Simulating single-photon-single-atom absorption experiments with an optical resonator

The absorption of a single photon by a single two-level system (TLS) in free space is a fundamental physical process. In this contribution, we present the experimental realization of a model system to simulate the dynamics of such an absorption experiment: Comparing the absorption process of a single photon by a single TLS and the incoupling dynamics of an optical resonator, one finds an analogy between the energy stored inside the resonator and the probability of the photon to be absorbed by the TLS [1]. Both systems, resonators and TLS, respond in an equivalent way to the temporal profile of the incident light pulse. The energy storage in a resonator as well as the absorption of a single photon by a TLS can reach an unit efficiency under idealized conditions. Such an optimized process is achieved by an exponentially rising incident light field with a time constant which matches the lifetime of the system [2]. For such a perfect coupling, the end mirror of the resonator needs to have a reflectivity of unity. This corresponds to the excitation of a TLS from the complete solid angle. For a smaller reflectivity of the end mirror, the cavity simulates the absorption dynamics in the case of excitation from only a part of the solid angle. However, using real mirrors, a good analogy with excitation from a large solid angle can still be obtained by using a strongly asymmetric resonator, i.e. an incoupling mirror which has a significantly lower reflectivity than the end mirror of the cavity.