4 ms·

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 Quan

by lmpdev 4y ago

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

sebzim4500 4y ago
>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 4y ago
  My prediction: it will never scale properly.
  - sebzim4500 4y ago
    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?
    - naasking 4y ago
      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 https://arxiv.org/pdf/2102.07795.pdf
      - sebzim4500 4y ago
        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 4y ago
        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 4y ago
        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.
      - ko27 4y ago
        Isn't the paper mentioned in the article literally proving that wrong? Google managed to scale error correction and achieved less noisy qbits.
        naasking 4y ago
        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 4y ago
        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.
    - klyrs 4y ago
      Here's a well-informed argument to that end: https://arxiv.org/abs/1908.02499 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-quantum/ https://gilkalai.wordpress.com/2022/05/26/waging-war-on-quan...
    - sampo 4y ago
      > 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 4y ago
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.
rdclds 4y ago
> 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 https://en.m.wikipedia.org/wiki/Topological_quantum_computer
- buddha420 4y ago
  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.
Strilanc 4y ago
> 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.
bowsamic 4y ago
> 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
whatshisface 4y ago
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.
ouid 4y ago
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.

by lmpdev 4y ago

sebzim4500 4y ago

>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 4y ago
My prediction: it will never scale properly.
- sebzim4500 4y ago
  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?
  - naasking 4y ago
    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 https://arxiv.org/pdf/2102.07795.pdf
    - sebzim4500 4y ago
      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 4y ago
        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 4y ago
        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.
    - ko27 4y ago
      Isn't the paper mentioned in the article literally proving that wrong? Google managed to scale error correction and achieved less noisy qbits.
      - naasking 4y ago
        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 4y ago
        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.
  - klyrs 4y ago
    Here's a well-informed argument to that end: https://arxiv.org/abs/1908.02499 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-quantum/ https://gilkalai.wordpress.com/2022/05/26/waging-war-on-quan...
  - sampo 4y ago
    > 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 4y ago

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.

rdclds 4y ago

> 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 https://en.m.wikipedia.org/wiki/Topological_quantum_computer

buddha420 4y ago
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.

Strilanc 4y ago

> 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.

bowsamic 4y ago

> 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

whatshisface 4y ago

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.

ouid 4y ago

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.