Friday, 10 June 2011

Negative Entropy and Quantum Observers

I've just read this interesting paper from arXiv:
The thermodynamic meaning of negative entropy, Rio et al.
Landauer's erasure principle exposes an intrinsic relation between thermodynamics and information theory: the erasure of information stored in a system, S, requires an amount of work proportional to the entropy of that system. This entropy, H(S|O), depends on the information that a given observer, O, has about S, and the work necessary to erase a system may therefore vary for different observers. Here, we consider a general setting where the information held by the observer may be quantum-mechanical, and show that an amount of work proportional to H(S|O) is still sufficient to erase S. Since the entropy H(S|O) can now become negative, erasing a system can result in a net gain of work (and a corresponding cooling of the environment).
The main point is an idea of the authors about Landauer's erasure principle applied to quantum memories. There is a less technical description, but with a no less catching title, in this article: Erasing data could keep quantum computers cool.

The authors suggest that a quantum observer can use Landauer's principle to extract heat from the environment instead of throwing heat on it, effectively cooling instead of heating. I'm not a specialist in quantum computing and I didn't go through the arguments of the paper in much detail, but I will be a little skeptical about that and I will explain why. As far as I understood, in order to derive the result the paper assume the existence of a "quantum observer" and give the example of a quantum memory. That is not very clear to me. In fact, if the final idea is to use it in real computers, the ultimate observer will have to do a sharp measurement in the end and will obtain a definite number. The very idea of a quantum observer seems strange to me in the sense that all observers are obviously quantum, but somehow the measurement of a property will cause the decoherence of the entanglement between the observer and the system.

If someone wants to share the thoughts about that paper, please feel free. It seems to be an interesting work, but I would like to understand it better.

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