Reaching the limit where a single atom couples strongly enough with a single photon to exhibit coherent behavior has been a major focus in quantum optics for much of the past two decades. That strong coupling limit is hard to achieve because photons and atoms interact so weakly in general. But by placing an atom inside a small optical cavity bounded by mirrors that are reflective enough to trap a photon for hundreds or thousands of roundtrips before it escapes, researchers can strengthen the interaction.

According to quantum electrodynamics (QED), the vacuum field of the cavity fluctuates, polarizing the atom that enters it; in return, the induced atomic dipole fluctuates, polarizing the cavity field, an interaction that prompts the exchange of a photon. As a result, an excited atom placed within an optical cavity tuned to the frequency of the atomic transition can repeatedly emit and reabsorb a photon at...

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