Quantum beats

In physics, quantum beats are simple examples of phenomena that cannot be described by semiclassical theory, but can be described by fully quantized calculation, especially quantum electrodynamics. In semiclassical theory (SCT), there is an interference or beat note term for both V-type and ${\displaystyle \Lambda }$-type atoms. However, in the quantum electrodynamic (QED) calculation, V-type atoms have a beat term but ${\displaystyle \Lambda }$-types do not. This is strong evidence in support of quantum electrodynamics.

The observation of quantum beats was first reported by A.T. Forrester, R.A. Gudmunsen and P.O. Johnson in 1955, in an experiment that was performed on the basis of an earlier proposal by A.T. Forrester, W.E. Parkins and E. Gerjuoy. This experiment involved the mixing of the Zeeman components of ordinary incoherent light, that is, the mixing of different components resulting from a split of the spectral line into several components in the presence of a magnetic field due to the Zeeman effect. These light components were mixed at a photoelectric surface, and the electrons emitted from that surface then excited a microwave cavity, which allowed the output signal to be measured in dependence on the magnetic field.

...
Wikipedia