Two-photon spectra of quantum systems

Summary form only given. We present our recent theory [1] of frequency-filtered and time-resolved N-photon correlations or N-photon spectra, g^(N)_Γ (ω₁, T₁;... ; ω_N T_N). There are an extension of the standard Nth order temporal correlation functions, g^(N) (T₁;... ;T_N) (that measure the probability of emission of N photons at times T₁,..., T_N), into the frequency domain. Heisenberg uncertainty principle requires the introduction of the detector or filter frequency resolution, Γ, in the theory.