Measurements with a noninvasive detector and dephasing mechanism

We study the dynamics of the measurement process in quantum-dot systems, where a particular state out of coherent superposition is observed. The ballistic point contact placed near one of the dots is taken as a noninvasive detector. We demonstrate that the measurement process is fully described by the Bloch-type equations applied to the whole system. These equations clearly reproduce the collapse of the density matrix into the statistical mixture in the course of the measurement process. The corresponding dephasing width is uniquely defined. We show that the continuous observation of one of the states in a coherent superposition may accelerate decay from this state-in contradiction with rapidly repeated observations, which slow down the transitions between quantum states (the quantum Zeno effect).