nonzero, it is clear to see that the Mollow triplet remains the spectrum for the fluoresced light even in a steady state solution. General two-time correlation functions and spectral density The study of correlation functions is critical to the study of quantum optics as the Fourier transform of the correlation function is the energy spectral density. Thus the two-time correlation function is a useful tool in the calculation of the energy spectrum for a given system. We take the parameter to be the difference between the two times in which the function is calculated. While correlation functions can more easily be described using limits of the strength of the field and limits placed on the time of the system, they can be found more generally as well. For resonance fluorescence, the most important correlation functions are , , , where , , . Two-time correlation functions are generally shown to be independent of , and instead rely on as . These functions can be used to find the spectral density by computing the transform , where K is a constant. The spectral density can be viewed as the rate of photon emission of photons of frequency at the given time , which is useful in determining the power output of a system at a given time. The correlation function associated with the spectral density of resonance fluorescence is reliant on the electric field. Thus once the constant K has been determined, the result is equivalent to This is related to the intensity by In the weak field limit when the power spectrum can be determined to be . In the strong field limit, the power spectrum is slight more complicated and found to be . From these two functions it is easy to see that in the weak field limit a single peak appears at in the spectral density due to the delta function, while in the strong field limit a Mollow triplet forms with sideband peaks at , and appropriate peak width of for the central peak and for the sideband peaks. Photon Anti-bunching Photon anti-bunching is the process in Resonance Fluorescence through which rate at which photons are emitted by a two-level atom is limited. A two-level atom is only capable of absorbing a photon from the driving electromagnetic field after a certain period of time has passed. This time period is modeled as a probability distribution where