Long-lived quantum coherence between macroscopically distinct states in superradiance

The dephasing influence of a dissipative environment reduces linear superpo sitions of macroscopically distinct quantum states (sometimes also called S chrodinger cat states) usually almost immediately to a statistical mixture. This process is called decoherence. Couplings to the environment with a ce rtain symmetry can lead to slow decoherence. In this letter we show that th e collective coupling of a large number of two-level atoms to an electromag netic field mode in a cavity that leads to the phenomena of superradiance h as such a symmetry, at least approximately. We construct superpositions of macroscopically distinct quantum states decohering only on a classical time scale and propose an experiment in which the extraordinarily slow decohere nce should be observable. (C) 2000 Elsevier Science B.V. All rights reserve d.