
Scott’s Supreme Quantum Supremacy FAQ - xmmrm
https://www.scottaaronson.com/blog/?p=4317
======
whatshisface
If it goes well, the history of quantum computing will be divided up in to
three eras: the era of twisty philosophical arguments that it's working ("the
molecule is simulating itself"), the era of academic arguments that it's
working ("we can solve this one carefully constructed problem") and the era of
practical arguments ("Amazon is selling QC time for $20/kilogate-bit, what do
you mean it's not possible?"). Quantum supremacy marks the transition from the
first era to the second.

~~~
smsm42
The third stage is the hardest. Thermonuclear energy production has been
sitting in "theoretically possible, but economically unviable" for decades,
tons of technologies like supersonic commercial flight has been achieved and
then scrapped because of practical concerns, many other technologies struggle
with passing economic viability barrier. Taking that into account, getting
from 2nd to 3rd stage would be the hardest task. Comparing to classic
computers, their practical usability has been clear since Babbage time, so far
it's not the case for QC.

~~~
lebed2045
I don't think it's true for quantum computers. Folding proteins and stuff is
good use-case of the cause but the real drive is the military. Same as for
gen-AI. If you don't r&d this but Chine does - you're fucked. So the only way
to do not become in the disadvantageous position - is to reason it yourself.

~~~
Judgmentality
If the military is paying for it then it's already in stage 3. Just because
something isn't readily available to the public doesn't mean it's a failure -
I can't buy a rocket but the various space programs still do lots of cool
things and they have money to pay for it.

~~~
smsm42
Military pays for a lot of stuff that then is not going anywhere. Just as
other venture funds do. You can't win without taking some risks. But that
doesn't mean everything the army pays for automatically is a great thing.

------
ClintEhrlich
I've been waiting for Scott Aaronson to put all of this into perspective since
the first leaks about Google's quantum supremacy started appearing in popular
media.

He has exceeded my expectations with this post, which cuts through all the
hype to communicate exactly what the results of this experiment mean for the
field. It's worth reading and sharing.

~~~
ClintEhrlich
On second reading, I have but one trivial gripe:

"Enormity" implies moral reprobation, so it's a poor way of describing the
significance of a computational discovery.

~~~
klipt
Isn't it just

Enormous : enormity :: huge : hugeness?

~~~
ClintEhrlich
Not quite.

Enormous : enormousness :: huge : hugeness.

Enormity = Immense scale of evil (e.g., the "enormity of the holocaust")

~~~
sho
It can be used in that way, but the neutral usage is valid too. I'd even argue
the "evil" undertones of the usage you describe borders on archaic.

"Enormousness" isn't a word in common usage that I'm aware of.

~~~
ClintEhrlich
I have no dog in the fight, and wish "enormity" had never developed a
normative undertone, but I strongly disagree that said usage is anything close
to archaic.

If you Google "enormity," the dictionary definitions it displays before the
results are: 1.the great or extreme scale, seriousness, or extent of something
perceived as bad or morally wrong. "a thorough search disclosed the full
enormity of the crime" 2\. a grave crime or sin. "the enormities of the
regime"

Merriam-Webster claims this is not the exclusive usage, and that enormity can
mean "immensity" without normative implications when the size is _unexpected_.
But the very example it cites, from Steinbeck, involves the "enormity" of a
situation in which a fire was started.

That said, I agree that "enormousness" is an awkward word, which I do not use.
I'm left to ponder the enormity of my own pedantry.

------
nexuist
Preface: I know nothing about quantum computing.

What exactly _is_ a qubit? I'm not asking what does it mean, because I know
there's superpositions and all that jazz, but as in...like, in an electronic
circuit, what is a qubit? Is it made out of logic gates? Which ones?

If we can make one qubit, can't we just make a bunch of them by copy and
pasting circuits similar to how we used vacuum tubes in the 60s and 70s? How
come our current limit is only around 54 or so?

Are qubits, and quantum computers by extension, not even electronic circuits?
If so...what the hell are they?

~~~
rolltiide
I'm in the same boat. A qubit can have more than the two states that a
transistor can have, got it.

Okay, now what can we do with that?

