  The dephasing influence of a dissipative environment reduces linear
superpositions of macroscopically distinct quantum states (sometimes also
called Schr\"odinger cat states) usually almost immediately to a statistical
mixture. This process is called decoherence. Couplings to the environment with
a certain symmetry can lead to slow decoherence. In this Letter we show that
the collective coupling of a large number of two-level atoms to an
electromagnetic field mode in a cavity that leads to the phenomena of
superradiance has 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 decoherence should be observable.
