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The Case Against Quantum Computing

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Re: The Case Against Quantum Computing

#41

Post by core-questions »

powera wrote: post #7
setquk wrote: post #4

Earlier quoted context omitted.

Sounds like Fusion. That’s still going on after 60 odd years.

With one major difference. We know that fusion in some form is possible (for example, in the sun, or in a hydrogen-bomb). The difficulty is doing it at a reasonable scale. It's less clear that quantum computing is possible.

You can literally sign up for D-Wave Leap and run problems on a quantum computer immediately. It's not gate-model, but it's a real quantum annealer that functions and can compute useful things.

Re: The Case Against Quantum Computing

#42

Post by sp332 »

pavelrub wrote: post #36
sp332 wrote: post #30

Earlier quoted context omitted.

They are not classical computers. We've had actual quantum logic gates since 1995. https://en.wikipedia.org/wiki/Timeline_of_quantum_computing And as linked above, IBM is renting out time on real quantum computers up to 17 qubits now. Intel, Google, and IBM had all announced plans for 49 or 50 qubit computers, which would have achieved quantum supremacy at the time, but IBM improved their simulator to 56 qubits last…

They are classical computers in the sense they are are no stronger than classical computers - they provide a different physical implementation, but computationally can't achieve anything that a classical computer can't. Everything those "quantum computers" do can be simulated with a classical computer. The entire motivation behind the development of QC is supremacy, and the existence of those "quantum computers" alon…

Well I don't want to get into a full-on argument in the comments here, but I 100% disagree with your definition of a quantum computer. If a computer uses quantum logic gates then it's a quantum computer.

Re: The Case Against Quantum Computing

#43

Post by krastanov »

core-questions wrote: post #40
krastanov wrote: post #15

Earlier quoted context omitted.

This is one of the standard complaints (the gist of it being that quantum computing is some form of analog computing, i.e. requiring near-infinite precision). For researchers in the field it becomes rather frustrating to have to repeat the same response without being heard, so I can understand the annoyance expressed in the parent comment. For what is worth, here is a good explanation of how this complaint misreprese…

> the gist of it being that quantum computing is some form of analog computing It absolutely is - at least, in the only practical, real-today, working instantiation of it which is in the form of quantum annealing.

This is an extremely misleading statement. Quantum annealing is most certainly not what quantum computing is about (it is not particularly "quantum" either). At most, you can argue that it is an important first step (which is also doubtful). Most of the interesting hardware currently being developed has nothing to do with quantum annealing and most researchers are fairly annoyed at D-wave for originally pushing this nonsense (D-wave is the company that started selling quantum annealing machines, which are little more than a classical analog computer).

See the ion traps developed in the UK/Maryland or the transmon qubits at Google/IBM/Yale for some well known prototypes of quantum computing hardware (which are admittedly quite far from being practical or usable).

Re: The Case Against Quantum Computing

#44

Post by tezthenerd »

atq2119 wrote: post #39
tezthenerd wrote: post #35

Here is a way to see the fallacy of the OMG, its 10^300 variables, thats crazy style of “argument”. Consider a probabilistic classical algorithm on 500 bits. Perhaps a Monte Carlo simulation of an Ising model for example. Note first that the most general probability distribution over the 500 classical bits takes 2^500 real numbers to specify. (You have to specify P(000…0) and P(000…1) and… P(111…1)). [You should comp…

Monte Carlo simulation doesn't generate all classical probability distributions though. Attempting to do that would be nuts. Monte Carlo simulation only samples from a single, fixed distribution (or maybe a small number of distribution). I'm not super convinced by the argument based on number of parameters either, but your analogy doesn't refute it at all.

Its not meant to be an analogy.

We also will not attempt to generate all quantum states on a quantum computer. As with classical monte carlo, we will only ever generate a tiny fraction of the possible quantum states/distributions, and will also sample from a fixed distribution (whatever the quantum circuit outputs, we measure it in a fixed basis and always draw samples from that single, fixed distribution).

We will also achieve robustness against the tyranny of the real numbers in our gate parameters in a very similar way that a classical computer does when it approximates some idealized Monte Carlo algorithm.

Re: The Case Against Quantum Computing

#45

Post by [deleted] »

nixpulvis wrote: post #6

I still need to gain the intuition for why a quantum computer can operate on some kinds of things "faster". I've read some of the math, but that did little to satisfy me (I need to study it more clearly). But all of this seems in a tragic state at the moment. Allowing rampant misinformation and hype as to what these machines are actually capable of.

