[HN Gopher] Physicists take a key step in correcting quantum com...
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Physicists take a key step in correcting quantum computer errors
Author : rbanffy
Score : 44 points
Date : 2021-10-17 12:34 UTC (10 hours ago)
(HTM) web link (www.science.org)
(TXT) w3m dump (www.science.org)
| pama wrote:
| To the authors of such blurbs about scientific breakthroughs:
| please cite the exact paper you refer to within the first
| paragraph, including a hyperlink. Using a generic phrase like:
| "... a team has demonstrated a way to detect errors in the
| setting of a quantum bit..." is not enough information to
| unambiguously understand what the author of this piece is talking
| about. In paragraph 7, the author of the blurb mentioned the name
| of the relevant scientists for the first time, after the blurb
| quoted a number of other scientists, included a link to a
| previous paper, and the reader still doesn't know which paper
| this blurb is referring to... I guess that even an AI-based blurb
| generator could learn some simple rules about being specific in
| crediting the source that inspired their writing.
| Strilanc wrote:
| This is the paper: https://arxiv.org/abs/2009.11482
|
| It's actually from last year, and other groups have done better
| experiments since then (in particular, experiments doing
| multiple rounds of correction instead of just immediately
| smashing the system and analyzing the pieces). It's quite funny
| to see a big deal being made of this paper _now_ rather than a
| year ago.
| stephc_int13 wrote:
| I sometimes wonder why the so-called quantum computing field is
| not considered fringe science by the mainstream media.
|
| It is, in my opinion, not much more credible than perpetual
| motion or free energy, and less credible than cold fusion...
| Causality1 wrote:
| Because it functions? It may never be the behemoth researchers
| expect it to someday be but it actually does function and
| produce results.
| stephc_int13 wrote:
| Where? I've been following this field for almost 30 years
| now, and I have yet to find a single publication where the
| produced result is useful in any way.
|
| Why do we think it is more valid than Alchemy?
| Causality1 wrote:
| https://journals.aps.org/prx/abstract/10.1103/PhysRevX.6.03
| 1...
| wrycoder wrote:
| Quoting from that paper's summary:
|
| _More work is needed to turn quantum enhanced
| optimization into a practical technology. The design of
| next- generation annealers must facilitate the embedding
| of problems of practical relevance._
|
| Like fusion, current progress is promising, but practical
| results lie in the future?
| cevi wrote:
| Unlike perpetual motion or free energy, the math and physics
| behind quantum computing is rock solid - the possibility is
| baked into the framework of quantum mechanics at an absolutely
| fundamental level. Even if it turned out that all of quantum
| field theory was completely bogus, I would still expect the
| basic framework of quantum mechanics (i.e. unitary evolution,
| the Born rule for converting amplitudes to probabilities,
| density matrices describing mixed states) to be correct.
|
| If quantum computing was _not_ possible, that would mean that
| everything we thought we knew about physics was hopelessly
| wrong. Physics would have to be _much_ stranger - we 'd have to
| give up on either locality (i.e. causation being limited by the
| speed of light), reversibility (closely related to conservation
| of energy), or nondeterminism in order to make the math work
| out. There are several papers in the literature that derive
| quantum mechanics from these or related assumptions (often
| together with some extra technical detail, that differs between
| approaches):
|
| https://arxiv.org/abs/quant-ph/0101012
| https://arxiv.org/abs/1011.6451 https://arxiv.org/abs/0911.0695
| stephc_int13 wrote:
| I think that quantum mechanics is a useful model and there is
| a lot of truth in it, but it is not _the_ truth.
|
| Using rock-solid math and physics I think you can do time
| travel and speculate about parallel universes, but I don't
| think we should blindly believe the models.
|
| We should never confuse the map and the territory.
| tsimionescu wrote:
| Then how do you explain the extraordinary predictive power
| of QM, and why do you believe that the same maths that
| gives us measurements accurate to 23 decimals is completely
| inaccurate in its predicition of QC?
|
| 30 years is nothing given how complex the engineering
| problem is of actually building a working QC. Fusion is
| much simpler and very well understood and yet it has been
| more than 50 years since research into fusion power plants
| has started, and the most optimistic estimates say we'll
| have the first ever research power plant 30 years from now,
| if everything goes well (EUfusion's DEMO plant).
