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Bytram (from IRC) writes:

Could a 'demon' help create a quantum computer? Physicists implement a version of Maxwell's famous thought experiment for reducing entropy:

The second law of thermodynamics states that the entropy -- sometimes thought of as disorder -- of a system cannot decrease over time. One of the consequences of this law is that it precludes the possibility of a perpetual motion device. Around 1870, James Clerk Maxwell proposed a thought experiment in which a demon could open and close a gate between two chambers of gas, allowing warmer atoms to pass in one direction and cooler atoms to pass in the other. This sorting, which required no energy input, would result in a reduction of entropy in the system and a temperature difference between the two chambers that could be used as a heat pump to perform work, thus violating the second law.

"Later work has shown that the demon doesn't actually violate the second law and subsequently there have been many attempts to devise experimental systems that behave like the demon," said [physics professor David] Weiss. "There have been some successes at very small scales, but we've created a system in which we can manipulate a large number of atoms, organizing them in a way that reduces the system's entropy, just like the demon."

The researchers use lasers to trap and cool atoms in a three-dimensional lattice with 125 positions arranged as a 5 by 5 by 5 cube. They then randomly fill about half of the positions in the lattice with atoms. By adjusting the polarization of the laser traps, the researchers can move atoms individually or in groups, reorganizing the randomly distributed atoms to fully fill either 5 by 5 by 2 or 4 by 4 by 3 subsets of the lattice.

"Because the atoms are cooled to almost as low a temperature as possible, the entropy of the system is almost entirely defined by the random configuration of the atoms within the lattice," said Weiss. "In systems where the atoms are not super-cooled, the vibration of the atoms makes up the majority of the system's entropy. In such a system, organizing the atoms does little to change the entropy, but in our experiment, we show that organizing the atoms lowers the entropy within the system by a factor of about 2.4."

Original Submission