Conventional laser cavities contain huge numbers of atoms and photons, each contributing to the radiation field by stimulated emission. The effect of an individual photon on the dynamics of such a system is negligible. But if reduced to its bare essentials, a laser would become a pumped single atom that interacts with a single electromagnetic mode in a cavity. In so minimal a system, quantum fluctuations on the scale of single photons can become dominant and the usually familiar behavior of a laser can become very unfamiliar.

In the mid-1980s, groups led by Herbert Walther (Max Planck Institute for Quantum Optics) and Serge Haroche (Ecole Normale Supérieure) approached that conceptual limit when they built single-atom micromasers based respectively, on one-and two-photon transitions. 1,1 The devices worked using a dilute beam of Rydberg atoms that transit a superconducting cavity a few centimeters in length. In a cavity tuned to the frequency...

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