
Quantum supremacy: the gloves are off - furcyd
https://www.scottaaronson.com/blog/?p=4372
======
leoh
Two key points:

> OK, so let’s carefully spell out what the IBM paper says. They argue that,
> by commandeering the full attention of Summit at Oak Ridge National Lab, the
> most powerful supercomputer that currently exists on Earth—one that fills
> the area of two basketball courts, and that (crucially) has 250 petabytes of
> hard disk space—one could just barely store the entire quantum state vector
> of Google’s 53-qubit Sycamore chip in hard disk. And once one had done that,
> one could simulate the chip in ~2.5 days, more-or-less just by updating the
> entire state vector by brute force, rather than the 10,000 years that Google
> had estimated on the basis of my and Lijie Chen’s “Schrödinger-Feynman
> algorithm” (which can get by with less memory).

> But does IBM’s analysis mean that “quantum supremacy” hasn’t been achieved?
> No, it doesn’t—at least, not under any definition of “quantum supremacy”
> that I’ve ever used.

~~~
revel
Process arguments are rarely convincing and particularly not when the process
is so technically complex.

------
ThePhysicist
Regardless of whether the Martinis team actually demonstrated quantum
supremacy, their achievement is undoubtedly worth the praise it received. I
built and operated a superconducting two-qubit processor for my PhD thesis in
2010, so I have a good understanding of the challenges invovled in operating
and tuning qubits. For me, seeing the 53 qubit chip, its wiring setup and the
control electronics is just breathtaking.

I think John's team neatly demonstrated that they are far ahead of the
competition (e.g. Rigetti, IBM) in terms of maturity and qubit control, and if
that's any indication of future success I would strongly bet on their team for
building the first actually working quantum computer.

~~~
rrss
What is a "first actually working quantum computer" if not this Sycamore
device that Google used to demonstrate quantum supremacy?

~~~
317070
I reckon one that has quantum error correction on qubits which present Boolean
values. I.e. one which could also run a classical program. That is the quantum
computer which is actually a quantum Turing machine and which can do speedups
of e.g. factorization. Unfortunately, it is hypothesized that such a quantum
Turing machine will require about ~1000 qubits of this machine per qubit with
error correction, maybe more.

~~~
kulahan
For what it's worth, the FAQ on his blog dances around this topic:

"Running Shor’s algorithm to break the RSA cryptosystem would require several
thousand logical qubits. With known error-correction methods, that could
easily translate into millions of physical qubits, and those probably of a
higher quality than any that exist today."

~~~
grubles
"How to factor 2048 bit RSA integers in 8 hours using 20 million noisy qubits"

[https://arxiv.org/abs/1905.09749](https://arxiv.org/abs/1905.09749)

------
knzhou
As always, thank god for Scott Aaronson's blog! There is a lot of
misinformation sown in popular science. Shtetl Optimized is just about the
_only_ science-related source that is simultaneously correct, accessible,
timely, and widely read. That's incredible in a world where 99% of sources
barely achieve two of these goals.

------
dang
There are three threads. The others are:

Google's post
[https://news.ycombinator.com/item?id=21332768](https://news.ycombinator.com/item?id=21332768)

IBM's critique
[https://news.ycombinator.com/item?id=21333105](https://news.ycombinator.com/item?id=21333105)

------
naringas
I still don't understand what the hell is "quantum computing". I think my
problem is my own conception of classical computing.

My best approximation for what a quantum computer actually is. Is that it's
some sort of device to set quantum states, then run a "quantum algorithm"
(i.e. a graph of "quantum logic gates"), and finally read out the "quantum
bits" resulting from the algorithm. That sounds a lot more like a sensor to
me. Also all the problems with noise and calibration reinforce my notion that
it's more like a sensor than a computer.

But to be fair I barely understand "classical computing". I think my personal
notion relies too much on badly defined ideas about "the run-time" of a
computer program (some software) that I have constructed from "industry"
experience.

To me big huge fundamental part of computing has to do with arbitrarily
representing anything with a symbolic alphabet. And then automatically
manipulating this symbolic alphabet to get a result with can be "mapped back"
through the initial arbitrary representation so we learn something about
whatever we initially chose to represent.

Maybe that's not computing? but then what is that? and then what is
computing??

Try as I might I cannot reconcile my understanding of classical computing with
quantum computing.

~~~
cdumler
In classical computing, a bit is a binary state. The implementation isn't
important. What is important is there are only two positions: true/false,
on/off, electricity flowing/no electricity flowing, a wooden stick pointed to
the left/right. If you treat combinations of state as binary numbers (0000=0,
0001=1, 0010=2, 0011=3, etc) and you create functions that manipulate these
bits (AND, OR, NOT, etc gates) such that they generate new patterns of bits
consistent with doing math. A classical computer relies on classical physics:
something exactly is something. You either have a 1 or 0, because the
implementation is either one thing or the other thing.

Quantum computing relies on quantum effects, say the spin of an electron. The
spin of the electron can be measured and when does it will either be up or
down. At first it seems like it's just like a classical computer; however,
it's possible to "entangle" multiple qbits together. The orientation becomes
fuzzy. In this fuzzy state is possible to create quantum gates that again do
math, but these results become "fuzzy."