| cevi wrote:
| There is no solid math or physics which supports time
| travel. There are a lot of cases where people claim to have
| found some loophole or another in the impossibility proofs
| for time travel, sketching out a model that looks plausible
| at first glance, but they all end up postulating some
| blatantly non-physical entity somewhere along the way (i.e.
| cosmic strings, negative energy density, ... - I've gotten
| my hopes up quite a few times).
|
| Speculating about parallel universes is a different story -
| even if they exist, we can't travel to them, so there is no
| sense in claiming that we can somehow harness their power.
| It's a philosophical question, not a physical one. In
| contrast, the rules of quantum mechanics (when phrased in
| terms of density matrices, at least) only describe things
| we can actually measure and observe, and people actually do
| test every single aspect of the predictions quantum
| mechanics makes.
| Crash0v3rid3 wrote:
| > There is no solid math or physics which supports time
| travel.
|
| This is false. Time travel to the future is possible and
| proven[0]. Perhaps you are referring to traveling to the
| past.
|
| [0] https://en.m.wikipedia.org/wiki/Experimental_testing_
| of_time...
| zachf wrote:
| As a nitpick, negative energy densities are indeed
| uncommon but they aren't unphysical. In fact, every
| quantum field theory admits at least one state with
| negative energy density--its a theorem. But even in
| quantum field theory there isn't enough negative energy
| density to actually form acausal (faster than light)
| shortcuts through the spacetime.
| cevi wrote:
| Oh, I didn't know about this! "Get past chapter four of
| any textbook on quantum field theory" has been on my to-
| do list for quite a while.
| zachf wrote:
| Sure, experiments will always be the ultimate arbiter of
| truth. But if you don't "visit the territory", you'll never
| know that your map is wrong. The map has been challenged in
| many many ways that seemed more impossible than large scale
| QC, and the map turned out to be right.
|
| Backwards time travel isn't possible in general relativity,
| though, making very mild and physically plausible
| assumptions about the spacetime. Same in semiclassical
| quantum gravity.
| rafale wrote:
| I found this dude he seems to understand how it works pretty
| well: https://youtu.be/Eak_ogYMprk
| targafarian wrote:
| Quantum computing has already been demonstrated, but on a small
| scale. The challenge in achieving all of what we know quantum
| computing can achieve (as well as the things we have yet to
| discover) is in scaling it up, for which error correction is
| one of the key missing ingredients.
|
| There are various applications. Some that you hear about most
| might be a little overblown (but still maybe not). For example,
| quantum simulation is an application within the physics field
| that already makes these systems worthwhile to pursue, without
| any mention of breaking encryption and the other things
| mentioned. Quantum logic, i.e. quantum computing on the
| smallest scale, has been employed to make cutting-edge atomic
| clocks (which are undeniably a worthwhile pursuit, with
| countless applications in the real world).
|
| Putting "scaling up quantum computing, the pieces of which have
| already been demonstrated" in the same sentence as perpetual
| motion and free energy (known to be in violation of laws of
| physics) is completely unjustified.
| stephc_int13 wrote:
| Perpetual motion and free energy is in violation of known
| laws of physics.
|
| My intuition is that large scale quantum computing is also in
| violation of laws of physics, but we don't have good
| models/theory about those yet.
|
| What I am calling for is a reality check on this field.
|
| Pretty much like string theory I am ready to bet it won't go
| anywhere.
| birktj wrote:
| Quantum computers aren't that powerful, in my understanding
| there should be no reason that a large quantum computer
| couldn't exist. [1] is a fun paper that discusses if
| efficiently solving NP-complete problems should be
| considered impossible in physics. In it it is also
| discussed why this does not exclude quantum computers.
|
| [1] https://www.scottaaronson.com/papers/npcomplete.pdf
| tsimionescu wrote:
| QC is a direct consequence of the known laws of physics. To
| discover QC doesn't work would be proof the QM is
| fundamentally wrong - not that there is some more
| fundamental theory, but that we have been accidentally
| getting the most accurate predicitons of any theory in the
| history of science.
|
| That possibility alone is worth all of the investment in
| this field.
|
| In contrast, String Theory is just an extension of QM,
| motivated by nothing except some cool looking math. It
| doesn't even make any definite predictions (supersimmetry
| is possible in string theory, but not required, at least
| not at any particular energy level).
