there be something that it's like to be us,
but nothing that it's like to be a conventional Turing machine. I.e. there's
Chalmers' "hard problem of consciousness."

Here I'm going to plead ignorance, with my extenuating circumstance being that
we're up against arguably the ultimate mystery of existence. :-)

Yes, I feel like there's something that it's like to be me. (Though if I were
a philosophical zombie, just a glorified Turing machine made of meat, I
could've told you exactly the same thing, so you should take whatever I have
to say about this with a grain of salt. :-) )

And yes, I take it as axiomatic that there's similarly something that it's
like to be you---or for that matter, to be a chimpanzee or a dog (but an ant?
a bacterium? unclear). No, I don't understand it. No, I don't know what
properties of a computational process are either necessary or sufficient to
cause there to be something that it's like to inhabit that process; I don't
know how we should build an AI either to ensure that it's conscious or to
ensure that it isn't. In practice, we'd probably _have_ to extend a
presumption of consciousness to anything that behaved sufficiently similarly
to us---that, famously, was Turing's point in 1950. But even here there are
many uncertainties: for example, would you still take a machine to have
"passed the Turing Test" if you could perfectly predict everything it would
say given a copy of its source code---not just in principle but in practice?

------
Tomte
Hi Scott,

what would you say are the main reasons why you in your mid-thirties are a
professor at a prestigious university?

Sheer determination and perseverance? Less partying and more learning? Your
upbringing?

I’m asking, because sometimes there’s an interesting story behind it, like
with Matt Might
([http://matt.might.net/articles/tenure/](http://matt.might.net/articles/tenure/))
who found himself in a objectively bad situation — a child believed to die
soon — making him reevaluate priorities and getting successful as a side
effect.

~~~
ScottAaronson
Well, the "less partying" part sounds accurate. :-) In grad school, for
example, I had some good friends, but very little "party life" and certainly
very little dating life---not out of choice, just because I didn't even know
how to start. As a result, I was often severely depressed. On the positive
side, though, I was more productive at research than I've ever been since! I
had unlimited time to think, and also enormous drive to succeed so that at
least something would be going right in my life. Just as important, though, I
loved _actually working_ on research problems (still do), as opposed to just
the recognition you get from other people after you've solved one. I could
form an infatuation with a particular problem that would last weeks or months.

Now that I have tenure and a wife and two kids, and get to travel the world
and study what interests me, I'd like to tell myself a story according to
which "all the hard work and sacrifice ultimately paid off." On the other
hand, I know people who had fantastic social lives all throughout college and
are now _also_ highly successful---an instance of the general phenomenon that
there's no justice in the universe. :-D

Anyway, if you're interested, I told some more of my story in my Scientific
American interview with John Horgan -
[https://blogs.scientificamerican.com/cross-check/scott-
aaron...](https://blogs.scientificamerican.com/cross-check/scott-aaronson-
answers-every-ridiculously-big-question-i-throw-at-him/)

------
amelius
Hi Scott,

Of the two main possible applications of QC, i.e. computational chemistry and
cryptography, it's often just cryptography that is mentioned in the media. The
Wikipedia entry on QC, in its "Potential" section, discusses mostly
cryptography, while only two sentences are dedicated to chemistry.
Cryptography is also used as the toy-problem for QC prototypes, i.e. factoring
integers.

However, as I understand it, QC-for-cryptography will break existing
techniques, and will not (from an application standpoint) bring anything new
to the table.

Therefore, my question: does QC have a PR-problem?

~~~
ScottAaronson
It has multiple PR problems, including the one you mentioned. :-)

Everyone who follows QC knows that simulating quantum chemistry could be
commercially important, while breaking RSA is not. And as far as I can tell,
no one cares anymore about doing tiny demonstrations of Shor's algorithm, to
factor 21 into 3x7 or whatever. Over the past 5 years, the experimental
interest has shifted to (1) demonstrating sampling-based quantum supremacy
(which has nothing to do with Shor's algorithm), (2) demonstrating the
building blocks of quantum error correction, and (3) the prospect of doing
useful simulations.

(A central reason for this is that, until you have a full fault-tolerant QC
with millions of physical qubits, you almost certainly _can 't_ run Shor's
algorithm in a way that will outperform a classical computer, even if you
wanted to. By contrast, people are excited right now that they might be able
to learn something new for physics or chemistry with just a few hundred good
physical qubits.)

So anyway, we all know all of this, but popular expositions still like to
concentrate on breaking public-key crypto because of its wow-factor (and, of
course, the undisputed theoretical importance of Shor's algorithm, and its
possible eventual security importance). In addition, there's often a time-lag
problem, where the people working in the field have one set of concerns, and
then the broader discussion is still stuck in the world of 1997.

------
Jommi
I have so many questions for you, but lets just go with one.

Can you explain to me the concept of "logical qubit", and the whole debacle
about qubit quality? I'm still getting confused again and again over this.

~~~
ScottAaronson
Qubits lose their quantum superposition states as they interact with their
environment---effectively getting "measured" by their surroundings. This is
called _decoherence_ , and is the central engineering obstacle to building
useful QCs.

The longer a qubit lasts while maintaining its quantum coherence, and
(especially) the more operations you can do on it while keeping it coherent,
the better that qubit's quality.

In practice, we will never be able to build qubits of perfect quality (i.e.,
ones that maintain their coherence forever except when we deliberately measure
them). In the 1990s, some people thought this would be a fatal obstacle to
scaling up QCs. But then two closely-related discoveries, called quantum
error-correction and quantum fault-tolerance, dramatically changed the
picture. These discoveries showed that, even if your physical qubits fall
short of perfection, as long as they have a high enough quality, you can glom
a bunch of them together into a single "logical qubit"\---that is, a qubit
that lives in the collective state of multiple physical qubits, and that can
still be recovered even if any small number of those physical qubits lose
their coherence. Furthermore, one can do an arbitrarily long quantum
computation on these encoded (logical) qubits, continuously monitoring to see
which of the physical qubits have suffered errors and correcting those errors
(but _not_ monitoring in a way that would collapse the logical qubits!). In
this way, one can in principle build a reliable quantum computer out of
unreliable parts---a generalization of John von Neumann's famous discovery
from the 1950s, which showed that the same was true of classical computation.

------
distorted_torus
Hi Scott,

I am interested in attending graduate school for a PhD in Computer Science and
am beginning to think about the areas of research I'd like to get involved in.

Below are a few sporadic questions.

1) I really like mathematics, so what are the foundational areas of
mathematics in quantum computing? What areas of mathematics should researchers
know very well?

2) Are there any exciting connections between machine learning and quantum
computing that you know about?

3) What is a problem that really excites you about the field at this very
moment?

4) Can you direct me to good books, papers, or resources for the absolute
beginner in quantum computing?

------
aswanson
Any recommended books on complexity theory?

