The End of an Entanglement

Science : In quantum physics, decoherence is a catch-all term that usually implies degradation of the purity of a quantum state. Over the past few decades it has been used as a guide to understand the loss of the two-body coherence called entanglement, which is an intrinsically quantum effect. In this context, it is relevant to fundamental questions such as: Why is the world mostly classical when we believe quantum theory provides all of the governing principles? The answer lies in the critical role of “largeness"; simply put, larger bodies lose coherence more quickly. This is the essential ingredient in producing nearly instantaneous decay of entanglement between two large bodies or between a large body and a small one. The role of largeness is seen when decoherence occurs increasingly faster with the size of the environment. Preservation of coherence is important in maintaining steady behavior of quantum systems whose coordinated action is critical, for example, among the working units of quantum computers when they become available.A small body (spin, photon, atom, exciton, quantum dot, Cooper pair, etc.), on the other hand, can continue to behave as a quantum mechanical unit, even if not macroscopically entangled. A topic that remains open in almost all decoherence discussions, however, is the preservation or destruction of two-body quantum coherence when both bodies are small. For example, it has been predicted only recently that the one-body and two-body responses to a noisy environment can follow surprisingly different pathways to complete decoherence. Experimental entry into this new domain is needed, and impressive results are now reported on page 579 of this week’s Science magazine. The researchers have devised an elegantly clean way to check and to confirm the existence of so-called “entanglement sudden death,” a two-body disentanglement that is novel among known relaxation effects because it has no lifetime in any usual sense--that is, entanglement terminates completely after a finite interval, without a smoothly diminishing long-time tail.