Atomic Memory

In 1872 Ludwig Boltzmann, a founder of modern thermodynamics, gave a lecture in which he said that the entropy, or disorder, of isolated systems increases irreversibly as time passes. On hearing this the physicist Joseph Loschmidt rose in protest. He argued that the laws governing the motions of all particles are symmetric with respect to time. Thus any system that had decayed from order to chaos could be made orderly once again simply by reversing the momentum of each particle, without affecting the total kinetic energy of the system. In defiance Boltzmann pointed his finger at Loschmidt and said, “You reverse the momenta.” This scholarly conflict illustrates the paradoxical nature of the second law of thermodynamics, which states that systems tend toward maximum entropy. Yet Loschmidt’s argument remains cogent. If one were able to film the motions of any small group of particles and show the film to a physicist, he or she would have no way of telling in principle whether the projector was running forward or backward. Consequently, according to Loschmidt’s criticism (which has come to be called the Loschmidt paradox), any law that governs the behavior of large collections of particles should be symmetric with respect to time. While the meaning and implications of the second law are still active topics of research and disagreement [see “The Arrow of Time,” by David Layzer; SCIENTIFIC AMERICAN, December, 1975], there now exist several methods by which Loschmidt’s time reversal can be realized. In other words, a system of particles that has apparently decayed from a highly ordered state can be returned to that state by reversing the motions (or some other degree of freedom) of its constituent particles. In effect an assembly of atoms is able to exhibit a kind of memory of its earlier condition. If a system is to display this kind of atomic memory, it must be prepared so that it has some kind of order, often hidden, in its apparently disordered state. In the atomic systems we shall discuss, this hidden order is provided by exposing samples (which may be solid, liquid or gaseous) to coherent electromagnetic radiation of various types, including radio waves, microwaves and laser beams. Sound waves can also play this role. The reemergence of an ordered state in such systems becomes evident when the sample emits its own coherent electromagnetic pulse, an echo of the earlier radiation. Apart from their inherent interest, these echo pulses and related forms of coherent emission provide novel ways to study the fundamental behavior of atomic interactions.