Topics: Entropy, Information, James C. Maxwell, Mark G. Raizen, Thermodynamics
Almost 25 years ago, Rolf Landauer argued in the pages of this magazine that information is physical (see Physics Today, May 1991, page 23). It is stored in physical systems such as books and memory sticks, transmitted by physical means—for instance, via electrical or optical signals—and processed in physical devices. Therefore, he concluded, it must obey the laws of physics, in particular the laws of thermodynamics.
But what is information? A simple, intuitive answer is “what you don’t already know.” If someone tells you that Earth is spherical, you surely would not learn much; the message has low information content. However, if you are told that the price of oil will double tomorrow, then, assuming that to be true, you would learn a great deal; the message has high information content.
Mathematically, a system’s information content can be quantified by the so-called information entropy H, introduced by Claude Shannon in 1948. The larger the information entropy, the greater the information content.1 Consider the simplest possible information-storage device: a system with two distinct states—for example, up and down, left and right, or magnetized and unmagnetized. If the system is known with certainty to be in a particular state, then no new information can be gained by probing the system, and the information entropy is zero.
Figure 3. Bringing Maxwell’s demon to life. A pair of laser beams can be tuned to atomic transitions and configured to create a one-way potential barrier; atoms may cross unimpeded in one direction—right to left in this figure—but not in the other. (a) When the barrier is introduced at the periphery of a V-shaped magnetic trap, the atoms that cross the barrier will be those that have converted nearly all their kinetic energy to potential energy—in other words, the cold ones. (b–c) By slowly sweeping the barrier across the trap, one can sort cold atoms (blue) from hot ones (red), reminiscent of James Clerk Maxwell’s famous thought experiment, or cool an entire atomic ensemble. Because the cold atoms do work against the optical barrier as it moves, their kinetic energy remains small even as they return to the deep portion of the potential well. (Adapted from ref. 8 , M. G. Raizen.)
However, owing to remarkable technological progress achieved in recent decades, experiments with atoms and small particles have now become feasible. Maxwell’s demon, Szilard’s engine, and Landauer’s erasure principle can now be rigorously studied in lab experiments.
One of the first such experiments was performed by Mark Raizen and coworkers at the University of Texas at Austin. 8 They confined an ensemble of atoms in a magnetic trap, as shown schematically in figure 3. Initially, all the atoms are in the same internal state. The group then introduced a one-way optical barrier, composed of two laser beams arranged side by side: One beam promotes atoms to an excited state, and the other is tuned such that it has no effect on excited atoms but repels atoms in the ground state. An atom (red) approaching from the excitation-beam side gets promoted to an excited state, passes unimpeded through the second beam, and then relaxes to the ground state by emitting a photon. An atom approaching from the other side, by contrast, encounters the repelling beam first and is turned around—it can’t get through. The two beams behave as an atom diode. Physics Today: Information: From Maxwell’s demon to Landauer’s eraser
Eric Lutz and Sergio Ciliberto
8. G. N. Price et al., Phys. Rev. Lett. 100, 093004 (2008); http://dx.doi.org/10.1103/PhysRevLett.100.093004
M. G. Raizen, Science 324, 1403 (2009). http://dx.doi.org/10.1126/science.1171506
