QUANTUM BEATS REVISITED - A QUANTUM JUMP APPROACH

The term quantum beats refers to a superposed oscillatory behaviour in the light intensity emitted by some suddenly excited atomic systems i n their subsequent decay. In this paper we reinvestigate quantum beats for systems where two upper levels may decay to a common ground state by using the recently developed approach of quantum jumps. The treatm ent turns out to be very simple and intuitive. It gives explicit expre ssions for the time-dependent intensity of the radiating system and al lows us to exhibit several phenomena which so far seem to have been ov erlooked. We show that quantum beats may also occur if only one of the upper levels is initially excited, a superposition is not needed. A p ossible experimental set-up for the verification of this effect is pro posed. It is also shown that in some cases the beat frequency may diff er considerably from the separation deltaomega of the upper levels. Fo r equal Einstein coefficients of the upper levels the beats are shown to be unexpectedly absent for small deltaomega.