Quantum jumps in a two-level atom: Simple theories versus quantum trajectories

A strongly driven (Omega much greater than gamma) two-level atom relaxes to wards an equilibrium state rho which is almost completely mixed. One interp retation of this state is that it represents an ensemble average, and that an individual atom is at any time in one of the eigenstates of rho. The the ory of Teich and Mahler [Phys. Rev. A 45, 3300 (1992)] makes this interpret ation concrete, with an individual atom jumping stochastically between the two eigenstates when a photon is emitted. The dressed-atom theory is also s upposed to describe the quantum jumps of an individual atom due to photoemi ssions. But the two pictures are contradictory because the dressed states o f the atom are almost orthogonal to the eigenstates of rho. In this paper w e investigate three ways of measuring the field radiated by the atom, which attempt to reproduce the simple quantum jump dynamics of the dressed state or Teich and Mahler models. These are spectral detection (using optical fi lters), two-state jumps (using adaptive homodyne detection), and orthogonal jumps (another adaptive homodyne scheme). We find that the three schemes c losely mimic the jumps of the dressed-state model, with errors of order 3/4 (gamma/Omega)(2/3), 1/4(gamma/Omega)(2), and 3/4(gamma/Omega)(2), respectiv ely. The significance of this result to the program of environmentally indu ced superselection is discus sed. [S1050-2947(99)04809-X].