~~~
ScottAaronson
A classic from 1994 is "Computational Complexity" by Papadimitriou.

Two good sources for newer material (from the past quarter-century) are
"Complexity Theory: A Modern Approach" by Arora and Barak, and "The Nature of
Computation" by Mertens and Moore.

A personal, idiosyncratic favorite is "Gems of Theoretical Computer Science"
by Uwe Schoning.

------
ttctciyf
Hi Scott,

If it turned out to be true that advances in advertising technology like
profiling and microtargeting (see, e.g. [1]) could effectively deliver the
likelihood of electoral victory to their highest paying and most ruthless
practicioners, would this be something to worry about? And if so, what action
should we take in order to preserve democratic ideals?

1: [https://medium.com/join-scout/the-rise-of-the-weaponized-
ai-...](https://medium.com/join-scout/the-rise-of-the-weaponized-ai-
propaganda-machine-86dac61668b)

------
slbenfica
Scott what do you think is going to be a successor to the scientific
paper/academic journal? What do you think of platforms like distill.pub and
Fermat's Library and the impact they might have?

~~~
ScottAaronson
I'd seen Fermat's Library but haven't used it. I hadn't heard of distill.pub
before your comment.

Honestly, I don't see scientific papers going away anytime soon. What's the
alternative to an individual or group setting out clearly in writing what they
discovered, the evidence that it's true, and the background and context to the
discovery, putting the writeup where interested people can easily find it, and
then taking full responsibility for it, so others can cite and build on the
work? As far as I'm concerned, that's all a "scientific paper" means---
regardless of whether it appears in a prestigious journal or just the arXiv or
somewhere else on the web, and whether it otherwise follows normal academic
writing conventions or flouts them. Even an answer on a website like
MathOverflow, were it sufficiently well-written and sourced, could as far as
I'm concerned be cited as a research paper and given academic credit.

But while "papers," for some definition, are probably here to stay, I'm
optimistic about deep reform to the system of _journals_. The first step, of
course, should be for academics to shake off the yoke of the truly brazen
predatory publishers like Elsevier. Many of us have already pledged never
again to review for or submit to those publishers, at least until they
fundamentally change their practices. For more about this, see my review of
"The Access Principle":
[https://www.scottaaronson.com/writings/journal.html](https://www.scottaaronson.com/writings/journal.html)

As the next step, we should break free even of society journals, insofar as
they put research results behind a paywall. Everything---certainly if it was
funded by the taxpayers---should be freely accessible on the web. (In math,
CS, and physics, we _already_ put essentially all our papers on the arXiv,
where they're freely available, and have been doing that for ~25 years. But
the fact that the paywalls even exist still rankles---and other fields, like
biology, have yet to catch up.)

Ultimately, we might converge on the model of journals as "arXiv overlays":
that is, stamps of approval that particular arXiv preprints have been peer-
reviewed (which is pretty much the only service that journals now provide
anyway). Or maybe we'll even handle peer review in some other way entirely.
E.g., people keep trying to experiment with peer reviews being public, but it
keeps failing---possibly because, lo and behold, most academics don't _want_
their frank commentary on the importance of each other's work to be made
public with their names on it! :-)

As it happens, my friend and colleague Michael Nielsen used to work in quantum
computing, but now spends full time at Y Combinator thinking about the future
of scientific communication. He wrote a book that had lots of interesting
ideas for how we could improve things, and surely there are many other good
ideas waiting to be proposed, possibly taking advantage of recent or near-
future technologies. While the concept of a "paper" (or treatise, or
monograph, or note, or other unit of written research) strikes me as mostly
determined by the nature of science itself, as far as I'm concerned almost
everything else is up for grabs.

------
noobermin
Hey Scott, thanks for doing this.

What do you think of the Copenhagen Interpretation especially in light of the
delayed choice quantum eraser experiment[0]?

I know interpretation of QM is something actual working scientists try to
avoid (I know I do such for my little corner of physics) but given how much
interpretation informs intuition it's worth considering once in a while.

[0]
[https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser](https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser)

~~~
ScottAaronson
I confess I get annoyed when people make arguments about the interpretation of
quantum mechanics using complicated thought experiments involving lasers and
mirrors. Why not just talk about it in the modern way, in terms of qubits and
states? :-)

For some of my recent thoughts about interpretation of QM, see this blog post,
and especially the discussion in the comments section:
[https://www.scottaaronson.com/blog/?p=3628](https://www.scottaaronson.com/blog/?p=3628)

~~~
noobermin
I'm a laser physicist, so I think exclusively in terms of lasers and mirrors,
but I get your point.

------
drostie
Hi Scott, I first stumbled across your blog while I was across the pond doing
my Master's in Delft; it looks like you've been relatively successful since
then.

I was wondering how you feel that success/celebrity has affected you. Does it
actively drive you to something other than research -- say, are you viewing it
as a building block to maybe publishing a book or so? Or does it feel like a
distraction from research in that "oh why do I have these arguments online"
way that we can often fall into?

~~~
ScottAaronson
There’s no question that celebrity can get to your head in this line of work.
Every single time I get off my private plane somewhere, I’m mobbed by quantum
complexity theory groupies, not to mention the Hacker News paparazzi, and the
women shrieking and throwing their panties at me ... oh god, the women are so
persistent! Don’t they realize I’m married?

But I do try hard to “keep it real”—or better, “keep it complex”—by setting
aside some quality time just for me and some pen and paper. As I like to say,
it’s all about the amplitudes.

(Seriously: I _do_ have a book, Quantum Computing Since Democritus! And I
_have_ been approached by people asking me to sign it. But typically only in a
few highly selected places, like Cambridge, MA or Berkeley, CA. :-) )

------
TrinaryWorksToo
I'm halfway through the first chapter of Neilsen and Chuang's book. I'm
enjoying reading about the subject and am at the quantum parallelism part.

Can you explain why Grover's algorithm has a runtime of root N? It seems like
the runtime should be log2(n) because of exponential qubits or 1 because there
must be a way for all the qubits to interfere.

