[HN Gopher] Towards quantum computers that are robust to errors
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Towards quantum computers that are robust to errors
Author : bookofjoe
Score : 72 points
Date : 2023-02-22 19:40 UTC (2 days ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| bookofjoe wrote:
| Paper discussed:
|
| >Suppressing quantum errors by scaling a surface code logical
| qubit [Open Access]
|
| https://www.nature.com/articles/s41586-022-05434-1
| lmpdev wrote:
| I generally avoid discussing Quantum Computing as I know next to
| nothing, but I've been wondering:
|
| Are there any inherently stable/noise free/non-Volatile Quantum
| Computing methods, _at all_?
|
| Even just maintaining the state of a single Qubit for long
| periods without an exotic lab setup, or reliably transforming a
| single Qubit from a state, to a state with a near zero error
| rate?
|
| My apologies if these are general knowledge
| bowsamic wrote:
| > Are there any inherently stable/noise free/non-Volatile
| Quantum Computing methods, at all?
|
| Why would there be? Such a thing is impossible, since quantum
| states naturally decohere upon interaction with the
| environment, and therefore are inherently volatile unless
| isolated from the environment
| ouid wrote:
| _Universal_ quantum computing is still gated behind a
| technology known as a "magic state". Maintaining coherence is
| but one technical challenge to the implementation of a quantum
| computer, and while we have not solved it, I think it is likely
| to be the lesser of the two hurdles. The main problem with
| magic states is that the larger our computation needs to be,
| the more pure our magic state needs to be. The claim is that we
| can take many "copies" of impure magic states, and leverage QEC
| to produce a single "copy" of a purer magic state. Fortunately,
| given this, the number of copies we need in order scale our
| computation to n qubits for n operations is polynomial in n.
|
| The big caveat, however, is that in order for this scheme to
| work, the impure copies need to have no entanglement between
| them. These are partial measurements of a larger quantum system
| which are _by definition_ not maximally mixed, and as far as I
| know, no one has come up with a compelling physical or
| mathematical argument why it should be easy to find n separable
| mixed states like this. In fact, I think the more natural claim
| would be that this is hard to do, since separable pure states
| are a measure zero subset of pure states.
| sebzim4500 wrote:
| >Are there any inherently stable/noise free/non-Volatile
| Quantum Computing methods, at all?
|
| No, but there are methods (quantum error correction) that allow
| you to simulate noise free qbits with multiple, slightly noisy
| qbits. The main challenge here is that you need to start with
| quite good qbits for this error correction to actually help.
| naasking wrote:
| My prediction: it will never scale properly.
| sebzim4500 wrote:
| Not disagreeing necessarily but why do you think this?
| Codes have got better, qbits have got massively better over
| the last few decades, why wouldn't the two lines eventually
| cross?
|
| Do you think there is some physical principle that will
| prevent getting a quantum speedup, or do you just think
| that the practical engineering challenges are simply too
| difficult?
| klyrs wrote:
| Here's a well-informed argument to that end:
| https://arxiv.org/abs/1908.02499
|
| and a nuanced lay-opinion by the same author:
| https://gilkalai.wordpress.com/2022/05/26/waging-war-on-
| quan...
| naasking wrote:
| I've become increasingly convinced that QM is incomplete
| for various reasons, and so I no longer have confidence
| that coherence will scale the way we expect. I think it
| will hit a hard limit that we can't yet see because of
| this incompleteness. Maybe that wall will become clear
| once we have quantum gravity, maybe some successor to QM
| like [1] will clarify what's happening.
|
| [1] https://arxiv.org/pdf/2102.07795.pdf
| ko27 wrote:
| Isn't the paper mentioned in the article literally
| proving that wrong? Google managed to scale error
| correction and achieved less noisy qbits.
| naasking wrote:
| Sure, they scaled it in the same way we're able to build
| bigger and more effective rockets than 50 years ago.
| We'll never travel faster than light though, because the
| speed of light is known to be the limit. I'm saying there
| will be some ultimate limit to coherence as well, but
| quantum mechanics is incomplete as a model of reality so
| we just don't see that in the math yet.
| gloriousduke wrote:
| If that's the case, the QC computer endeavors are still
| worthwhile since while failing to scale them we may
| develop another piece of a more complete model of
| reality.
| parton wrote:
| Could you briefly summarize one of the reasons you think
| QM is incomplete? I would agree with you that there is no
| reason to expect coherence to scale, but I don't know of
| any reason for it not to scale either.
| sebzim4500 wrote:
| If there is a phsyical principle that stops QCs from
| working, then trying to build QCs is probably the best
| way to make progress in fundamental physics.
| naasking wrote:
| Yes, quantum computers are good physics research in the
| same way that ITER can be considered good plasma
| research. Neither will produce a working product that
| some people are expecting. I'm not sure that QC will
| discover the problem before we infer it theoretically due
| to the many engineering challenges, but it's possible. If
| any real progress is made in QC, my bet's on boson
| sampling.
| bawolff wrote:
| Yeah, seems very win-win. Either we get a working quantum
| computer or we make revolutionary new discoveries in
| physics. Either option would be pretty exciting.
| sampo wrote:
| > qbits have got massively better over the last few
| decades
|
| Progress has been rather slow:
|
| 2001: Shor's algorithm was used to factor 15
|
| 2012: Shor's algorithm was used to factor 21
|
| 2019: Shor's algorithm was attempted at factoring 35, but
| failed due to too much error accumulation
| mjburgess wrote:
| I don't believe there is a quantum computer that exists with a
| single stable logical qubit. I may be wrong, but if so, at best
| I think it's in the single-digits.
