Intensity fluctuations in a driven Dicke model

Exact analytic results for the steady-state atomic correlation functions of arbitrary order n are given for a system of N superimposed two-level atoms (the Dicke model) collectively interacting with an imposed C.W. laser field. Photon statistical studies through the normalised intensity-intensity correlation function, g(2)(0), show that when both N and the driving field become large, g(2)(0) → 1.2. This compares with an earlier approximated calculation1) which allows an independent atomic decay mechanism giving rise to g(2)(0) ≈ 2. Cooperative interactions thus reduce intensity fluctuations. Photon anti-bunching occurs for finite N. There is a second-order phase transition critical bifurcation point in a thermodynamic limit in which N → ∞; a critical exponent is determined.