| zachf wrote:
| I agree with everything you said in the first two
| paragraphs. But string theory is not "motivated by
| nothing but cool looking math". String theory is the
| generalization of quantum mechanics to higher dimensional
| fundamental objects, i.e. beyond particles, but in many
| senses the string theories that are well understood are
| just quantum mechanics in a different suit and hat.
|
| It is already known that many of the most physically
| important quantum field theories are actually string
| theories if you rewrite them in different variables (this
| is called AdS/CFT duality), especially for understanding
| their large-coupling behavior string theory is the only
| tool available to analytically understand the theory. So
| string theory being completely wrong would be as
| surprising as quantum computing being impossible, for the
| same reason as you gave for QC. It would indicate
| something profoundly wrong about something we think we
| understand well. Its not impossible but the case is much,
| much more robust than the general internet understands.
| tsimionescu wrote:
| My point about string theory is exactly what you're
| saying: string theory is a different matehmatical
| formulation of QM, that happens to permit additional
| phenomena that have not been observed. It being a
| generalization to higher-dimensional spaces is exactly my
| point: it is mathematically motivated, not motivated by
| observation.
|
| And yes, to the extent that it's the same formulae as QM,
| finding counter-example would be fascinating. But
| (higher-dimensional) string theory could be wrong with no
| impact on QM/QFT.
| zachf wrote:
| Well my main point is that AdS/CFT duality is asserting
| an exact equivalence between quantum field theories and
| string theories. So for example N=4 SYM, a quantum field
| theory viewed from one perspective, is exactly equal to a
| string theory from another perspective. That's why I say
| it would be a spectacularly weird thing if string theory
| turned out to be unphysical but SYM was, because it would
| turn out then that they weren't equal, and we really
| strongly believe they are.
|
| In other words I'm not saying that string theory is
| (only) a generalization of known correct physics, but
| that many string theories are equal to (and therefore by
| definition as physically valid as) physically important
| quantum field theories. Then there are other string
| theories that are truly interesting and novel, too.
| dang wrote:
| " _Please don 't post shallow dismissals, especially of other
| people's work. A good critical comment teaches us something._"
|
| https://news.ycombinator.com/newsguidelines.html
| stephc_int13 wrote:
| I respectfully disagree with your disapproval of my comment
| here, I agree that my opinion is quite strong but I don't
| think it is offensive.
|
| Given that Quantum Computing has yet to deliver on its
| promise after decades of work and huge investment, I think it
| is safe to compare the field to other fringe science that
| once occupied some of the best minds on the planet (like
| Newton and his obsession with Alchemy).
| v37p wrote:
| Serious question for you. Have you read any recent papers?
| Some quantum computing applications have outperformed
| classical computers already[0] and will get better with
| more qubits and less noise. This stuff is in its infancy
| and making small baby steps. The phone in your pocket
| didn't get here without punch card programming or computers
| the size of rooms. This is that and you're around to
| witness it. Have some patience!
|
| [0] http://arxiv.org/abs/1905.02860
| dilawar wrote:
| I share the sentiment that quantum computer folks hasn't
| delivered but I dont feel its a fringe science. Its just
| hard to deal with noise at that level.
|
| To me, string theory feels like a fringe science; and
| irritating one at best.
| [deleted]
| ThePhysicist wrote:
| Quantum computing isn't fringe science and almost no one in
| the physics community questions the validity of the
| underlying theories. It is a large engineering challenge so
| it's unclear whether a working large-scale quantum computer
| will be realized anytime soon, but that doesn't make it
| fringe science.
| amelius wrote:
| How do you know it's a "key" step unless you've solved the
| problem and proved there's no other way to do it?
| m101 wrote:
| How sure are we that quantum computing will ever work? I think
| this boils down to whether the wave function collapses as
| expected(?)
|
| Perhaps reality is actually only approximated by the wave
| function but reality is something else under the surface which
| means quantum computers may need to be reconsidered.
| andi999 wrote:
| How sure are we with fusion reactors? Probably both 50/50.
| Azsy wrote:
| They are not similar at all.
|
| We know its possible to have a fusion reactor, we would just
| have to dump enough mass together to build a sun. Hopefully
| we can make it more cost effective.
