Quantum dynamics of a driven two-level molecule with variable dephasing

The longitudinal (Gamma(1)) and transverse (Gamma(2)) decay rates of a two-level quantum system have a profound influence on its evolution. Atomic systems with Gamma(2) = 1/2 Gamma(1) have been studied extensively, but with the rise of solid-state quantum devices it is also important to consider the effect of stronger transverse relaxation due to interactions with the solid environment. Here we study the quantum dynamics of a single organic dye molecule driven by a laser. We measure the variation of Gamma(2) with temperature and determine the activation energy for thermal dephasing of the optical dipole. Then we measure the second-order correlation function g((2))(tau) of the light emitted by the molecule for various ratios Gamma(2)/ Gamma(1) and saturation parameters S. We show that the general solution to the optical Bloch equations accurately describes the observed quantum dynamics over a wide range of these parameters, and we discuss the limitations of the various approximate expressions for g((2))(tau) that appear in the literature.