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A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an `` atom+measuringapparatus'' system in which the ``meter'' was pointing simultaneously towards two different directions---a ``Schr\"odinger cat. '' The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement.
Brune et al. (Mon,) studied this question.