"crack encryption by simulating a state!" yeah but what? is that something I
should be concerned about now? "hahaha no no no silly normie we'd need _two
thousand_ qubits for that, this machine only has 53!" oooookay, and you did
that number in your head, how??? "we just solved the first unsolvable problem
that a mere bit bound supercomputer couldn't solve, look at this math
formula!" but that didn't explain "we are celebrating, are you not
celebrating"

There just seems to be a lack of non-introductory but non-PhD level
information. Where is the "explain it like I've been accepted into college at
all".

~~~
rini17
Schrodinger's cat in unopened box is 1 qubit = it's alive and dead at the same
time. When the box is opened to observe the result, the quantum state
"decoheres" \- decays to 1 bit result.

Now imagine 53 such boxes, interconnected by quantum gates. The 53 qubits
combined are in all of 2^53 states at once. The gates can be set up such that
some combinations like "cat 1 alive", "cat 2 dead", etc. are much more likely
result than others, after the boxes are opened. And all this computation is
done in one step, whereby the classical computer must do 2^53 steps to get the
same result.

To have 53 cats all undisturbed in these dead/alive states so that computation
is done without errors is very technically challenging :)

~~~
drdeca
I don’t think the cat thing helps to explain this, especially with the
finality association of “dead”.

~~~
rini17
It can't really be explained, we kind of accept it works like it has been in
both states at once until the box was opened. Similarly, we don't really know
how to explain how particles travel by both slits at once in the double-slit
experiment.

~~~
drdeca
Idk, it seems to me like [https://www.smbc-comics.com/comic/the-
talk-3](https://www.smbc-comics.com/comic/the-talk-3) is a pretty good
explanation.

“A new ontological category”.

What’s the problem?

~~~
rolltiide
satire and educational

this has been the best thing I've seen so far

------
6gvONxR4sf7o
>Q12. Even so, there are countless examples of materials and chemical
reactions that are hard to classically simulate, as well as special-purpose
quantum simulators (like those of Lukin’s group at Harvard). Why don’t these
already count as quantum computational supremacy?

>Under some people’s definitions of “quantum computational supremacy,” they
do! The key difference with Google’s effort is that they have a fully
programmable device—one that you can program with an arbitrary sequence of
nearest-neighbor 2-qubit gates, just by sending the appropriate signals from
your classical computer.

>In other words, it’s no longer open to the QC skeptics to sneer that, sure,
there are quantum systems that are hard to simulate classically, but that’s
just because nature is hard to simulate, and you don’t get to arbitrarily
redefine whatever random chemical you find in the wild to be a “computer for
simulating itself.” Under any sane definition, the superconducting devices
that Google, IBM, and others are now building are indeed “computers.”

This is the core of it to me. It's a question of 'some people’s definitions of
"quantum computational supremacy."' Many people say that the definition here
is a crappy one, and sure this shows a form of it, but not the kind to justify
the hype. Sure, it's fully programmable, but not so programmable as to do
anything that anyone cares about (even a teeny tiny few-bit version of
something people care about) better than we can otherwise.

To appeal back to his analogy to the wright brothers, it's like they carved a
frisbee from a stick while working towards airplanes. It's amazing that they
carved a log into a neat shape you can throw further than another log, and the
hype train is saying that it's fully carve-able, so it counts as "airplane-
supremacy," but that's a crappy definition and not what we're waiting for.

~~~
whatshisface
At the time of the Wright brothers, which was incidentally before Frisbees
were invented, there were some naysayers that thought heavier than air flight
was simply impossible. A Frisbee, which can be chucked much further than a
balloon, would for them serve as actual proof of "wooden supremacy."

~~~
6gvONxR4sf7o
Interesting. Any idea how they responded to the obvious retorts about heavier
than air birds?

~~~
toxicFork
If they weigh the same as a duck then that should explain things, the birds
are witches

------
nabla9
> But others, including my good friend Gil Kalai, are on record, right here on
> this blog predicting that even quantum supremacy can never be achieved for
> fundamental reasons. I won’t let them wiggle out of it now.

