QUANTUM TRAJECTORIES AND QUANTUM MEASUREMENT THEORY

Beyond their use as numerical tools, quantum trajectories can be ascri bed a degree of reality in terms of quantum measurement theory. In fac t, they arise naturally from considering continuous observation of a d amped quantum system. A particularly useful form of quantum trajectori es is as linear (but non-unitary) stochastic Schrodinger equations. In the limit where a strong local oscillator is used in the detection, a nd where the system is not driven, these quantum trajectories can be s olved. This gives an alternative derivation of the probability distrib utions for completed homodyne and heterodyne detection schemes. It als o allows the previously intractable problem of real-time adaptive meas urements to be treated. The results for an analytically soluble exampl e of adaptive phase measurements are presented, and future development s discussed.