Also, What resources do you reccomend for self study? Are there quantum
computing meetups in San Francisco that you can recommend?

~~~
ScottAaronson
The reason why the running time of Grover’s algorithm involves sqrt(n) has to
do with the Pythagorean theorem—or if you like, the fact that quantum
mechanics is based on the 2-norm, in contrast to classical probability theory
which is based on the 1-norm. Classically, each time you pick one item out of
N to query, you can add ~1/N probability to the marked item—so the probability
of having found the marked item after T queries goes like T/N. Quantumly, you
can add ~1/sqrt(N) _amplitude_ to the marked item with each query, so the
amplitude on the marked item after T queries goes like ~T/sqrt(N), and hence
the probability of observing the marked item when you measure goes like
~T^2/N.

A fundamental result from the 1990s, called the BBBV Theorem, shows that not
even a quantum computer can solve the unordered search problem any faster than
Grover’s algorithm solves it. I won’t prove the theorem in this comment :-),
but the intuition is simply that quantum mechanics is a norm-preserving and
linear theory. So you actually need to do something to gradually put more and
more amplitude onto the marked item; you can’t just instantly and magically
give an amplitude of 1 to whichever branch of your superposition happened to
hit the marked item.

I’m not sure if there are QC meetups in SF (does anyone else?). But certainly
nearby Berkeley is one of the centers of the world for QC—home to Umesh
Vazirani’s group, the Simons Institute for Theory of Computing, and now also
the startup Rigetti.

~~~
TrinaryWorksToo
That makes some sense with the extra dimension providing more space to store
information about all the information. I appreciate the detailed response with
some jumping off points. Thank you!

------
robertjoec
What are some of the most interesting research problems in your field? How
promising would you say topological quantum computers are to allowing for
fault-tolerant systems?

------
JakeTyo
Why don't we call Qbits just "qits". Since Quantum-Binary-Digit doesn't really
make sense. Do people in the field refer to them in other ways?

~~~
andbberger
Sure it makes sense - a qubit is canonically a quantum spin system, which has
two eigenstates, up and down.

~~~
JakeTyo
What about superposition? Or is that not considered a 'state'?

~~~
OscarCunningham
They're states but not eigenstates. It's like the difference between RGB
colour and greyscale. In both cases there are infinitely many possible
colours, but in greyscale they're all mixtures of two "primary" colours (black
and white) whereas in RGB they're mixture of four (black, red, green and
blue).

In a qubit the infinitely many superposition states are all mixtures of just
two eigenstates.

~~~
JakeTyo
Thank you! This makes perfect sense.

------
adrianN
When do you think will we have the first quantum computer that can solve
useful problems faster than the classical competition, for example in quantum
chemistry?

~~~
ScottAaronson
I _hope_ in 5-10 years, but really I don’t know, and no one else does either.

------
glifchits
In my opinion, you're one of the most entertaining and approachable writers on
mathematical topics like QC. How did you get good at writing?

~~~
ScottAaronson
It’s funny: when Philip Roth passed away recently, I was rereading some of his
stuff, and thinking to myself, “why am I so terrible at writing?”

If I have any tips, I guess they’d be bend-over-backwards honesty, willingness
to make an ass of yourself, practice, and more practice.

------
Zophike1
For those wanting to undergraduate research basically in REU in QIT what's
your advice for them?,what do they need to take on such an endeavor?,what are
researchers looking for in prospective students?, and finally would a
researcher take on a student who hasn't had much in terms of coursework but
has been teaching themselves ?

------
Leibniz0
Scott, I loved your book (QCSD). What are the chances that overcoming noise in
quantum gate systems as they get larger is a practical impossibility beyond
some threshold - for instance in the same way that inverting a gaussian
convolution requires super-exponentially less noise above a given sampling
density / stdev ?

~~~
ScottAaronson
If anything like that turned out to be true, and fundamental (rather than just
an engineering limitation), I’d regard it as a shocking discovery that would
overturn our current understanding of QM. After all, physics is local—each
particle couples mostly to its near neighbors—so there doesn’t seem to be any
_inherent_ reason why noise per qubit needs to keep increasing as you
integrate more and more qubits.

For more, see my other answers on this thread about QC skepticism.

------
nicklaf
Hi Scott,

If quantum computing ever becomes commonplace, and quantum computer programs
widely written and understood, do you envision that the teaching of quantum
mechanics itself will be significantly impacted? Or instead will we simply see
quantum complexity theory begin to redefine the undergraduate computer science
curriculum?

~~~
ScottAaronson
I think that the teaching of QM has _already_ been impacted by quantum
information—and I hope it gets impacted more, _regardless_ of if and when we
get useful QCs! To my mind, it’s just infinitely clearer to start with the
basic rules and properties of QM—states, unitary transformations,
measurements, tensor products, entanglement, density matrices,
etc.—illustrated with the simplest systems to which those rules apply, namely
collections of qubits (or more generally, finite-dimensional Hilbert spaces).
This already lets the students fully understand no-cloning, quantum
teleportation, quantum key distribution, basic quantum algorithms like
Bernstein-Vazirani and Simon, and other cool things from quantum information,
and it already lets them explore the conceptual questions (the measurement
problem and so forth) that interest most of them. After this material has been
mastered, and _only_ after, one could see how it gets applied to real physical
systems like the harmonic oscillator, the hydrogen atom, or a particle in a 1D
potential well. So, all the mathematical complications of infinite-dimensional
Hilbert spaces—which are _not_ essential to understanding QM itself—could be
deterred to this part of the course. This is the reverse of the historical
order in which the ideas were discovered in the 1920s, but I think it’s the
much more logical order in which to learn them—even if quantum information
_weren’t_ a big thing that people now care about.

If you want to see a recent text by a bona fide physicist that presents QM in
_exactly_ this way—what I think of as simply the “modern” way—then check out
Lenny Susskind and Art Friedman’s remarkable “The Theoretical Minimum” series.
Or you could look at pretty much any introductory book or course lecture notes
about quantum information, including mine. We do it this way as well, except
that we never _do_ get to the harmonic oscillator or the hydrogen atom. :-) I
can say from experience that the material is then totally accessible to any
bright undergrad who’s done math up through linear algebra.

~~~
nicklaf
OK, now I'm out of excuses for having not read _Quantum Computing since
Democritus_ ;-) (and _The Theoretical Minimum_ as well, why not both?).

Thanks for the enlightening response!

------
mygo
you’ve written about P vs NP in the past. What are your current thoughts on P
vs NP as of today? Do you have any speculation as to whether it’s one or the
other, and why? Do you think there have been any major recent breakthroughs
(say, in the past 5 years) to move us closer to a solution?

------
selimthegrim
Hi Scott -

What was the best part/most favorite thing about your sabbatical?

Can you explain why Forrelation is a guiding light for quantum deep learning
these days?

As a bonus - it would be nice to see a walkthrough - commentary of the recent
oracle separation proof of Tal and Raz and how they used Fourier transforms
specifically.

