Photon anti-bunching

Figure 2.12 summarizes the contributions of these common processes to the autocorrelation function in an FCS experiment. Other processes such as photon anti-bunching [48], saturation of the excitation emission cycle [38] and protonation of chromophores [49] will not be discussed in detail here. 

In a complete quantum theory of radiation several interesting phenomena regarding photon statistics are predicted. One such effect is photon anti-bunching for a two-level system. When an atom has just emitted a photon it cannot immediately radiate a second photon since it is in the lower state [4.17,18]. This behaviour is experimentally observed in photon correlation experiments. 

Fig. 4.3. Excited states correlation function using stationary illumination (anti-bunching). Observe the amplitude is related to 1/N the number of emitted photons...

We treat here the durations of intervals between pulses as independant variables. This is correct since two successive intervals are uncorrelated. At the end of a given interval, the detection of a photon projects the atom in the ground state so that any information concerning the length of this interval is lost. This is to be contrasted with the fact that two successive pulses are correlated (anti-bunching effect for example). 

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