Abstract

Recently a new experimental scheme for a quantum nondemolition measurement of the number of photons contained in a high-Q microwave cavity has been discussed by Brune et al.^[1] They proposed to use a beam of Rydberg atoms crossing two separated nearly resonant Ramsey fields before and after passing through the cavity. Inside the cavity a nonresonant dynamical Stark shift proportional to the photon number is exerted on one of the Rydberg levels and causes a phase shift in the atomic dipole oscillation. This shift can be detected with the help of the Ramsey technique by measuring the energy of the outgoing atoms, thereby finding them either in the upper or in the lower level of the dipole transition. By a numerical simulation of the measuring process Brune et al found that the initial photon distribution collapses into a Fock state after detecting about 20 atoms, provided the atomic beam has a certain velocity spread and the velocity of each atom is detected in addition to its energy. By means of analytically studying the decay of the nondiagonal elements of the density matrix Paul^[2] gave an estimate of the minimum number of atoms N_min needed for a complete measurement and determined the optimum experimental parameters.

© 1992 IQEC