~~~
ScottAaronson
My favorite part of my sabbatical in Tel Aviv was having some actual time to
do research. My second-favorite part was the hummus.

Forrelation is _not_ a “guiding light for quantum deep learning.” I’m not even
sure if there’s such a thing as “quantum deep learning” that’s sufficiently
well-developed to deserve the name. You can use Forrelation as a separating
example for some other quantum learning problems, e.g. k-means clustering, but
so far that mostly just shows that you can artifically shoehorn an exponential
quantum speedup into those tasks, rather than that the tasks will give us
useful quantum speedups in “real life.”

In the Forrelation problem, I conjecture that the only thing that really
matters about the Fourier transform is that it’s a unitary matrix all of whose
entries have small absolute values. As such, it lets you relate two Boolean
functions in a way that a quantum computer can notice, but that’s extremely
“global” in nature and doesn’t show up locally. Raz and Tal may have used some
additional technical properties of Forrelation in their proof (now that I
write that, I’m actually not sure—I’ll need to check!). But if they did, then
I conjecture that another proof could avoid the use of those properties.

~~~
ScottAaronson
Just as a quick followup: Avishay Tal has confirmed for me that, indeed, their
proof still goes through if you replace the Fourier transform by any other
unitary matrix with bounded entries.

~~~
selimthegrim
Thanks, Scott! I thought you’d missed this. Maybe we’ll get to have you give a
colloquium here at Tulane someday soon!

------
evanb
Hi Scott,

What do you think of radical Mathematical Platonism (eg. a la Tegmark)? As a
computational physicist I tend to tell people that in my hands a computer is
like a telescope, but it lets me see into mathematics---but that seems to
tacitly assume the reality of mathematical objects.

------
zitterbewegung
How long do you think that we will have a quantum computer that will out
compete traditional computation?

What do you think about Topological Quantum Computation?

How do you think we can solve the problem that many people getting their PhD
are vastly underpaid and also adjunct professors?

------
geraltofrivia
Hi Scott, I got interested in Quantum Computing, and fundamental mathematics a
week ago. Ever since, I'm reading your book `Quantum Computing since
Democritus`. I discovered your blog (oh and it's such a joy to read!). Today I
run into your podcasts and AMA. This is going to get overwhelming very soon, I
feel.

Couple things: your book is an amazing repertoire of abstractions, theorems,
phenomenon and whatnots. Does it get overwhelming sometimes walking down the
street like normal people, with that deep an understanding of these domains?

I personally think that your book is a nice place to start with, in the
domain(s) of fundamental mathematics. The text gives a nice idea of the depth
I'll encounter in these fields (I think). Keepin' it real helps me, an
engineering grad, to not pass it up as ravings of men with too much time on
their hands. Do you think your book should be read in this context? What else
would you recommend, if any?

~~~
ScottAaronson
Knowing more math than most of the people who you pass on the street, but at
the same time having vastly inferior social skills to them, is something that
you sort of get used to in childhood. :-) You should keep in mind, also, that
as a theoretical computer scientist, I spend a lot of my time hanging around
people who are abnormal in more-or-less the same ways I am, and in many cases
more so.

Besides QCSD, I’d try Mermin’s Quantum Computer Science: An Introduction for
QC, or any of my 50 favorite books from this list

[https://www.scottaaronson.com/blog/?p=3679](https://www.scottaaronson.com/blog/?p=3679)

if you’re just looking for some interesting reading material.

------
jderick
Do you think that quantum physics can tell us anything about 'the hard
problem' in philosophy? Is it possible that the mind could somehow control how
quantum states collapse in situations where randomness would be the typical
explanation?

~~~
ahussain
It doesn't answer this question directly, but Aaronson's paper "The Ghost in
the Quantum Turing Machine"[1] is a great read. It talks about the
relationship between quantum mechanics and free will.

[1]
[https://www.scottaaronson.com/papers/giqtm3.pdf](https://www.scottaaronson.com/papers/giqtm3.pdf)

~~~
ttctciyf
It is a great read! Also, in the comments on his blog post introducing that
paper, he says:

> Well, yes, “the fact of experience” is a toughie! :) In fact, I regard the
> “hard problem of consciousness” as so far beyond us, that it’s not even
> clear that science or rational argument give us any sort of toehold.

\-
[https://www.scottaaronson.com/blog/?p=1438#comment-80656](https://www.scottaaronson.com/blog/?p=1438#comment-80656)

------
fpoling
Hi Scott,

If somebody claims that there is no freedom of will, what what you accept as a
reasonable proof? It seems that one can be sceptical even in presence of
extremely precize predictions as it my be easier to alter memory about
predictions than to produce them.

~~~
ScottAaronson
I’m not sure I fully understand your argument, but if someone showed that they
could perfectly predict the actions of the people around them, say hours or
days into future, placing the predictions into a sealed envelope or a
cryptographic commitment and revealing them later, then _to whatever extent
fraud and the like had been ruled out as explanations_ (in the ordinary
experimental ways, and possibly with the help of James Randi :-) ), I’d have
to say that the person had successfully unmasked free will as an illusion.

~~~
fpoling
thanks for a reference about James Randi! Now I reliazed that indeed if the
claims survives exteremely sofisiticated scrutiny, then it really does not
matter if it was a fraud or not, and one would need to accept one lives in a
world where one is predictable in one way or another.

------
xTouny
Hello Scott, I have gathered from YouTube that you enrolled in CS Major in
order to make your own video game, How does that fit a prodigious math geek
who started learning calculus aged 11 and lately one of the pioneering quantum
theorists?

------
bem94
Do you think that humanity is well placed to be responsible wielders of
quantum computing as a tool, and what do you feel your role (as a scientist
with a large public following) is in shaping those future perceptions of
responsibility?

~~~
ScottAaronson
No, I don’t think humanity is well placed to be responsible wielders of
quantum computing. But the thing is, we’re even _less_ well-placed to be
responsible wielders of nuclear weapons, or climate-destroying combustion
engines, or probably even guns or fire! :-) So with all the more dangerous
technologies out there, it would be weird to fret about _quantum computers_ of
all things: something that seems unlikely to do much harm (at least if we take
care to upgrade our crypto), and that _might_ even do significant good for the
world, for example if quantum simulation lets us design more efficient
batteries and solar cells.

