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

The longitudinal ($Γ_1$) and transverse ($Γ_2$) decay rates of a two-level quantum system have profound influence on its evolution. Atomic systems with $Γ_2=\tfrac{1}{2}Γ_1$ 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 $Γ_2$ with temperature and determine the activation energy for thermal dephasing of the optical dipole. Then we measure the second-order correlation function $g^{(2)}(τ)$ of the light emitted by the molecule for various ratios $Γ_2/Γ_1$ 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 $g^{(2)}(τ)$ that appear in the literature.

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