Here is Gil Kalai's post from yesterday: "Quantum computers: amazing progress
(Google & IBM), and extraordinary but probably false supremacy claims
(Google)." [https://gilkalai.wordpress.com/2019/09/23/quantum-
computers-...](https://gilkalai.wordpress.com/2019/09/23/quantum-computers-
amazing-progress-google-ibm-and-extraordinary-but-probably-false-supremacy-
claims-google/)

~~~
n4r9
Note also Kalai's comment on this very blog post:

> Scott is correct that inability to achieve quantum supremacy is quite
> central to my argument (since 2014), so naturally I don’t expect that the
> recent claims by the Google team will stand. Of course, if these claims (or
> any other quantum supremacy claim) are correct then this would defeat my
> theory. It goes without saying that the claims are so fantastic that also
> responsible believers in quantum computers should examine these specific
> claims (like an alleged NP=! P proof) carefully and skeptically. Also, it
> would be nice to hear some details about the precise claims and methodology
> of the Google team.

[https://www.scottaaronson.com/blog/?p=4317#comment-1819916](https://www.scottaaronson.com/blog/?p=4317#comment-1819916)

------
calhoun137
I think we need to comes to grips with a hard truth about the reality of
academic life. Once you invest decades of your life into a research subject,
if it turns out the entire thing is never going to work, there are major
social and financial pressures to deceive the public about the true nature of
the subject.

I saw this happen with string theory first hand, and my experience with string
theory was a major factor that led to changing paths to pure mathematics and
computer science.

As someone who has spent a lot of time researching QC, I do not believe it
will ever be possible to build a practical quantum computer. We have been over
this so many times on this site. Here is a good link from a serious
professional who takes the same position [1]

In fact, my personal opinion is that quantum computers are functionally, a
hoax, the main purpose of which is to generate hype, secure research grants,
ensure career stability for academics, and give science reporters something to
write about to get clicks while deceiving the public.

[1] [https://www.quantamagazine.org/gil-kalais-argument-
against-q...](https://www.quantamagazine.org/gil-kalais-argument-against-
quantum-computers-20180207/)

~~~
dcposch
> Once you invest decades of your life into a research subject, if it turns
> out the entire thing is never going to work, there are major social and
> financial pressures to deceive the public about the true nature of the
> subject.

You're implying that Scott Aaronson is being deceptive.

I think that needs better evidence. The story you linked to is two years old
and it's about QC skeptic Gil Kalai.

Scott addresses him directly in his post:

> If quantum supremacy was achieved, what would it mean for the QC skeptics?

> I wouldn’t want to be them right now! They could of course retreat to the
> position that of course quantum supremacy is possible (who ever claimed that
> it wasn’t??), that the real issue has always been quantum error-correction.
> And indeed, some of them have consistently maintained that position all
> along. But others, including my good friend Gil Kalai, are on record, right
> here on this blog predicting that even quantum supremacy can never be
> achieved for fundamental reasons. I won’t let them wiggle out of it now.

~~~
calhoun137
I am not very familiar with Scott Aaronson and am just making a general
observation about the subject of QC, I have no idea what his motivations or
intentions are. If Scott believes QC will day be practical and I don't, only
time can tell who is right and who is wrong. I have seen this kind of goal
post moving in string theory, and its been going on for 20 years with QC in a
strikingly similar way imo.

There is no way to argue against this kind of goal post moving style of debate
because even if __another __20 years go by and QC still don 't exist on a
practical level, the goal posts will just keep getting moved.

I am extremely confident that this will continue until the public gets bored
of hearing about it.

~~~
sanxiyn
I think Scott is agnostic about whether QC will be practical or not. To quote
the original article, "we have no idea how long it will take".

~~~
DebtDeflation
>"we have no idea how long it will take"

IMO, this is a reasonable position to take. However, we have to accept that
even if a universal QC is possible, "how long" could conceivably be centuries.

Our perspective with regard to technology has been significantly biased by our
experience with Moore's Law/Dennard Scaling. The speed of progress in ICs from
1960 to 2010 (and especially 1980-2000) is an outlier in the history of
science and technology, yet we base expectations in everything from AI to QC
on it.

------
amai
It should be mentioned that IBM managed to simulate a 56 qubit version of the
same problem Google describes in the paper (see
[https://pastebin.com/RfUMXJZE](https://pastebin.com/RfUMXJZE)) on a classical
super computer:

\- [https://www.ibm.com/blogs/research/2017/10/quantum-
computing...](https://www.ibm.com/blogs/research/2017/10/quantum-computing-
barrier/)

\- [https://www.newscientist.com/article/2151032-googles-
quantum...](https://www.newscientist.com/article/2151032-googles-quantum-
computing-plans-threatened-by-ibm-curveball/)

So technically Google cannot claim to reach quantum supremancy with only 54
qubits as described in the paper.

------
bjornsing
I have the greatest respect for Scott, but I do think he’s being a bit too
enthusiastic here in comparison with the D-wave. At the very least I think he
should have included this question in his list:

Q: Why can the D-wave not be used to illustrate “quantum supremacy” in a
similar way?

(As I understand it the D-wave can sample from the solutions to ”ising model-
like” problems, which I assume would be extremely difficult for a classical
computer to do (but probably possible to verify).)

