Quantum Zeno Effect states that a system cannot alter its state while being observed. It is a feature of the quantum world, where frequent measurement of a particle can stop its time evolution. It can be explained in simple words on the basis of daily observations. For example,  it feels like it takes longer to complete a minute than usual when you are observing a clock. Another example is that it takes more time for water to boil in a boiling container when it is being observed.

To explain the Zeno-effect we can take the example of an electron transition. When an electron jumps from higher to lower energy level it emits light. This is called spontaneous emission. But, when the movement of the electron is “observed” the Quantum Wave Function collapses and the electron returns to its excited state. The frequent measurement of the electron slows down the decaying process. But,   Zeno effect be acquired, the measurement has to be done really fast, around a thousand times a second.

In the year 1977, another possibility was put forth that the frequent measurement could also accelerate the decaying process, called Anti-Zeno Effect. A system can acquire either Zeno effect or Anti-Zeno effect. It is much like Schrödinger’s Cat. Imagine an unstable quantum system. Before being measured, it is in superposition of two states. Take as an example for the two states Up and Down. But the moment it is measured the wave function collapses and it acquires a definite state, either Up or Down. 

Zeno effect in experiments

In 1989, an experiment was performed where 5000 Beryllium cations were surrounded by an electric and magnetic field in a Penning trap and laser cooled to below 250 microkelvins (near absolute zero).  A resonant radio frequency magnetic field was applied, causing a jump from ground state to excited state. A bunch of ultraviolet pulses were fired and the ion traps were observed. The results were in favor; there was a significant reduction in the transition.

In 2001, Mark Raizen successfully observed the quantum Zeno effect as well as the anti-Zeno effect. They used ultracold sodium atoms to perform quantum tunneling and observed suppression as well as acceleration.

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