What effect slows a quantum system's evolution through frequent measurement?

Frequent measurement slows a quantum system’s evolution through the quantum Zeno effect.

In quantum mechanics, a system can begin in one state and gradually evolve into another. If it is measured repeatedly at very short intervals, each measurement projects the system back into a state compatible with the measurement. Because the probability of transition is proportional to the square of a very short time interval, sufficiently frequent measurements can strongly suppress the transition.

The name refers to Zeno of Elea’s ancient arrow paradox, in which an arrow appears motionless at every instant. Physicists Baidyanath Misra and E. C. George Sudarshan gave the effect its modern formulation in 1977. An influential experiment followed in 1990, using laser pulses to monitor transitions in trapped beryllium ions.

The effect does not mean that human attention magically freezes matter. A measurement can be a physical interaction, such as absorption or coupling to an environment. It is also distinct from ordinary quantum decoherence, although environmental interactions can produce Zeno-like behavior. Under different timing conditions, measurements can instead accelerate decay; this is called the quantum anti-Zeno effect.