------
EtherTyper
Will non-linear quantum computers ever be possible? If so, how would they be
made, and what would programming them be like? Do we still model non-linear
states with vectors and transformations with unitary matrices?

~~~
jey
What does "linear" mean to you? Integration is linear[1]. The Hilbert space of
all the functions you care about is linear[2]. Life is locally linear[3]. So
maybe "non-linear" is just a red herring.

1\.
[https://en.wikipedia.org/wiki/Linearity_of_integration](https://en.wikipedia.org/wiki/Linearity_of_integration)

2\. Not really but close enough.

3\. I don't know what I mean either, but
[https://en.wikipedia.org/wiki/Linear_time-
invariant_theory](https://en.wikipedia.org/wiki/Linear_time-invariant_theory)
is interesting anyway.

~~~
EtherTyper
I mean quantum computers that harness the NLSE in their computations somehow.

~~~
jey
Interesting. That's way more physics than I know. :)

------
tombert
I don't have much to say, except I really enjoyed Quantum Computing Since
Democritus. The lighthearted nature of the book made it a lot more
approachable for someone who is mostly self-learning this stuff.

------
jaddood
Hello!

Could you give an overview of the proceedings of your job as a researcher in
quantum computing?

For instance, how do you start your day job, what do you do during the day
(how exactly do you do research,) how do you end your day, etc...

~~~
ScottAaronson
While I was on sabbatical:

7am - Wake up, eat breakfast, check email, help get daughter ready and walk
her to school

9am - Back to sleep

2pm - Wake up again, shower, eat lunch, check email. Possibly coffee with
friend or colleague.

4pm - Time to pick daughter up again!

4-6pm - Play with kids, eat more, check email

6-8pm - Read news and social media, get depressed

8-9:30pm - Help get kids ready for bed

9:30pm-4am: NOW it’s time to do some research!!! Tools: pen, paper, sofa,
LaTeX editor, and sometimes web browser to look up papers (but this last tool
is _extremely_ dangerous and can lead to procrastination)

4am - Collapse

Being back in Austin and teaching will severely affect the above hours. But in
any case, it’s not like I consciously decided on them, as part of some plan to
maximize productivity (!!) - this is just what I fall into when I don’t have
other obligations.

------
SurfaceCode
Scott,

Do you think it's a good idea that all the teams working on quantum computing
have put their money on the surface code? What are your thoughts? Do you
believe that we are putting all our eggs in one basket?

~~~
ScottAaronson
I'm not an expert on this, but I think the surface code is so popular because
it's 2-dimensionally local and has great parameters and speed of error
correction. But people certainly work on other codes too, and if someone
invents something that's unequivocally better than the surface code---or
better for what the experimentalists are building now---I guarantee you
they'll take notice! They don't always care about finer points of complexity
theory, but this they _do_ care about.

------
Nokinside
What kind of computer architectures are realistic or possible with quantum
computers?

You can't have quantum von Neumann computer or can you? Are realistic quantum
computers always going to be quantum circuits?

------
MrXOR
Hi Scott, What is the current status of PvsNP problem? P=NP or P!=NP? Is any
relationship between PvsNP problem and Riemann Hypothesis? Who to follow to
know the progress of solving PvNP problem?

~~~
FartyMcFarter
[https://www.scottaaronson.com/papers/pnp.pdf](https://www.scottaaronson.com/papers/pnp.pdf)

~~~
ScottAaronson
+1

There's no clear relationship between P vs. NP and the Riemann hypothesis,
except that the generalized Riemann hypothesis (GRH) would let you derandomize
certain randomized algorithms, and also Mulmuley's Geometric Complexity Theory
(a speculative approach to P vs. NP using algebraic geometry and
representation theory) involves some of the same mathematical objects that
were used to prove the analogues of RH over finite fields.

------
vikR0001
Given that observing the state of a qubit changes it, and (I guess) that
changes all entangled qubits, what methodology is used to read out the result
of a quantum computer's calculation?

~~~
ScottAaronson
When the quantum computation is finished, you suck it up and you measure. The
entire point of the quantum algorithm was to set things up in such a way that,
even as the measurement destroys the superposition state, the right answer is
observed with a high probability (since you used constructive and destructive
interference to boost its amplitude, while suppressing the amplitudes of the
wrong answers).

------
wokawoka
Can you describe how different the operating system would be from the
traditional operating system we use today for quantum computing? Broad
question but any specifics you can answer....

~~~
Koshkin
More importantly, what would quantum web browsing look like.

------
cc-d
Do you believe that nation state actors are currently utilizing quantum
computing, and if not now, do you see this happening in the near future? If
so, what use cases do you foresee?

------
navidfarhadi
Hi Scott. Thank you for doing this AMA.

In your opinion what are some good universities across the world to look into
if one wants to do graduate or post-graduate level research in quantum
computing?

~~~
ScottAaronson
I already answered that in another comment, but briefly: Waterloo/Perimeter,
Caltech, MIT, Berkeley, U. Maryland, Singapore, Oxford, Cambridge, Bristol,
CWI Amsterdam, Hebrew University, Tsinghua, UTS Sydney, McGill/Montreal, LRI
Paris, and don't count out UT Austin -- we're planning to expand a lot! And
many, many other places have at least one or two people in the field.

~~~
vymague
How about Germany? It seems people doing optics here like to connect their
research to quantum computing.

~~~
ScottAaronson
Yes, there's some great stuff going on at the Max Planck Institute for Quantum
Optics outside Munich.

Also the groups in Innsbruck and Vienna in Austria.

------
master_yoda_1
What is your suggestion for introductory quantum computing book?

------
ttctciyf
Is there a model of a computation that stands to quantum computation in the
same way a universal Turing machine stands to classical computers, or is the
UTM already enough?

~~~
ScottAaronson
Yes, you can define a universal quantum Turing machine, which is a single
quantum Turing machine U able to simulate any other quantum Turing machine M
(at least, to arbitrary precision) given a coded description of M on its tape.
This is one of the main observations David Deutsch made in his famous paper
from 1985.

~~~
ttctciyf
Thanks, and thanks for spending a big chunk of your time here!

------
jkabrg
Do you think there's any hope of finding a _practical_ algorithm for USTON
that runs in logarithmic space? Reingold's algorithm needs O(n^64^32)
time(!!!)

------
mcnamaratw
Hi Scott,

Can you recommend a nice technical intro on what this is all about? (Level:
I'm an EE, fine with undergrad quantum mechanics but never understood field
theory at all.)

