Degree of coherence
![Figure 1: This is a plot of g(1) as a function of the delay normalized to the coherence length τ/τc. The blue curve is for a coherent state (an ideal laser or a single frequency). The red curve is for Lorentzian chaotic light (e.g. collision broadened). The green curve is for Gaussian chaotic light (e.g. Doppler broadened).](/Images/godic/202501/08/Coherence_g13617.png")
![Figure 2: This is a plot of g(2) as a function of the delay normalized to the coherence length τ/τc. The blue curve is for a coherent state (an ideal laser or a single frequency). The red curve is for Lorentzian chaotic light (e.g. collision broadened). The green curve is for Gaussian chaotic light (e.g. Doppler broadened). The chaotic light is super-Poissonian and bunched.](/Images/godic/202501/08/Coherence_g23617.png")
![Figure 3: This is a plot of g(2) as a function of the delay normalized to the coherence length τ/τc. A value of g(2) below the dashed black line can only occur in a quantum mechanical model of light. The red curve shows the g(2) of the antibunched and sub-Poissonian light emitted from a single atom driven by a laser beam.](/Images/godic/202501/08/Subpoissonian3617.png")
![Figure 4: This is a plot of g(2) as a function of the delay normalized to the coherence length τ/τc. This is an example of a g(2) that indicates antibunched light but not sub-Poissonian light.](/Images/godic/202501/08/Antibunched3617.png")
In quantum optics, correlation functions are used to characterize the statistical and coherence properties of an electromagnetic field. The degree of coherence is the normalized correlation of electric fields. In its simplest form, termed ![g^{(1)}](/Images/godic/202501/08/f07ff35d85fe2f09eee3d24f7ae759aa3617.png")
, it is useful for quantifying the coherence between two electric fields, as measured in a Michelson or other linear optical interferometer. The correlation between pairs of fields, ![g^{(2)}](/Images/godic/202501/08/711493674a8c4be67c725a0aa7178dbe3617.png")
, typically is used to find the statistical character of intensity fluctuations. First order correlation is actually the amplitude-amplitude correlation and the second order correlation is the intensity-intensity correlation. It is also used to differentiate between states of light that require a quantum mechanical description and those for which classical fields are sufficient. Analogous considerations apply to any Bose field in subatomic physics, in particular to mesons (cf. Bose–Einstein correlations).