Quantum optics with large /spl chi//sup (3)/ nonlinearities

Summary form only given. For applications in quantum optics it is necessary that the quantum noise resulting from spontaneous emission by the atoms be small. The reduction of absorption in these systems effectively reduces the spontaneous emission. The use of quantum coherence effects to reduce quantum noise in atomic systems was first proposed by Dalton, Reid and Walls. An analysis of quantum noise in three level atoms interacting with 2 light fields by Gheri et al. demonstrated that a nonlinear phase shift could be imposed on the probe beam due to the signal. This particular configuration utilised a "ghost transition" where the population in one transition was nearly zero, thus the quantum noise due to spontaneous emission was negligible and the conditions for a good QND measurement were satisfied. This was verified in a recent experiment by Roch et al who using cold trapped atoms and the "ghost transition" scheme, obtained the best QND correlation scheme to date.