Thanks! Tom McNamara

------
ClintEhrlich
Howdy Scott,

Is anyone in your field working on the implications of computational
complexity for normative ethics? Hume's guillotine relies on the impossibility
of any evidence for or against moral facts, but one could stipulate to that
while using e.g. Kolmogorov complexity to select moral facts with the highest
prior probability to a naive computational oracle.

Not saying it's how I'd necessarily choose my ethics, but if AGI employs
algorithmic inference (e.g., approximating Solomonoff induction) for
conventional epistemology, the potential may exist for it to extend those
algorithms to normative judgments, for better or worse.

------
Ono-Sendai
Hi Scott, What's your current position on the likelihood of quantum computing
working/being possible?

P.s. I enjoyed your book 'Quantum Computing Since Democritus'.

~~~
dsr_
That's a very wide question. How about something like:

What's the likelihood of quantum computing being a practical method to solve
some class of problem in the next ten years?

~~~
fiatjaf
You're assuming it's possible without explicitly saying it?

~~~
dsr_
If it's not possible, then the answer is clearly 0.

If it is possible but not practical for solving any problems, that's a thing
one would want to know.

If it's possible but unlikely to be practical within the next ten years,
that's even more useful.

Perhaps it's possible and already practical for some problem. I don't know
what that is, so it would be good information.

In all of these cases, it's Scott's presumably better-informed opinion that's
being requested.

~~~
fiatjaf
I want to know if it is possible.

------
Y_Y
Hi Scott,

Why do we live in a universe where the Halting Problem is unsolvable?

~~~
schoen
(Not Scott!)

The Halting Problem's unsolvability is proven in a fully abstract mathematical
way, and so the truth of this result is as logically necessary as the truth of
other mathematical theorems. It doesn't refer to anything about physics. So it
seems like this question might be better bifurcated into

Why do we live in a world in which Turing machines are a good model for what
computations we can physically perform?

and

Would the laws of mathematics be the same in any universe?

Each of these questions might then be a good one to pose to Scott.

~~~
ScottAaronson
Yup. :-)

Or better yet, we could bifurcate into:

(1) Why is our universe apparently unable to solve the halting problem for
_Turing machines_?

The answer, presumably, is a combination of the physical Church-Turing Thesis
(specifically, the apparent impossibility of Turing-uncomputable processes in
our world), with Church and Turing and Post's theorem on the unsolvability of
the halting problem.

Of course one could then push back and ask why the Church-Turing Thesis should
be true of our world: i.e., why _shouldn 't_ there be physical
"hypercomputers"? That's an enormous question, but my old survey "NP-Complete
Problems and Physical Reality" contains some thoughts about it:
[https://arxiv.org/abs/quant-ph/0502072](https://arxiv.org/abs/quant-
ph/0502072)

And then there's:

(2) Why should we live in a universe that's unable to solve "its own" halting
problem?

I.e., even if super-Turing computers _were_ physically possible, we could
presumably formulate a halting problem for _those_ computers, which would then
require a still more powerful computer to solve (a super-duper-Turing
computer?), and so on forever. This is because we could simply repeat Turing's
diagonalization argument at a higher-level up -- given very minimal properties
of computation, such as the ability to feed one program to another program as
code, the ability of one program to emulate another one, and the ability to
compute the NOT function. Once you have those properties, the inability of any
given computational model (even a super-Turing model) to solve its own halting
problem is just a matter of logic, as schoen said.

~~~
danbruc
_Once you have those properties, the inability of any given computational
model (even a super-Turing model) to solve its own halting problem is just a
matter of logic, as schoen said._

This seems to assume that logic provides some form of undeniable truth but I
am not sure that this is the case. Sure, every system of logic is hopefully
consistent and you can use it to derive true statements within it by simply
playing a game of symbol manipulation, and in that sense the system is a
source of undeniable truth. But when we are using logic to reason about the
real world, we need the real world and the logic we use to be compatible.
Classical logic works great in the classical world but quantum mechanics seems
to push it to its limits. You can certainly still use it if you are careful,
but it seems no longer a really good fit once you no longer have your
particles either here or there, either spin up or spin down.

So logic seems much more like physics if you want to apply it to the real
world instead of just using it for the fun of symbol manipulation. There are
systems of logic that are useful to describe the world and there are ones that
are less useful or probably even ones yielding wrong conclusions about the
real world. Long story short, I am not sure that we can conclude that there
will be halting problems for super-Turing machines because we can not be sure
what kind of logic would be adequate to describe those machines. Not that I
consider this a likely scenario or whatever, I just think we have to be more
careful when saying something is just a matter of logic.

------
beefman
Will the power of 2^72 complex numbers working for _me_ cause gravitational
collapse? Has anyone calculated holographic bound limitations on quantum
computers?

~~~
beefman
(can't edit)

There's a related comment here[1]. The simpler version of the bound doesn't
depend on mass or energy, just surface area (or radius for a Schwarzschild
black hole). It seems to me that 2^72 of "storage" is already pushing it, but
anyway I don't understand why the number of bits needed to "describe" the
state isn't the relevant quantity. IIRC Bekenstein had at least one completely
classical derivation of the bound.

[1]
[https://www.scottaaronson.com/blog/?p=2428#comment-786973](https://www.scottaaronson.com/blog/?p=2428#comment-786973)

~~~
beefman
Scott briefly discusses this issue in his book[1], where he cites a paper by
Davies[2], which in turn cites one of Scott's talks[3]. I'm not sure about
Davies' idea that the holographic bound should be formulated in terms of
Kolmorgorov complexity instead of Shannon entropy, but the general question of
holographic limitations on quantum systems deserves further study, it seems to
me.

[1]
[https://www.amazon.com/dp/0521199565/](https://www.amazon.com/dp/0521199565/)

[2] [https://arxiv.org/abs/quant-ph/0703041](https://arxiv.org/abs/quant-
ph/0703041)

[3] [https://arxiv.org/abs/quant-ph/0507242](https://arxiv.org/abs/quant-
ph/0507242)

------
deepnotderp
What do you think about the silicon photonic approach to quantum computing? In
particular, thoughts on scalability? Photonic qubits are unique wrt
decoherence, no?

------
marchdown
What are some research areas complementary to the quantum
computing/computational complexity where additional effort would remove
roadblocks in QC research?

------
deveac
Hi Scott. I'd love your thoughts on quantum encryption's viability vs.
supremacy, and how those two concepts might unfold in relation to each other.

------
soberhoff
How's that quantum supremacy coming along? I heard it was supposed to take a
year. And it's been a year.

Also, are you planning to write another book at some point?

------
virgil_disgr4ce
Thanks for doing this AMA, Scott. My question is: Could I ever get a job in
quantum computing (or related) being 100% completely self-taught?

------
soberhoff
Do you ever still do exercises? What's the last thing you learned that other
people might have learned as students?