~~~
bjornsing
Another reason I feel this is oversold: This quantum “computer” cannot run
Shores factorization algorithm. But if it could it would only be able to
factor integers up to 2^53. The time required to factor a 2^60 to 2^80 integer
on a classical computer is measured in _milliseconds_ [1]...

Quantum supremacy in any reasonable sense of the word supremacy is a long way
off.

1\.
[https://hal.inria.fr/file/index/docid/451604/filename/smalli...](https://hal.inria.fr/file/index/docid/451604/filename/smallint_expcomp_draft_02_1.pdf)

~~~
itcrowd
You may not agree with the authors' or Aaronsons' definition of quantum
supremacy, that's ok. I think his definition is very reasonable. Aaronson
argues that this experiment _will_ prove quantum computing supremacy in great
length and defined it in the first entry of the FAQ as follows:

> [quantum computing supremacy] term refers to the use of a quantum computer
> to solve some well-defined set of problems that would take orders of
> magnitude longer to solve with any currently known algorithms running on
> existing classical computers

.. and continues to explain why this setup does exactly that.

~~~
bjornsing
Yea... I’m not suggesting of course that Scott is wrong about this being an
illustration of “quantum supremacy”... (Did you think I was...?)

But comparing it to manned flight or the first nuclear reactor... In those two
cases there was a clear path to something very useful. I’m my mind this
experiment changes very little as to how probable it is that we will soon have
useful quantum computers, or even that we will ever have them. I guess that’s
another question for Scott’s list:

Q: If what we care about is computing solutions to difficult real world
problems, in what way is this a meaningful milestone?

~~~
j1vms
> But comparing it to manned flight or the first nuclear reactor (...) this
> experiment changes very little as to how probable it is that we will soon
> have useful quantum computers, or even that we will ever have them.

Not a lot of people directly own & operate their own airplane, let alone a
nuclear reactor, and yet both have had a significant impact on modern life. As
for this experiment & QC, only time will tell of course.

~~~
sanxiyn
I think you misinterpreted "we will ever have them" part. bjornsing is not
talking about whether there will be personal quantum computers. It is about
whether there will be useful quantum computers at all. That is still open, for
example because of issues related to quantum error correction.

------
gtirloni
I've read almost all comments here and, although I've grown up with lots of
technology, this is the kind of stuff that will turn me into my
parents/grandparents. It's like when I was teaching them how to operate the
VCR years ago.

~~~
ramraj07
Your analogy explains itself away - maybe you knew how everything inside a vcr
worked but I didn't, I just knew what the buttons did and how to clean the
head. I'm sure we will figure our way around these quantum knobs as well!

(Seriously though, why the hell did those vcrs have such a large green
board??)

------
yters
Most major breakthroughs were pretty low tech in origin, and the high tech was
all about optimizing the idea. I think we get fundamental research backwards
nowadays.

~~~
khawkins
At the very least, we shouldn't be comparing quantum computing to air flight.
The Wright brothers were basically hobbyists, incomparable to the global
leading technology corporations in the history of human civilization.

This seems more comparable to the moon landing or LHC Higgs-Boson discovery.
Challenging to pull off once, and the next steps seem to increase
exponentially in man-power and cost.

~~~
yters
Those haven't really brought us much benefit. The benefit to effort
relationship seems to be inverse.

------
someotherone
So it looks like QC is good for speeding up simulations, and with far less
overhead for ECC and such than it needs for things like factoring. So is
simulation the QC "killer app"?

Does it only do certain kinds of simulations well, i.e. simulations of quantum
systems, or can that be generalized? Can it be applied to economic,
environmental, traffic, etc as well?

~~~
OscarCunningham
>Can it be applied to economic, environmental, traffic, etc as well?

Our current understanding is that quantum computers won't offer a speedup for
the simulation of nonquantum systems. The only simulations they'll be faster
for are systems for which quantum effects are important.

