What's the News: Anyone who has had their thighs baked by a laptop knows that computing releases heat. And it's more than a common-sense maxim: physicists have shown that heat released by information processing is bound by a physical law, where a bit of information processed must cause a corresponding rise in temperature. But could quantum mechanics allow computations that actually cool computers down? In a recent Nature paper
, researchers describe how this paradox is possible. How the Heck:
In this paper, the team describes how, using the quantum mechanical property of entanglement, an observer can actually drain heat from a system while deleting information.
How, you say? It all comes down to a question of entropy. The second law of thermodynamics states that the entropy of a system is always increasing or remaining the same, but never decreasing. And the way we usually experience it, entropy is heat. Landauer's Principle, which arises from the second law, links heat and information processing: any irreversible computation, the principle says, is going to add entropy to the universe.
But if a computation is reversible---if a 1 or 0 is deleted while its state is recorded somewhere, thus making it possible to recreate it---then no entropy should be released at all. Physicists have confirmed this math in the past.
Now researchers have shown that in quantum mechanics, entropy can be seen as a lack of knowledge on the part of the observer (that is, the experimenter) about the state of the 1s and 0s. In a quantum quirk, when an observer is entangled with bits of information, he has a great deal of detailed knowledge about those bits---so much, in fact, that entropy in the system goes into the negative numbers. Thus, when the observer makes a reversible deletion, he is actually siphoning heat off from the system. Voila: A computation that cools.
What's the Context:
The idea of reversible computing and entropy has been hot stuff in the last half-century---it sits at the intersection of thermodynamics, the physics of heat, and information theory, the study of information. The senior author of this paper wrote an article on it way back in 1985 in Scientific American.
This result is likely to cause some heated discussion among physicists, as it comes very close to breaking the second law of thermodynamics. As the lead researcher warns in a blog post at Scientific American, "For many physicists, this is tantamount to saying that perpetual motion is possible!" (And we all know how they feel about that.)
The team argues, though, that because of the strange effects of entanglement, the second law is preserved.
The Future Holds: The team has described a theoretical situation, and now someone needs to step up to the plate and test it experimentally. Should it work out in the real world, the researchers think it could eventually be used to cool down supercomputers. Reference: Lídia del Rio, Johan Åberg, Renato Renner, Oscar Dahlsten, Vlatko Vedral. The thermodynamic meaning of negative entropy. Nature, 2011; 474 (7349): 61 DOI: 10.1038/nature10123
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