~~~
ScottAaronson
I'm _constantly_ learning new things that to people from other fields would be
freshman-level trivialities! The most recent such example would need to be
something I learned today. For example, maybe a little tidbit that I picked up
from Sanjeev Arora's STOC'2018 tutorial on deep learning (where I was this
morning): it turns out, backpropagation is just a dynamic programming
algorithm for calculating a gradient in time linear in the size of the
network, improving over the quadratic time that one would've needed naively.

I feel bad that I rarely have the time (or willpower!) to do exercises
anymore, though I'd probably benefit if I did.

------
henryaj
Do you have any thoughts to offer on the "culture wars", especially the way
you were treated by the online mob when you wrote about your personal struggle
with your sexual identity[0,1]?

0\.
[http://slatestarcodex.com/2015/01/01/untitled/](http://slatestarcodex.com/2015/01/01/untitled/)

1\.
[https://www.scottaaronson.com/blog/?p=2091#comment-326664](https://www.scottaaronson.com/blog/?p=2091#comment-326664)

~~~
ScottAaronson
Dude, if you'd just asked me a more specific question, you could surely have
baited me into commenting on this or that culture war issue. Since you asked a
general question, though, I'll simply answer with the generality that I wish
we could all make more of an effort to show empathy for each other's problems,
and (as I said on the podcast) carefully articulate where we agree and
disagree with each other rather than just flinging 140-character excrement.

OK, I'll also give you a few links. Here's my Quora answer about what it felt
like to be widely attacked on social media, for trying to host a conversation
about gender and teenage nerdery that got past platitudes to the roots of
people's feelings:

[https://www.quora.com/What-was-it-like-for-you-to-be-
widely-...](https://www.quora.com/What-was-it-like-for-you-to-be-widely-
attacked-online)

Second, here's my friend Sarah Constantin's blog post about the changing
nature of social media, the one I mentioned in the podcast:

[https://srconstantin.wordpress.com/2016/11/27/hiding-in-
plai...](https://srconstantin.wordpress.com/2016/11/27/hiding-in-plain-sight/)

Third, here's a recent post called "On Culture War Bubbles," from the blog
"Thing of Things":

[https://thingofthings.wordpress.com/2018/06/27/on-culture-
wa...](https://thingofthings.wordpress.com/2018/06/27/on-culture-war-bubbles/)

This is much easier said than done, but as Ozy writes in that post, may we all
be granted the mental strength to ignore culture war bullshit, _until and
unless_ we decide with the rational parts of our brains that it's important or
worth responding to.

~~~
henryaj
That Quora post is wonderful - thanks for sharing. Glad you were able to rise
above the attacks made on your character :)

------
vymague
Most people were not able to succeed in academia. What do you think you did
differently than most people?

~~~
ScottAaronson
Uhh, most people don't even _try_ to succeed in academia! So no surprise if
they don't. It's not like I made it to the NBA or something. :-)

Let me speak only about academic CS, since that's what I know best. Of the
students who enter the major CS PhD programs in the US, I think something on
the order of half of them (maybe a bit less) end up in academic positions,
with the rest going to startups and industry and government. So, of the PhD
students who really want academic positions in CS, fortunately a large
fraction are still able to get them---just not necessarily at a top-tier
school. Of course, many students outright prefer industry -- Google and
Microsoft can, after all, offer compensation packages that blow academic ones
out of the water -- or else they prefer industry to the academic options that
are available to them. And that's fine. I feel lucky to work in a field where
PhD students have multiple career options, and when a student comes to me for
advice, I try to help them figure out what's best for them, rather than
imposing some global preconception on it.

From what I know, the situation is worse in other academic fields, like math
and high-energy physics and certainly philosophy and other humanities fields
-- where you can get dozens of well-qualified applicants for every academic
opening, with all the cutthroat competition and other pathologies you'd
imagine that would lead to.

~~~
vymague
Thank you very much for your answer. I suppose the situation is a bit
different in CS.

------
sixdimensional
Is it possible to simulate the function of a quantum computer in a traditional
computing system, even if it is slower, and would doing so be of any value?

Do you know what the Google Bristlecone quantum processor is? Is it a "real"
or simulated quantum computer?

Do you think quantum computers (or perhaps alternate computing paradigms like
trinary) are better at fuzzy problems than traditional binary systems?

------
Raed667
Hi Scott,

Would you see, in the future, quantum computing used by end-users? If so in
what possible applications?

------
naiv
Hello Scott,

can encrypted data on blockchains easily be decrypted with quantum computers
in the coming decade?

~~~
ScottAaronson
No, almost certainly not in the coming decade. See my answer here:
[https://news.ycombinator.com/item?id=17429547](https://news.ycombinator.com/item?id=17429547)

------
mherrmann
What would you recommend to an amateur wanting to tackle P vs NP in his spare
time?

~~~
tzahola
Algorithms 101

~~~
mherrmann
And beyond that?

~~~
137things
The most accessible route would probably to try to think up a polynomial time
algorithm for an NP-Complete problem. There are a lot of problems to choose
from (e.g. Sudoku, Battleship are some fun ones) which you can find a list of
on wiki or somewhere. Indeed you need some study on algorithm design (I guess
Algorithms 101), but really it all takes is some creativity. Hope you solve
the problem and win that million $s

------
angel_j
Is anybody attempting to build Quantum Computers that use light as the medium?

------
marcelluspye
Hi Scott,

Do you think some understanding of the implications of quantum computers is
important for laypeople, and what do you think is the best way to communicate
this kind of information? You can see the eyes of `regular people' glaze over
mid-sentence if you use the word quantum, and associate any quantum-anything
to magic. In the era of clickbait and fake news, this makes misinformation all
the more prevalent, especially when it comes to pop-sci/`mainstream' coverage.

tl;dr a) How much of a problem is it that people will believe anything with
the word `quantum' in it? and b) What do you think is the best way to remedy
this?

------
ryao
What books do you recommend on those topics?

~~~
ScottAaronson
Which topics?

------
jcmoscon
Do you believe in the simulation hypothesis?

