Towards quantum computers that are robust to errors
nature.com
nature.com
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
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
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.
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.
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.
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?
For my part, I don't think quantum foundations are on a firm footing. I think the measurement problem is still unresolved to a satisfactory degree, which means our understanding of decoherence is incomplete.
Per the link I posted above, there's also a good chance that gravity is inherently decoherent, which means entanglement will naturally breakdown in various conditions around mass.
I've also developed skepticism of quantum field theories. For one, the supposedly "most precise calculation in physics" has been marred by numerous mistakes and even fraud (electron magnetic moment). Renormalization is also sketchy business.
A more fringe reason, but one I think will resolve some of those issues is a growing skepticism of continuity [1], even for classical mechanics; continuous formalisms just seem to lead to logical and physical absurdities, like Norton's Dome and singularities in GR. Discrete theories are only now getting a little attention, but they're promising [2] because they seem to eliminate some of the formal structure (gauges), and the infinities disappear.
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
and a nuanced lay-opinion by the same author: https://gilkalai.wordpress.com/2022/05/26/waging-war-on-quan...
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.
What the media reports is physical qubits (and hence not useful for computation), rather than logical.
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
>Suppressing quantum errors by scaling a surface code logical qubit [Open Access]
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).
(jk, the comment quality is what I come here for)
https://www.techradar.com/news/scientists-say-theyve-debunke...
What do we have to be excited about then? Please explain.
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.
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.