  We present a protocol for probing the state of a quantum system by its
resonant coupling and entanglement with a meter system. By continuous
measurement of a time evolving meter observable, we infer the evolution of the
entangled systems and, ultimately, the state and dynamics of the system of
interest. The photon number in a cavity field is thus resolved by simulated
monitoring of the time dependent excited state population of a resonantly
coupled two-level system, and we propose to regard this as an extension of
quantum non-demolition measurements with potential applications in quantum
metrology and quantum computing.