|
| There isn't a natural phenomena that solves a problem by
| encoding it and (ab)using quantum physics.
| gaze wrote:
| 1. We aren't
|
| 2. Everything after the first sentence doesn't make sense. Any
| system that implements a restricted class of unitaries and
| features local measurement as described by Born's rule can
| efficiently solve problems in BQP. We have no reason this
| should apply to 10 qubits and not 1000.
|
| The theory of quantum mechanics has been completely predictive
| in materials and has resulted in the most accurate predictions
| in nature (Anomalous magnetic dipole moment to a part in a
| trillion.) It doesn't matter if the theory is "Macroscopic" or
| whatever, it's predictive and within the constraints of its
| predictions you have known computational power up to problems
| in BQP. Whether BQP=P is an open question, but as is P=NP.
|
| Known problems are things like wiring, materials
| science/sources of noise -- eventually including things like
| cosmic rays. It should work and we have no reason to think it
| shouldn't due to fundamental limitations of the natural world.
| It is, however, one of the hardest scientific things humans
| have ever tried to do so far. To say it'll definitely work is
| hubristic.
| m101 wrote:
| As you can see I don't really know much about this stuff.
|
| There's apparently noise in the system - how do we know that
| the reality is different to the model, and what we are
| putting down to noise is actually reality being different to
| the model? Kind like Einstein's correction of Newton.
| lisper wrote:
| This is not quite true. There are spontaneous collapse models
| [1] that are consistent with the data, like the GRW model
| [2]. Personally I give long odds against these being true,
| but you can't rule them out on the basis of current data. And
| _if_ they are true then they would present a fundamental
| limit on how big you can make a quantum computer before it
| fails.
|
| [1] https://plato.stanford.edu/entries/qm-
| collapse/#ContSponLoca...
|
| [2] https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%
| 80%...
| gameswithgo wrote:
| The only barriers to them working are engineering challenges
| which may prove insurmountable, may not. There are not any
| theoretical considerations that would make them not work.
| stephc_int13 wrote:
| We can also consider Alcubierre drive to be an engineering
| problem, don't you think?
| tsimionescu wrote:
| Not really, the Alcubierre drive depends on things we have
| no reason to think exist in the universe (negative energy).
| It's also likely that the Alcubierre drive doesn't actually
| allow accelerating from slower than light speeds to higher
| than light speeds.
|
| QC in contrast must exist or else QM is fundamentally wrong
| - not incomplete, but wrong.
| ericpauley wrote:
| There are certainly possible reasons why quantum computing
| could be impossible. To me the most compelling of these would
| be that the error rate of physical qubits might fundamentally
| always exceed the threshold for correction with any real ECC
| as the number of logical qubits increases.
|
| I highly recommend reading [1] for an overview of the various
| skeptical viewpoints on quantum computing. Note that, while
| Aaronson argues most of the initial arguments are trivially
| wrong, he admits that whether or not error correction is
| fundamentally impossible is an open question.
|
| [1] https://www.scottaaronson.com/democritus/lec14.html
| Jensson wrote:
| Trying to build a quantum computer is probably a good way
| to learn more about quantum interactions, since it would
| stress those to its limits to work.
| Azsy wrote:
| I've always seen it the other way around.
|
| We can keep solving the engineering challenges, but the
| universe might simply not allow us to get a return on
| investment with every additional qubit.
| Strilanc wrote:
| This is part of the reason I like that the first phase of
| Google's current quantum computer roadmap is "physics
| derisking" [1]. There's no known reason that it should be
| impossible to make a thousand qubits dance together in a highly
| controlled way and get the expected results, but maybe there
| are some unexpected or unknown obstacles.
|
| That being said, if physics derisking failed, it seems really
| unlikely to me that it would be because quantum mechanics
| itself was wrong as opposed to there just being some unforseen
| problematic error mechanism that was consistent with quantum
| mechanics. Finding a flaw in quantum mechanics itself would be
| a scientific revolution, which would be awesome, but those are
| also pretty rare.
|
| I also think it's a little telling that people keep thinking
| it's the philosophically unsatisfying parts of quantum
| mechanics that are going to break, when entering a new regime.
| It smells of confirmation bias.
|
| Disclaimer: am on google quantum team.
|
| 1: https://youtu.be/VvHh6GoNhy8?t=311
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