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Scott’s Supreme Quantum Supremacy FAQ

scottaaronson.com

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Re: Scott’s Supreme Quantum Supremacy FAQ

#81

Post by grumpy8 »

nexuist wrote: post #2

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 c…

Extremely simplified:

>> in an electronic circuit, what is a qubit?

It's not an electronic circuit, it literally is an electron or a photon. And it's using that particle's properties to do "superposition and all that jazz".

You can't put too many next to each others because they start interacting together because that's what electrons do when they get close.

Re: Scott’s Supreme Quantum Supremacy FAQ

#82

Post by jessriedel »

sanxiyn wrote: post #69
gowld wrote: post #58

Earlier quoted context omitted.

What are the theoretical models for the energy cost of computing on a qubit? I'll be excited for QC when there is known way (even with some handwaving and future-tech plans) to compute a non-trivial result for a reasonable sum, such as "crack someone's private RSA key for under $10M of compute cost"

This depends on your definition of "trivial". But if we restrict it to factoring, no, there is no known path to QC factoring at the moment, none at all.

I don't understand. Factoring composite numbers into their primes is definitely a predicted use for QC in the future. Are you just saying the architecture of the machine powerful enough to do that is still uncertain?

Re: Scott’s Supreme Quantum Supremacy FAQ

#83

Post by jessriedel »

bjornsing wrote: post #62
sanxiyn wrote: post #60

Earlier quoted context omitted.

In principle, there is no reason why D-wave also can't achieve quantum supremacy. It is just that D-wave hasn't, so far. As for Ising model, D-wave didn't outperform classical algorithm after classical algorithm was tuned (it was a new problem, so existing classical algorithm wasn't the best possible), see https://arxiv.org/abs/1401.1084 , also there are reasons to suspect why Ising model will not provide any quantum…

Well D-wave is evaluated on an optimization task. This Google thing isn’t even trying to solve a real problem. What says you can’t get some very hard to replicate random bits out of a D-wave?

If D-Wave could achieve quantum supremacy, they definitely would, and would heavily publicize it. They haven't, which gives us strong evidence that they can't right now.

Re: Scott’s Supreme Quantum Supremacy FAQ

#84

Post by jessriedel »

reilly3000 wrote: post #54
daxfohl wrote: post #52

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?

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

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.

Re: Scott’s Supreme Quantum Supremacy FAQ

#85

Post by jessriedel »

galimaufry wrote: post #48
teraflop wrote: post #17

Earlier quoted context omitted.

That sounds like question 12 from the FAQ ;)

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?

> 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.

Re: Scott’s Supreme Quantum Supremacy FAQ

#86

Post by 3PS »

galaxyLogic wrote: post #51
3PS wrote: post #32

Earlier quoted context omitted.

Short answer, a qubit is a unit vector in a 2D complex Hilbert space. Now, that doesn't actually say much about why we care or how they're useful. In practical terms, you can think of qubits as complex unit vectors along two axes, with one axis corresponding to |0⟩ (the zero qubit) and one axis corresponding to |1⟩, or the one qubit. So for example, you could have a qubit called |+⟩, which is just shorthand for (|0⟩…

> Schrödinger's cat must be alive or dead once we open the box I recently had a conversation about this in another thread. It seems to me, and nobody tried to convince me otherwise, that the Cat would be the Observer. Therefore it would be dead, NOT dead AND alive, as soon as it observes the poisonous gas in its box.

So this is a little nuanced. It's true that quantum systems collapse on "observation", but that doesn't actually mean "observation by a sentient entity". It could really just be any interaction with the outside environment. (This is why qubits have to be kept incredibly well-isolated.) We don't really fully know exactly how this collapse works, and related speculation is generally classified under the measurement problem [1]. But it's true that one of the key points of Schrödinger when proposing his thought experiment was that the notion of measurement or observation was not fully defined under the Copenhagen interpretation.

Side note, it's not that the cat is observing poisonous gas, but rather, that a Geiger counter is set to detect whether a radioactive atom decays or not and triggers the release of some poisonous gas if so. So, classically, Schrödinger's cat would be either alive or dead 50% of the time, not 100% dead. There are plenty of alternative ways to reconcile this classical view with quantum mechanics. Perhaps the simplest and most well-known is the many-worlds interpretation [2], which states that both events occur, just in different timelines, and we don't know what timeline we ended up in until we open the box. (Of course, it is ridiculous to speculate as to which timeline "we" end up in before the experiment is carried out, because the people in both timelines would still be "us" - this can get awkward to think about.)

[1] https://en.wikipedia.org/wiki/Measurement_problem

[2] https://en.wikipedia.org/wiki/Many-worlds_interpretation

Re: Scott’s Supreme Quantum Supremacy FAQ

#87

Post by ClintEhrlich »

ClintEhrlich wrote: post #3

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.

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.

Re: Scott’s Supreme Quantum Supremacy FAQ

#88

Post by sanxiyn »

jessriedel wrote: post #82
sanxiyn wrote: post #69

Earlier quoted context omitted.

This depends on your definition of "trivial". But if we restrict it to factoring, no, there is no known path to QC factoring at the moment, none at all.

I don't understand. Factoring composite numbers into their primes is definitely a predicted use for QC in the future. Are you just saying the architecture of the machine powerful enough to do that is still uncertain?

I think it is analogous to fusion. Fusion definitely works (the Sun and hydrogen bomb) and power generation is a predicted use of fusion in the future. That doesn't mean we have known path to fusion for power generation.

To extend the analogy, quantum supremacy is like fusion for neutron source. There are commercial fusion devices to be used as neutron source.

Re: Scott’s Supreme Quantum Supremacy FAQ

#90

Post by tagrun »

itcrowd wrote: post #73
tagrun wrote: post #65

Earlier quoted context omitted.

There is a scalability problem but due to different reasons. See my other comment in this thread for details. Classical circuitry is an issue, but not as much as you think. What happened is Martinis' group and others moved forward with a quick & dirty design which worked well for their device but can't be scaled (they basically didn't have the expertise like silicon people had). Nevertheless, it's not a fundamental p…

I agree on your other points and trust that you are more qualified to judge what the biggest obstacles are. Do you have a paper I can look into that goes into the chip architectures in more detail (not specifically for this new device)? Otherwise I'll await the science / nature paper of this demonstration. The classical circuit is mostly outside the cryo I assume since it's GHz's and LNAs are available. Do you know i…

When you say chip architecture, I assume you mean how to assemble together qubits like an integrated circuit. Here's one proposal for silicon based qubits: https://www.nature.com/articles/s41467-017-01905-6

Microwave signals are typically used for control rather than readout. I don't know if this experiment does it or not, and I am not an experimentalist. Multiplexing is more typically required for reducing timing errors of simultaneously driven signals (for better synchronization) and it really depends on the device and the mode of operation, plus whether the experiment they're doing needs it or not. The same experimental group sometimes do it one experiment and not do it in another, despite using the same device.

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