Hybrid quantum-classical computation

I’ve been working for the past year on a startup company that’s going to be doing some things quantum-related (obviously).  In the process I’ve been designing some circuits to make some measurements of quantum phenomena.  In the process I got to thinking about various classical circuit components and spent a lot of time understanding op amps.  As it so happens, a basic op amp can be used to make logic gates, notably the OR and NOR gates.  Naturally that got me thinking about quantum logic gates and I began to wonder why no one had ever thought to develop some kind of hybrid computing system in which certain quantum logic gates were incorporated into classical circuits in some way.  Well, what is one of the first things you’re taught (or you should be taught) when embarking on a new idea or line of research?  Perform a literature search!  And lo and behold, I discovered this recent gem that proposes a framework for hybrid computation (note: I’m not quite done reading it yet).

What I hope to get out of it is three-fold:

  1. a framework that will allow me to dust off my – gasp – experimenter’s “hat” and get back in the lab (I’ve been playing around with a breadboard in my living room for half the summer and the house is still standing…) where I can create some of these hybrid circuits and see how they run;
  2. some new ideas for my start-up company that might improve our products or provide some new (and perhaps unusual!) products; and
  3. some kind of generalized circuit analysis framework (a bit more than is given in the above paper).

The burning question at the heart of all of this is: what would such hybrid circuits be good for, i.e. could they be used for more than mere “computation” (at least in the traditional sense) and serve as improvements over traditional classical circuits in electronic devices and components?

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9 Responses to “Hybrid quantum-classical computation”

  1. Were you thinking of hybrid architectures on-chip? The problem with this is that you would need an entirely different architecture for your quantum components than you would for your classical ones. I think the practicalities of making such a circuit would require a prohibitive amount of funding unless you were really sure it was going to do something cool. Even if you were using, say, semiconductor quantum dots (as opposed to superconducting, ion-trap or some other exotic hardware), it would be very difficult to integrate this with CMOS.

    I’d love to hear your views on how you think the hybrid approaches would be realised. It is something I find particularly interesting :) I didn’t find any information in the paper above about how this could be practically achieved.

  2. quantummoxie Says:

    Hey Suz,

    How’s Vancouver? Actually, you and Geordie were two people I was particularly interested to talk to about this! And I partly disagree with your assessment – we must, by dint of the fact that we are classical beings, interact classically with quantum things so there has to be a way to put them side-by-side. Besides, isn’t that what D-Wave’s chip does? It must interface with classical computers if Google used it!

    You are right about the above paper – despite professing to be more of a “physics” paper than a “computer science” paper, it didn’t seem to offer many practical physical insights, at least to my mind.

    But, here’s the thing. I don’t think it would be quite as difficult as one might think – and I’m thinking about trying to implement something (yes, the theorist is getting his hands dirty).

    So, first think of a simple quantum setup like a Mach-Zender interferometer (MZI). In an MZI, the beamsplitters essentially behave like Hadamard gates (they have the same mathematical form). So, step one is to create a micro-sized MZI (which is something that has actually been proposed as the basis for a new type of bio-sensor). Step two is then to incorporate this into a standard micro-circuit.

    How does one implement a micro-MZI? Well, I’m thinking fiber-optics is one approach to try. This relieves one from the need for micro-mirrors (the fiber’s walls serve that purpose). The big thing, from my perspective, is making a micro-beamsplitter, but I work with some guys who might be able to pull it off (or know whether it could be done).

    Now, that’s working specifically with the gate model (something I know Geordie is not fond of). Most classical circuits that aren’t computationally-based are analog (e.g. TV remotes, microwave ovens, etc.) and you would know better than anyone else what the quantum “analogue” of analog circuitry is: adiabatic circuits!

    So, in my mind, I see some of this already happening in quantum electronics and in new nanotechnology applications. There’s a really interesting – though brief – discussion about this in this paper. In particular, see the part on CMOS technology.

    I think what’s been hindering progress, in some regard, is the fact that everyone’s focused on a quantum computer, i.e. the final product. It’s like we’ve leap-frogged a bunch of steps. Pull apart a classical computer and see what it’s made of – resistors, transistors, capacitors, switches, ICs, etc. mounted on PCBs. The only quantum corollary to a PCB (or, rather, an IC) that I’m aware of is the D-Wave chip.

    If quantum computers are ever going to be scaled down, we need to start thinking about how to do some of this stuff on a micro-scale. And I’m rambling, but I hope it makes sense.

    • physicsandcake Says:

      Hi Ian,

      I didn’t mean that it wasn’t possible to do this. I was just thinking that at the moment quantum circuits tend to be ‘a bunch of quantum things’ surrounded by ‘a bunch of classical things’, mainly for pre-and-post processing of the quantum things, and thus of course they are interfaced. So maybe I misunderstand the idea of a hybrid architecture, but I imagine it to be more like arrays of quantum and classical components, very tightly interwoven and integrated.

      In which case I still think it will be hard to do ;)

      I think doing things benchtop style with small ICs + optoelectronic components will be fine as a proof of concept, it is just when you want to start miniaturising and putting these things all on the same bit of silicon that it becomes tricky. But maybe that doesn’t matter at the moment.

      • physicsandcake Says:

        To be fair I am nit-picking a little here…. The general idea of investigating hybrid architectures is really interesting, and it would be cool to talk more about it.

        I didn’t mean to be at all negative, I just have a reflex reaction when people envisage that getting a few qubits to work (in any architecture) and then making something VLSI is a kind of simple, linear ‘next step’ ;)

      • quantummoxie Says:

        Oh, right, I think I misunderstood you. I absolutely agree that transitioning to micro-size and trying to mount these things on the same IC will be difficult, but I think one of the first steps is to actually pick something and try to miniaturize it. I think an MZI might be a logical first step. There are a couple of interesting papers out there proposing them as unique biosensors, at least one of which proposes a miniaturized version.

  3. I wonder if you could use these arrangements to do “weak measurements” in some sense (find out something about the wavefunction and not just get hit statistics, as per Y. Aharonov)? I describe a way to maybe do that at name link, not related to your concept.

    BTW Ian I am still having trouble posting from my own computer despite tricks with cookies and starting reply info over, maybe this library computer will do it.

  4. quantummoxie Says:

    Neil,

    That’s really, really bizarre. I honestly have no idea why you’re having trouble posting. I’ll look around to see if there’s some setting I can change. Maybe the Internet gods don’t like you … ;)

    Regarding your proposal, is that the post from Jan. 31 on your blog?

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