Robust unravelings for resonance fluorescence - art. no. 023805

Monitoring the fluorescent radiation of an atom unravels the master equatio n evolution by collapsing the atomic state into a pure state which evolves stochastically. A robust unraveling is one that gives pure states that, on average, are relatively unaffected by the master-equation evolution (which applies once the monitoring ceases). The ensemble of pure states arising fr om the maximally robust unraveling has been suggested to be the most natura l way of representing the system [H.M.. Wiseman and J.A. Vaccaro, Phys. Let t. A 250, 241 (1998)]. We find that the maximally robust unraveling of a re sonantly driven atom requires an adaptive interferometric measurement propo sed by Wiseman and Toombes [Phys. Rev. A 60, 2474 (1999)]. The resultant en semble consists of just two purr states which, in the high driving limit, a re close to the eigenstates of the driving Hamiltonian Omega sigma(x)/2. Th is ensemble is the closest thing to a classical limit for a strongly driven atom We also find that it is possible to reasonably approximate this ensem ble using just homodyne detection, an example of a continuous Markovian unr aveling. This has implications for other systems, for which it may be neces sary in practice to consider only continuous Markovian unravelings.