Archive for August, 2010

Hybrid quantum-classical computation

Posted in Uncategorized on August 25, 2010 by quantummoxie

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?


The quantum-classical transition

Posted in Uncategorized on August 14, 2010 by quantummoxie

Given the excellent comments on my recent Eliztur-Vaidman bomb test post and a discussion about Berry’s phase on another site, I got to thinking about this topic.  In particular, there was a bit of discussion surrounding something Aharonov and Rohrlich said in the context of a much larger discussion of Berry’s phase in their book <em>Quantum Paradoxes</em>.  They said,

We can continuously observe any quantum state. If we continuously observe a decaying atom, it never decays; if we continuously check whether a particle has crossed a barrier, it never crosses. We can make a free particle scatter off a force-free region just by constantly checking whether the particle has entered the region. Indeed, we can induce any evolution we want in a quantum system.

But can we really?  I realize that what they’re trying to get at (I think) is that the act of observation in QM requires an interaction between observer and observed. I don’t deny that. But when you think about the mechanism by which an observer does this interacting on the most fundamental level – an exchange of some boson depending on the type of interaction – either the uncertainty principle limits the observer to a certain level of discreteness or the interaction must involve virtual bosons. As I am unaware of any device that has an unbroken chain of virtual exchanges from the observed quantum process to the observer’s mind, truly continuous observation must not be possible, at least given existing technology. Whether it is fundamentally prevented by QM is an open question. Perhaps things like entanglement are really just strings of virtual processes (which would make it possible for some people to interpret them as violating SR).

But it seems to me that this gets at the heart of whether anything is truly classical or not.  Perhaps Bob Griffiths is right and the classical world – and continuity in particular – is simply an illusion (Freudian slip: I almost typed ‘allusion’) born of ‘coarse graining.’