Dephasing
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Dephasing is a process in which coherence in a substance caused by perturbation decays over time, and the system returns to the state before perturbation.
The coherence of a sample is explained by the off-diagonal elements of a density matrix. An external electric or magnetic field can create coherence between two quantum states in a sample if the frequency corresponds to the energy gap between the two states. The coherence terms decay with the dephasing time, T2.
After coherence is created in a sample by light, the sample emits a polarization wave, the frequency of which is equal to and the phase of which is inverted from the incident light. In addition, the sample is excited by the incident light and a population of molecules in the excited state is generated. The light passing through the sample is absorbed because of these two processes, and it is expressed by an absorption spectrum. The coherence decays with the time constant, T2, and the intensity of the polarization wave is reduced. The population of the excited state also decays with the time constant of the longitudinal relaxation, T1. The time constant T2 is usually much smaller than T1, and the bandwidth of the absorption spectrum is related to these time constants by the Fourier transform, so the time constant T2 is a main contributor to the bandwidth. Recently, the time constant T2 has been measured with ultrafast time-resolved spectroscopy directly, such as in photon echo experiments.