|
| What the media reports is physical qubits (and hence not useful
| for computation), rather than logical.
| Strilanc wrote:
| > _Are there any inherently stable /noise free/non-Volatile
| Quantum Computing methods, at all?_
|
| No. With quantum computers, you'd be delighted to have physical
| qubits where all gate error rates stayed below 1 in a thousand
| as you scaled up. Finding a qubit with massively better error
| rates, like one error per million gates, would be tantamount to
| inventing the quantum transistor.
|
| Error correction should be able to reach arbitrarily low error
| rates. But it has a lot of overhead so, in terms of amount-of-
| stuff, it'll be more like building your computer out of cogs
| and gears than like building it out of transistors.
| whatshisface wrote:
| There is a tension between qubits being able to interact with
| your quantum gates quickly, and not being able to interact with
| the environment quickly. Superconducting qbits can work well
| with gates but last for hundreds of microseconds, while nuclear
| spin qbits can last for seconds - but don't work so well with
| gates.
| rdclds wrote:
| > Are there any inherently stable/noise free/non-Volatile
| Quantum Computing methods, _at all_?
|
| topological quantum computers are based on braid theory which
| is invariant to almost any kind of environmental noise, thus
| qubita stay in a coherent state much longer without error
| correction
|
| https://en.m.wikipedia.org/wiki/Topological_quantum_computer
| buddha420 wrote:
| This is the correct answer. Surface codes, like the ones
| discussed in the article, are essentially a way to simulate a
| kind of topological quantum computer on other architectures.
|
| Nobody has yet built a topological qubit though, but
| Microsoft has claimed to be close for at least 5+ years now.
| On the other hand, TQC is supposed to be able to scale much
| faster, since the way in which you create a new qubit (by
| creating several anyons) doesn't necessarily require
| additional hardware - you could move the anyons for one qubit
| out of the way, and then use the same device you used to make
| them to make another[0]. Of course more hardware for
| manipulating additional qubits simultaneously may be desired
| - but the point is that the scaling problem is theoretically
| easier for TQC, even if creating the first qubit seems to be
| much more difficult.
|
| They are not immune to all forms of errors - for instance,
| cosmic rays could cause unwanted anyons to form. But they are
| immune to most typical errors.
|
| [0] This is a bit of an oversimplification. When talking
| about theoretical TQC, we are often talking about actually
| moving anyons confined to a 2D surface around. However, in
| the real world, the medium on which this happens is very
| disordered, so due to Anderson localization, anyons are
| actually trapped where they spawned. So this is where
| Majorana fermions and nanowires come in as a realistic
| approach where anyons can be moved, or alternatively,
| "measurement-based TQC" which relies on teleporting anyons
| instead of actually moving them.
| naturemodelfan wrote:
| [dead]
| pjs_ wrote:
| -
| resource0x wrote:
| Hasn't the claim of supremacy been debunked already? That's
| what this article seems to be saying:
|
| https://www.techradar.com/news/scientists-say-theyve-debunke...
|
| What do we have to be excited about then? Please explain.
| sebzim4500 wrote:
| Depends how you measure quantum supremacy. If you measure by
| seconds of computation, then the claim has been debunked
| since if you rent the most powerful supercomputer in the
| world you can match the performance of the experiment.
|
| If you measure by e.g. energy usage then the claim remains,
| since it would cost orders of magnitude more energy to run
| the algorithm described in the paper than run Google's
| experiment.
| meltyness wrote:
| That there's still shreds of evidence for transformational
| improvements to the dreadfully arcane, inefficient,
| proprietary machines that are currently "the best".
|
| Personally, my favorite is the Deutsch-Josza algorithm which
| illustrates a task which can be trivially understood, and
| provides exponential speedup over any classical algorithm
| that I can fathom to perform the same task.
|
| Consider there was something like a half-century delay
| between the discovery of the photoelectric effect and the
| invention of the triode.
|
| What's most striking to me is that these inherently optical
| phenomena apparently haven't been well-investigated for how
| they can provide speed-up to graphics processing.
| jethkl wrote:
| "Towards" in a title usually means the authors didn't do what
| follows in the title. I have no opinion on the article (and no
| expertise in the field), but whenever I see "towards", my aim in
| reading the article often shifts to understand where the authors
| got stuck.
| sfortis wrote:
| Why an article which asks 32$ to read is #7 on HN as we speak?
| IncRnd wrote:
| You don't need to pay $32. Just click on the article link from
| the summary page. It loads just fine.
|
| The part to click is this link at the top of the summary page:
| This is a summary of: Google Quantum AI. Suppressing quantum
| errors by scaling a surface code logical qubit. Nature
| 614, 676-681 (2023).
|
| That points to:
| https://www.nature.com/articles/s41586-022-05434-1
| tiagod wrote:
| The title is free
| yccs27 wrote:
| And the comments only cost your sanity!
|
| (jk, the comment quality is what I come here for)
| [deleted]
| dd36 wrote:
| Nature has a paywall?
| pathOf_aFineMan wrote:
| The actual article that this summarizes is free and linked on
| the briefing
| sampo wrote:
| Always has.
| dachryn wrote:
| yes thats their business model: have universities and
| institutions pay for access.
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