Of course it's possible that someone will discover an algorithm that gives
quantum computers an exponential speedup in the simulation of any system. But
I think that's pretty unlikely because it would imply that quantum computers
were exponentially faster than classical computers computers for _every_
problem, because you could just use your quantum computer to simulate a
classical one.

~~~
nullc
> Our current understanding is that quantum computers won't offer a speedup
> for the simulation of nonquantum systems.

The speedup from grover's algorithm is essentially universal it's just not an
exponential speedup. So for example in your non-quantum simulation task, you
want to search for an input to a cellular automata that makes it spell your
name after 5000 timestemps. You can use grover's algorithm to find that input
with a sqrt() the number of simulation runs that a classical machine would
need (and perhaps fewer, since there are likely multiple solutions).

To me that's one of the most interesting things about quantum computation:
There is a very broad class of things where it offers a modest speedup (sqrt)
and a narrow (but useful!) class of things where it offers an exponential
speedup. But if you imagine almost any kind of 'superpowered quantum
computation'\-- something that is a little more powerful than the laws of
physics as we know them allow-- you almost always get an exponential speedup
for everything (or even more absurd results, like every problem being solvable
in constant time). QC has this interesting property of being just strictly
more powerful than classical computing, but without being magic-instant-
computing.

[Which is also why many of the common incorrect descriptions of quantum
computing are sad, stuff like "testing all values in paralle"\-- if it worked
like that description it would be magic instant computing.]

~~~
OscarCunningham
> The speedup from grover's algorithm is essentially universal it's just not
> an exponential speedup. So for example in your non-quantum simulation task,
> you want to search for an input to a cellular automata that makes it spell
> your name after 5000 timestemps. You can use grover's algorithm to find that
> input with a sqrt() the number of simulation runs that a classical machine
> would need (and perhaps fewer, since there are likely multiple solutions).

I'd view this as using fewer simulations, rather than doing the simulations
faster.

Incidentally, I happen to be the author of some software for finding CA
predecessors: [https://github.com/OscarCunningham/logic-life-
search/](https://github.com/OscarCunningham/logic-life-search/). Good luck
getting it to do 5000 or even sqrt(5000) generations though! It's SAT solver
based, so as soon as someone does invent a quantum SAT solver it'll plug right
in.

> [Which is also why many of the common incorrect descriptions of quantum
> computing are sad, stuff like "testing all values in paralle"\-- if it
> worked like that description it would be magic instant computing.]

To be fair, many quantum algorithms do begin by preparing a superposition of
all possible intputs and then applying the unitary corresponding to some
classical function. The difficulty arises when you have to extract information
from the resulting output superposition.

------
daxfohl
What's e.g. China and Russia doing in this space? Or the US Gov't for that
matter? I can't imagine that national governments are just waiting for Google
and IBM to do the research and publish their results. Is it possible/likely
that NSA or some other national equivalent is way beyond these results
already?

~~~
reilly3000
I don't know what the NSA spends on R&D, but I have to imagine they don't have
a multi-billion dollar budget for secret R&D facilities and talent. It would
seem more likely they are monitoring developments in public and private
research and responding opportunistically. I'm just making assumptions here,
but also I don't think somebody with clearance could correct me here in public
:P

~~~
jessriedel
The NSA has the money and a history of massive secret projects. There are more
mathematicians at the NSA than in all of academic cryptography. However, my
general impression is that most people don't think the NSA has a secret QC
project because there hasn't been a noticeable disappearance of the best young
experimental QC researchers from the pipeline like there has been for
cryptography. It's the promising people quietly leaving that is hard to hide;
massive construction projects are comparatively much easier to hide.

~~~
muraiki
A recent NYT opinion piece from the general counsel of the NSA discusses many
of these issues, including quantum computing:
[https://www.nytimes.com/2019/09/10/opinion/nsa-
privacy.html](https://www.nytimes.com/2019/09/10/opinion/nsa-privacy.html)

~~~
jessriedel
Thanks much. For others, here are the relevant bits I could find (my
emphasis):

> ...It is by no means assured that our national security sector will be able
> to attract on a sufficient scale the scarce engineering, mathematical and
> scientific talent that would supply the necessary expertise. That challenge
> will require investment, enlightened strategic management and an innovative
> approach to luring a different type of expert out of the private sector into
> government. Meeting this challenge will require a greater reliance in
> general on the private sector, since government alone does not possess the
> requisite expertise. A large portion of the intelligence community’s experts
> on the military capabilities and plans of Russia and China joined government
> during the Reagan administration; other experts on counterterrorism and new
> technology burnished their technical skills following the Sept. 11 attacks.
> Many of those experts are nearing retirement or have already left to join an
> attractive private sector. _With millennials believing that technology in
> the private sector now allows them to help change the world — previously the
> idea of a mission had been largely the province of public service — it is
> not clear that the intelligence community will be able to attract and retain
> the necessary talent needed to make sense of how our adversaries will make
> use of the new technology_...

