Theory vs. reality — the throwdown

I was reading a post on Dave Bacon’s blog about a recent quant-ph article. The article questions whether or not fault tolerant quantum error correction can be run on gate model quantum computers, and offers some arguments as to why the answer may be no.

I believe that all of these arguments can (and probably will) individually be refuted. However I think most of the folks eager to begin the rebuttal may be missing the point.

People actually trying to build real QCs have to deal with a large number of real-life problems that are ignored in most theoretical QC. While they may understand that theory has value, they don’t like being told “this is the way things are” by people who have never had to fix a solder joint (yes, there are solder joints in real QCs).

I was involved with a particular case of this recently. A lot of the literature on adiabatic quantum computing (AQC) is wrong, for a very basic physical reason–the omission of realistic environments. Adding a simple thermal environment produces theory that matches with experiment. So when I see a “theorem” about AQC usually it doesn’t apply in practice – ie. it doesn’t match experiment – because the underlying model was not correct.

This experience isn’t unique. A large number of people actually building things take theory with a grain of salt, especially theory originating with a community that may not know what should be included (or not) in the underlying analyses. You may poke fun at Dyakonov because he doesn’t like his physics to be written in Lemmas; I think his point is that if you write physical statements as Lemmas you’re probably a few steps removed from the lab and may not appreciate the subtleties in a real condensed matter physics situation.

The gate model (which Dyakonov attacks) is obviously highly unrealistic for several reasons. Whether FT/QEC can ever be made to work may not even be the worst of these. This lack of realism doesn’t have to matter to theorists because to a theorist how difficult it might be to enable is not their problem. I think if QC theorists really wanted QCs to be built, there would be a lot more communication between these communities as to what is realistic and what isn’t, which would have led to the abandonment of the gate model many years ago.

5 thoughts on “Theory vs. reality — the throwdown

  1. I agree that academics are very often out of touch with reality. As to the abandonment of the gate model, I don’t see it. If an adiabatic QC is supposed, in theory, to evolve according to a Hamiltonian H(t), but, in practice, it evolves according to a Hamiltonian H(t) +V(t), doesn’t the perturbation V(t) become a serious, hard to control source of error? I think the gate model is intended to minimize this source of error.

  2. On the other side of the coin, experimentalists need to pay more attention to the limitations of the threshold results, rather than assuming that the highest number published is the only error level they need to consider. For example, I did not until recently appreciate the fact that the classic result that one qubit rotations plus CNOT is universal for QC is an error-free result that, strictly speaking, applies *after* error correction, not before. This means one cannot necessarily assume this when implementing the error correction.

    This could be seen as an example of theorists being out of touch with experimental reality, but it is just indicative of experimentalists being out of touch with with what the theory actually says. The fault lies on both sides.

  3. @rrtucci

    Although, V(t) (noise, pick your flavor) is a serious source of error for both AQC and the gate model, error introduced in AQC results in a worse approximation. In the gate method, error correction isn’t there to minimize error, it’s there to remove error. Unfortunately, if you reach a certain amount of noise and your error correction can’t remove it all, your answer isn’t approximate, it’s invalid.

    Though noise is a major concern in both approaches, it seems like less of a concern in AQC. After all, an approximate answer seems better than no answer at all.

    I suggest reading Geordie’s previous QC post where he discusses this issue, I thought he did a good job of explaining it there.

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