Zeno effect

The quantum Zeno effect (also known as the Turing paradox) is a situation in which an unstable particle, if observations are repeatedly made i.e., by shining a laser on the sample to take its measurement under a microscope or using some other form of atomic measurement, the unstable particle will never decay. Sometimes this is stated as "a system can't change while you are watching it." One can "freeze" the evolution of the system by measuring it frequently enough in its known initial state. The meaning of the term has since expanded, leading to a more technical definition in which time evolution can be suppressed not only by measurement: the quantum Zeno effect is the suppression of unitary time evolution in quantum systems provided by a variety of sources: measurement, interactions with the environment, stochastic fields, and so on. As an outgrowth of study of the quantum Zeno effect, it has become clear that applying a series of sufficiently strong and fast pulses with appropriate symmetry can also decouple a system from its decohering environment.

The name comes from Zeno's arrow paradox, which states that because an arrow in flight is not seen to move during any single instant, it cannot possibly be moving at all. The comparison with Zeno's paradox is due to a 1977 paper by George Sudarshan and Baidyanath Misra.

The first rigorous and general derivation of this effect was presented in 1974 by Degasperis et al. It had previously been described by Alan Turing in 1954:

It is easy to show using standard theory that if a system starts in an eigenstate of some observable, and measurements are made of that observable N times a second, then, even if the state is not a stationary one, the probability that the system will be in the same state after, say, one second, tends to one as N tends to infinity; that is, that continual observations will prevent motion …

resulting in the earlier name Turing paradox. The idea is contained in the early work by John von Neumann, sometimes called the reduction postulate. It was shown that the quantum Zeno effect of a single system is equivalent to the indetermination of the quantum state of a single system.

According to the reduction postulate, each measurement causes the wavefunction to "collapse" to a pure eigenstate of the measurement basis. In the context of this effect, an "observation" can simply be the absorption of a particle, without an observer in any conventional sense. However, there is controversy over the interpretation of the effect, sometimes referred to as the "measurement problem" in traversing the interface between microscopic and macroscopic.

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