~~~
gooseus
So I've been thinking about this simulation thing a bunch lately and while I'm
curious about Scott's answer here, I'll preemptively post my followup question
in case anyone else has any ideas or he wants to weigh in here:

Can you use the premises of the Simulation Argument to make similar statements
regarding the availability/capability of sufficiently advanced technology that
would potentially impact our coming into existence?

For instance, directed panspermia. Would at least one of the following have to
be true?

1\. The human species is very likely to go extinct before we can develop the
capability to successfully seed the galaxy with R/DNA-based life and spawn
another biosphere.

2\. Any sufficiently advanced civilization is unlikely to attempt to seed
other worlds with the basic building blocks of their living system so as to
initiate other biospheres.

3\. Our biosphere was almost certainly initiated by way of directed panspermia
from another sufficiently advanced R/DNA-based civilization that has existed.

For this argument, imo, (1) seems less likely than in the simulation argument
since I'd think we're closer to being able to send out small ships with
hibernating microscopic life than we are to building Universe-sized
simulations. (2) also seems less likely since the cost/benefit analysis of
seeding other worlds seems much higher to me than building and maintaining a
universe-sized simulation.

So for (3) if we assume abiogenesis started a civilization which decided to
start seeding other planets which then have a chance of leading to other
seeding civilizations, then we'd need to compare the likelihood of seeded life
vs abiogenesis to determine how likely it is that we are an abiogenesis
civilization vs a seeded civilization.

------
mjfl
Do you think modern PhDs overspecialize?

------
_rpd
What's our best understanding of the process of quantum decoherence? Are there
open questions? Problematic engineering issues?

------
fiatjaf
How does quantum computing fit in the quantum worldview of Wolfgang Smith?

~~~
ScottAaronson
Sorry, I don't know who that is.

~~~
Koshkin
I got curious and found out that it's this guy who "raises the questions of
whether the scientific method is in fact dependent on the scientistic
philosophy and, if it is not, whether linking it to other philosophical
frameworks would provide better solutions to the way [quantum] physical
phenomena are interpreted."

------
mathattack
No question. Just to comment that I enjoy your writing.

------
MartianSquirrel
Which do you prefer: Crêpes or waffles?

------
kingofking
ELI5 quantum computing since I’m a total noob at it.

------
nonamenoslogan
How long until quantum computing can put an end to those bullshit
cryptocurrencies?

------
mkagenius
If related, Have you heard about qubits being able to explain dark energy
mystery? What is you view about that?

------
internetman55
Hi Scott, hope you are doing well. A friend of friend worked with you and said
that you are nice and they enjoyed it a lot. Hope you have a wonderful day

------
liamcardenas
Thoughts on this comment?

"You don't actually want qubits, you want an analog computer with
differentiable signals. Most likely photonic. Qubits are a dead evolution
branch.

I've been recently exploring computational metamaterials for photonic
computation.

[http://users.ece.utexas.edu/~aalu/research%20-%20page%203.ht...](http://users.ece.utexas.edu/~aalu/research%20-%20page%203.htm).
(there's quite a few papers on this but unfortunately they are all paywalled.
Spoiler alert, they seem to be based entirely on Fourier transform).

These computational metamaterials don't need electricity to be powered (you
need something that will shoot the photons on them and read back the values
off tho).

Machine learning would be much, much faster on these as you have O(1)
differential calculus.

They don't heat up. You can possibly build a house sized CPU out of these. I
can see it, a city block sized CPU and a nuclear reactor next to it.

Did you know that on an analog machine, you can do sort in O(n)?

[https://en.wikipedia.org/wiki/Spaghetti_sort](https://en.wikipedia.org/wiki/Spaghetti_sort)

Hit me up if you wanna chat about this. I've seen the "light" (xdddd) now and
can't go back to stupid bits.

I'm not like super married to the metamaterials but analog photonic trumps
quantum for just about every task I can think of."

\- adamnemecek

Source:
[https://news.ycombinator.com/item?id=14674333](https://news.ycombinator.com/item?id=14674333)

~~~
rspeer
I'm not Scott, but spaghetti sort isn't any kind of computational
breakthrough, it's just trading off measurement error for computation time.

Here's the digital version of spaghetti sort:

1\. Enumerate the possible lengths of spaghetti that your spaghetti sorter can
distinguish above a certain probability. This enumeration will be small and
finite.

2\. Round your values to one of these lengths.

3\. Radix sort or bucket sort those values in O(n).

------
logeek
What are the prime factors of this number?
1847699703211741474306835620200164403018549
3386634101714717857749106516967111612498593
3768430543574458561606154457179405222971773
2524660960646946071249623720442022269756756
6873784275623895087646784409332851574965788
4341508847552829818672645133986336493190808
4671990431874381283363502795470282653297802
9349161558118810498449083195450098483937752
2725705257859194499387007369575568843693381
2779613089230392569695253261620823676490316 036551371447913932347169566988069

~~~
Koshkin
This number is prime.

~~~
schoen
I doubt it.

    
    
      >>> gmpy.is_prime(1847699703211741474306835620200164403018549338663410171471785774910651696711161249859337684305435744585616061544571794052229717732524660960646946071249623720442022269756756687378427562389508764678440933285157496578843415088475528298186726451339863364931908084671990431874381283363502795470282653297802934916155811881049844908319545009848393775227257052578591944993870073695755688436933812779613089230392569695253261620823676490316036551371447913932347169566988069)
      0

------
godelmachine
Hi Scott,

Its been my longtime dream to work in Electronics Engineering - more
specifically, design integrated circuits for Artificial Intelligence/ Machine
Learning applications. I read about FPGA's a lot and love the concept of
Reconfigurable Computing. Its because of my love for FPGA's I want to carve
out a career in VLSI. Right now, I work in Enterprise Software (BMC Software -
as a Support Engineer and in administrator role).

I tried to get a job in VLSI after attending a 6 month finishing school from
CDAC, but no luck. However, I still dream of getting an opportunity in
companies like Intel/ Xilinx and being able to develop some FPGA based
solutions for AI/ ML.

In the next 18 months, I see myself getting my Canada PR and searching for a
job there. My current job requires me to be in touch with Java and Java
related frameworks to a very small extent (primarily for reading logs, and our
development platform is build up on Eclipse.) I am hoping to get a job in
Canada based on these skillsets.

What do you reckon is the future of the hardware industry? I heard there's a
lot of scarcity of jobs there?

Or should I stick to Enterprise Software?

Honestly need your opinion here.

Thanks in advance, and great job taking up this AMA !! :)

