QUANTUM MEASUREMENT AND PURE DEPHASING

The prerequisite of quantum measurement is a transformation of an init ially off-diagonal density matrix rho(m alpha;n beta) describing an in teracting measured object and measuring device into a diagonal density matrix rho(m alpha;m alpha)delta(mn)delta(alpha beta). The latter den sity matrix describes a proper mixture of states having definite m-val ues. On the other hand, the irreversible relaxation (towards the therm odynamic equilibrium) is also characterized by transformation of an in itially off-diagonal matrix into a diagonal one. It has been shown tha t the process of irreversible relaxation can be used to perform quantu m measurement, provided the duration Delta t of the measurement is muc h larger than T-2, the phase relaxation time, and much smaller than T- 1, the population relaxation time: T-2 much less than at Delta t much less than T-1. A gedanken experiment describing this kind of measureme nt is provided. A pi/2-pulse transforms an initial s(z) = -1/2 state i nto superposition of s(z) = +/- 1/2 states. The irreversible relaxatio n leads to the proper mixture of s(z) = 1/2 and s(z) = -1/2 state. Res ults of the measurements are verified by the second electromagnetic pu lse.