So I create the function: "Give x such that x has the reminder of 1 after
dividing by some huge number?" In classical physics there is no one answer to
modulus. There could be an infinite number of possible answers. Solving it
with a classical computer does not mean much more than trying values and
seeing what works. But, asking that question with a quantum computer does
something different.

Being "fuzzy," it exists across all possible states at the same time until you
measure it. When you measure it, you will get a random answer but that answer
will work for the equation. Say you ask the question, "Give me x where x
divided by 5 has a remainder of 1." The quantum computer might first spit out
56 then next time 91 then next time 6.

~~~
thaumasiotes
> however, it's possible to "entangle" multiple qbits together. The
> orientation becomes fuzzy. In this fuzzy state is possible to create quantum
> gates that again do math, but these results become "fuzzy."

The fuzzy results, in my understanding, are a property of quantum mechanics
and don't require multiple qubits. A single electron has some probability of
spinning up and a complementary probability of spinning down, and until you go
ahead and measure the spin, the actual spin value hasn't coalesced to either
of those states, instead being some fuzzy intermediate probability spectrum.

Entangling multiple particles means (again, as far as I understand) that their
measurements cease to be independent -- if I have several pairs of entangled
electrons all with spin of 50% up / 50% down, then I might expect the results
of measuring the spin of those pairs to look something like this table:

    
    
        +/+   25%
        +/-   25%
        -/+   25%
        -/-   25%
    

when in fact, because these are entangled pairs, I will either get

    
    
        +/+   50%
        +/-   0
        -/+   0
        -/-   50%
    

or

    
    
        +/+   0
        +/-   50%
        -/+   50%
        -/-   0
    

What am I missing here?

~~~
thethirdone
> What am I missing here?

I am unsure what you are missing because what you explained is approximately
correct.

Minor correction:

Entangled qubits (each of 50% propbaility to be up) can have more possible
measurement distributions that the two you mentioned.

Distributions like the following exist.

    
    
        +/+ 20%
        +/- 30%
        -/+ 30%
        -/- 20%
       

> if I have several pairs of entangled electrons all with spin of 50% up / 50%
> down, then I might expect the results of measuring the spin of those pairs
> to look something like this table:

I think you may misunderstand how to get qubits entangled. You would have to
pass two qubits through a two-qubit gate to get them entangled. And doing so
would leave you with only one measurement distribution.

For example if you have a qubit in 50% |1>, 50% |0> and pass it through CNOT
with a qubit in |1>. You get:

    
    
        |11>   0
        |10>   50%
        |01>   50%
        |00>   0
    

But if the second qubit was in state |0>, you get

    
    
        |11>   50%
        |10>   0
        |01>   0
        |00>   50%
    

> if I have several pairs of entangled electrons all with spin of 50% up / 50%
> down

Also just in case you don't know qubits have phase so there is more than one
way to have a qubit that when measured will be up 50% of the time.

------
muizelaar
[http://archive.is/3ajcQ](http://archive.is/3ajcQ)

~~~
quakeguy
Thx! Blog is already down for me.

~~~
j1vms
He might just need a static-site blog from this point on.

------
X6S1x6Okd1st
So assuming that people take the economic loss and switch to public key
encryption schemes that appear quantum hardened what's the next most
interesting thing to do with a quantum computer?

~~~
deepnotderp
Quadratic speedup over any black box function is pretty nice. Could certainly
help ML a lot.

~~~
X6S1x6Okd1st
Could you elaborate? If the function is XOR I'm pretty sure you won't see a
speed up

~~~
hatsunearu
He's referring to Grover's algorithm.

------
dmix
> we cared about the increase in the speedup as D-Wave upgraded its hardware,
> and the trouble was that we never saw a convincing case that there would be
> one.

Is D-Wave still doing useful work in the field?

I've been meaning to read up on them and recent progress quantum in general...
I still see D-Wave in the news often in the tech press and Canadian media.

------
pyentropy
Scott Aaronson is probably one of the most underrated scientists of our time.
He's very approachable and has responded to comments and emails of undergrads
like me in a few hours. Not only is he part of the team that practically
invented quantum supremacy, but his blog is _the_ most informative blog out
there for ethics, morality, nerdiness, theoretical computer science and the
philosophical implications of complexity theory.

Y Combinator had a podcast with him which you can watch here:
[https://www.youtube.com/watch?v=0jrybODBUpA](https://www.youtube.com/watch?v=0jrybODBUpA)

~~~
ivalm
I enjoy Aaronson's blog/interviews, but he is not an "underrated scientist",
he is perhaps an underrated science communicator. Being a science
communicator, however, is somewhat unrelated to being a scientist. Looking at
his actual scientific work, he is in fact doing good work, but he is not in
the same league as, let's say, Peter Shor.

~~~
pyentropy
I think his BosonSampling method comes very close to Shor's algorithm, and I
don't think "leagues" really exist in academic output - but he's not doing
_good_ work - he's doing _great_ work.