> ... the government no longer possesses the lead in complex technology, at
> least in many areas relevant to national security. Arguably, the most
> powerful computing and sophisticated algorithm development now occurs not in
> the Pentagon or the N.S.A. but in university research labs and in the
> Googles and Amazons of the commercial world. (To be sure, the government
> still maintains its superiority in important areas ranging from nuclear
> energy to cryptography.)...

> ... our national security agencies for the first time must amass the talent
> and systems to understand not simply a military challenge but also
> challenges across a broad range of technology and global finance issues. The
> capacity for such understanding currently resides principally in the private
> sector and our universities, not the federal government.

------
19ylram49
Much needed perspective in the age of overly dramatic headlines and straight
up misinformation.

------
age_bronze
I posted this on scott's blog, still awaiting moderation:

"I’m trying to understand the chain of inference from Google’s leaked result
of quantum supremacy to theoretical computer-science “hardness” of the
computation.

Computing the exact probabilities of a random quantum circuit is proven hard,
but computing the exact probability of a random algorithm is also an open
problem, so what you really care about is approximation of computing the
probability (up to epsilon), or even weaker, just sampling from a probability
(also up to epsilon under some metric of comparing distributions).

Their computer implements Random Circuit Sampling, and their cited
“theoretical” hardness results are your paper from 2017, of QUATH => HOG, and
as far as I understood from your paper, proving that approximation / sampling
from a (random) quantum computer/circuit is hard is still an open problem (Am
I wrong here? I’m not up to date with everything), and a difficult one. But
you did make a compelling argument that even if QUATH was solvable, it will
lead to new insights.

Their actual benchmark uses cross-entropy benchmarking, called xeb in their
paper, and defined as 2^n* [P(x_i)] _i-1 (Can’t type brackets). I could not
find any ‘theoretical hardness’ paper at all using this benchmark, some
results talk about different XEB using log but they prove these are not strong
enough and can be reached classically.

Are there any hardness results for their benchmark? I wonder why they would
use that instead of the proven HOG, even for smaller input sizes, I would see
more value in a benchmark which has theoretical roots. I feel intuitively like
their benchmark is much more similar to the log variation of XEB than to HOG,
but didn’t think it through completely.

As for their gates, I understand they are not general random quantum gates,
but instead they have a variation of iSWAP and controlled phase CZ. The
hardness results that do exist also don’t address the limited gates, in how it
changes the distribution of random circuits. Are those two gates at least
universal, so that we have hope that this could be proved? Their statement was
“but reliably generating a target unitary remains an active area of research”,
so I assume they aren’t universal. I would love to see some heuristic argument
as to why those two in particular are hard, especially given results like
Gottesman–Knill theorem, it seems like some surprising gates do have classical
simulation. iSWAP is just swap and phase gates, and CZ is phase gate only on
the 11 state. Doesn’t feel like it could be universal to me but maybe I’m
wrong.

I probably missed some things, so I’d love it if you could point to papers
filling the gaps between their results and a real theoretical statement. I
don’t have the intuition to tell which gaps are important and which are not. I
also don’t know which papers/results already exist and I can’t really search
as I am not an academic, and don’t have full access to many papers, and you
probably know the state of the art results. "

~~~
tagrun
\- Cross-entropy is used for roughly estimating the fidelity (see
arXiv:1608.00263), which is independent of the problem they're solving. You're
confusing it with the overall distribution of the readout states, which is the
hard problem.

\- CZ is a perfect entangler. No, iSWAP and CZ do not form a universal set.
They typically combine virtual Z rotations (which has a continuous angle
parameter) with an additional one-qubit gate, such as a X or Y gate (or their
square root, which they mention in the paper) which gives a universal set.
Although they don't make use of continuous Z rotations in that particular
experiment, the device is capable of doing that, and is a (noisy) universal
quantum computer.

------
Robotbeat
Practically speaking, how long until the Quantum Jubilee, when (much of)
encryption is broken?

------
person_of_color
Where does this put Rigetti Computing?

------
billions
Most things that get years of speculation and press never come to fruition

~~~
smolder
Like flying machines, speculated about from DaVincis time?