[deleted]

Re: The Case Against Quantum Computing

#46

Post by nixpulvis »

Jach wrote: post #27
nixpulvis wrote: post #6

I still need to gain the intuition for why a quantum computer can operate on some kinds of things "faster". I've read some of the math, but that did little to satisfy me (I need to study it more clearly). But all of this seems in a tragic state at the moment. Allowing rampant misinformation and hype as to what these machines are actually capable of.

Scott's book referenced is very good, but you might also want to see if his layman's blog explanation of Shor's Algorithm is useful to you: https://www.scottaaronson.com/blog/?p=208

This was quite good at filling in some intuition.

Re: The Case Against Quantum Computing

#47

Post by krastanov »

klodolph wrote: post #31
TheOtherHobbes wrote: post #5

Earlier quoted context omitted.

The technical argument is absolutely clear: quantum computing cannot work because it relies on manipulating and measuring an absolutely astronomical number of continuous variables with near-infinite precision. The argument may or may not be correct, but it deserves something more thoughtful than a dismissive response that doesn't even recognise the basic point the author is making.

The argument that quantum computing relies on manipulating continuous variables with "near-infinite precision" is flatly incorrect, if my understanding of the threshold theorem is correct. So no, quantum computers do not require extreme amounts of precision or extremely low error rates because it is possible to correct errors by making the computer larger. https://en.wikipedia.org/wiki/Quantum_threshold_theorem

Amusing historical note: The same arguments were made about classical computers and a similar theorem (by Von Neuman) exists there.

Re: The Case Against Quantum Computing

#48

Post by krastanov »

core-questions wrote: post #41
powera wrote: post #7

Earlier quoted context omitted.

With one major difference. We know that fusion in some form is possible (for example, in the sun, or in a hydrogen-bomb). The difficulty is doing it at a reasonable scale. It's less clear that quantum computing is possible.

You can literally sign up for D-Wave Leap and run problems on a quantum computer immediately. It's not gate-model, but it's a real quantum annealer that functions and can compute useful things.

Quantum annealers are not quantum computers. They are not more powerful than classical computers. You either need the "gate model" or the equivalent "adiabatic quantum computing model" to achieve something infeasible on classical computers.

It is a common misconception to think that adiabatic quantum computing (which is equivalent to the gate model) and quantum annealing is the same thing. It is not, and quantum annealing is mostly a buzzword that D-Wave use to sell their devices.

Re: The Case Against Quantum Computing

#49

Post by [deleted] »

atq2119 wrote: post #39
tezthenerd wrote: post #35

Here is a way to see the fallacy of the OMG, its 10^300 variables, thats crazy style of “argument”. Consider a probabilistic classical algorithm on 500 bits. Perhaps a Monte Carlo simulation of an Ising model for example. Note first that the most general probability distribution over the 500 classical bits takes 2^500 real numbers to specify. (You have to specify P(000…0) and P(000…1) and… P(111…1)). [You should comp…

Monte Carlo simulation doesn't generate all classical probability distributions though. Attempting to do that would be nuts. Monte Carlo simulation only samples from a single, fixed distribution (or maybe a small number of distribution). I'm not super convinced by the argument based on number of parameters either, but your analogy doesn't refute it at all.

[deleted]

Re: The Case Against Quantum Computing

#50

Post by pavelrub »

sp332 wrote: post #42
pavelrub wrote: post #36

Earlier quoted context omitted.

They are classical computers in the sense they are are no stronger than classical computers - they provide a different physical implementation, but computationally can't achieve anything that a classical computer can't. Everything those "quantum computers" do can be simulated with a classical computer. The entire motivation behind the development of QC is supremacy, and the existence of those "quantum computers" alon…

Well I don't want to get into a full-on argument in the comments here, but I 100% disagree with your definition of a quantum computer. If a computer uses quantum logic gates then it's a quantum computer.

It’s not particularly important how you define the term “quantum computer”, nor have I presented any definition of a "quantum computer" that you can disagree with. The point is that, as I wrote before, regardless of your definition - currently there is no discernible advantage of quantum computers over classical computers in the only metric that matters: computational power. This is what people refer to when they talk about the impossibility of QC. Nobody has demonstrated that physically realizable quantum computers can solve BQP problems in polynomial time, or provide any superpolynomial speedups over classical computers, and there is certainly no proof that this is simply a matter of scale.
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