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 Λ {displaystyle Lambda } -type atoms. However, in the quantum electrodynamic (QED) calculation, V-type atoms have a beat term but Λ {displaystyle Lambda } -types do not. This is strong evidence in support of quantum electrodynamics. 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 Λ {displaystyle Lambda } -type atoms. However, in the quantum electrodynamic (QED) calculation, V-type atoms have a beat term but Λ {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. Since the invention of the laser, quantum beats can be demonstrated by using light originating from two different laser sources. In 2017 quantum beats in single photon emission from the atomic collective excitation have been observed. Observed collective beats were not due to superposition of excitation between two different energy levels of the atoms, as in usual single-atom quantum beats in V {displaystyle V} -type atoms. Instead, single photon was stored as excitation of the same atomic energy level, but this time two groups of atoms with different velocities have been coherently excited. These collective beats originate from motion between entangled pairs of atoms, that acquire relative phase due to Doppler effect. There is a figure in Quantum Optics that describes V {displaystyle V} -type and Λ {displaystyle Lambda } -type atoms clearly. Simply, V-type atoms have 3 states: | a ⟩ {displaystyle |a angle } , | b ⟩ {displaystyle |b angle } , and | c ⟩ {displaystyle |c angle } . The energy levels of | a ⟩ {displaystyle |a angle } and | b ⟩ {displaystyle |b angle } are higher than that of | c ⟩ {displaystyle |c angle } . When electrons in states | a ⟩ {displaystyle |a angle } and : | b ⟩ {displaystyle |b angle } subsequently decay to state | c ⟩ {displaystyle |c angle } , two kinds of emission are radiated. In Λ {displaystyle Lambda } -type atoms, there are also 3 states: | a ⟩ {displaystyle |a angle } , | b ⟩ {displaystyle |b angle } , and : | c ⟩ {displaystyle |c angle } . However, in this type, | a ⟩ {displaystyle |a angle } is at the highest energy level, while | b ⟩ {displaystyle |b angle } and : | c ⟩ {displaystyle |c angle } are at lower levels. When two electrons in state | a ⟩ {displaystyle |a angle } decay to states | b ⟩ {displaystyle |b angle } and : | c ⟩ {displaystyle |c angle } , respectively, two kinds of emission are also radiated. The derivation below follows the reference Quantum Optics In the semiclassical picture, the state vector of electrons is If the nonvanishing dipole matrix elements are described by