CONTINUOUS MEASUREMENTS OF QUANTUM PHASE

The question of phase measurement for a quantum oscillator using the q uantum non-demolition principle (NDP), which does not need the use of the wave packet reduction postulate, is studied. It is shown that this approach is appropriate to introduce in a proper way the relevant obs ervables for unlocalized (unsharp) measurement of quantum phase. Our a nalysis suggests that in contrast to other quantum-mechanical problems (e.g. the position and the momentum of a particle), the operator to r epresent the phase of a quantum field cannot be separated from the mea surement process. Thus, the quantum phase operator must be defined for a corresponding measurement scheme. Moreover, it allows us to describ e the time continuous process of quantum phase observations and also t o study the stochastic dynamics of the oscillator under such measureme nt.