------
tomerbd
Can I use it for my new ruby on rails webapp?

------
kwakuDompreh
Google achieved this some few days ago and I’m very much impressed. They
indicated the quantum machine could process data in 3mins and that same data
will take 20years for the most advance Computer to process successfully.
Impressive

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ragerino
I recently told a friend, that the perfect quantum computer application would
be a machine that can apply all possible operations simultaneously and
consistently filter out one or more of the methods which results in a valid
computer program producing a predefined result.

------
codeulike
My cat can behave as a cat would be expected to behave. And if we verify the
measurements of her behaviour using a classical computing cluster - to make
sure her behaviour really falls within the distribution of expected cat
behaviour - thats a very complicated calculation that will take many
processor-days. But my cat can just do that stuff in real time. Has my cat
achieved Quantum Supremacy, and is there a trophy or something I can pick up
somewhere for that?

~~~
teraflop
That sounds like question 12 from the FAQ ;)

~~~
galimaufry
I didn't really understand the answer though. The computer is programmable,
but the way supremacy is demonstrated is orthogonal to its programmability -
it is still demonstrating superiority on just the one problem of simulating
itself starting from any initial condition, no?

~~~
jessriedel
> it is still demonstrating superiority on just the one problem of simulating
> itself starting from any initial condition, no?

It's the "any initial condition" part that is hard. You cannot put a cat, or
even a small molecule, in any one of it's possible initial states, let it
evolve, measure the outcome, and get reliable answers. (If you could, that
would make it a computer! That's all a computer does, assuming the initial
states and evolution are rich enough to, e.g., be Turing complete.)

> The computer is programmable, but the way supremacy is demonstrated is
> orthogonal to its programmability

The fact that the problem posed by the challenger C is "run an arbitrary
quantum circuit" is in fact directly connected to the programmability.

------
westurner
Who even asked these questions?

I question this. All of this.

~~~
fastball
Literally people on HN have been asking many of these questions for years
whenever QC is discussed.

~~~
westurner
I believe Feynman originally asked the QC question many many years ago. What
an exciting milestone and a great FAQ.

"Always naysaying! Everything I create!"

------
erichocean
> _It’s like, if you believed that useful air travel was fundamentally
> impossible_

Uh, there are birds. Literally everyone thought useful air travel was
possible, and not only possible, but so easy that a Darwinian process was able
to produce it, not once, but literally thousands of times, in thousands of
ways.

\----

But looking at the actual "experiment", I don't count that as computation in
any meaningful sense, and morally equivalent to the following:

Set up a digital camera and point it at a scene. Take a picture. Now take
millions of additional pictures, without moving the camera.

Measure the values at each pixel. See how they correspond to "amplitudes"?
_That 's our computation._ <= (This is the part of the article that should
raise eyebrows...)

Article (smugly): How about you simulate (render) the scene using an actual
computer? Measure the amplitudes of the resulting millions of images. _OMG,
that took you so long!!! Loser._

Are we being punk'd here?

The digital camera is the quantum computer. The "scene" is the random initial
state _C_. The scene is then translated into a renderable scene for the
classical computing version, and rendered with an unbiased physically based
renderer, which produces the same result.

I fail to see how any of this is even remotely exciting, much less
interesting. A camera is not a computer, no matter how many measurements you
make with it, nor how "random" the scene you are taking pictures of is.

And yes, simulating reality takes more cycles than just measuring whatever
happened with a camera. Photos are "faster" to get than the equivalently
rendered scene—news at 11!

Again: _Are we being punk 'd here?_

~~~
alanbernstein
I have no comment on the QC claims, but regarding the flight analogy, it seems
you're being disingenuous: [https://www.xaprb.com/blog/flight-is-
impossible/](https://www.xaprb.com/blog/flight-is-impossible/)

~~~
erichocean
Curious: Did you Google that just now, in order to comment here?

~~~
alanbernstein
Yes I did. Did you invent a historical "fact" to comment here?

