Continuous weak measurement of quantum coherent oscillations - art. no. 165310

We consider the problem of continuous quantum measurement of coherent oscil lations between two quantum states of an individual two-state system. It is shown that-the interplay between the information acquisition and the backa ction dephasing of the oscillations by the detector imposes a fundamental l imit, equal to four, on the signal-to-noise ratio of the measurement. The l imit is universal, e.g., independent of the coupling strength between the d etector and system, and results from the tendency of quantum measurement to localize the system in one of the measured eigenstates.