Ordinarily, an atom hardly notices the presence of light. But when it’s placed in a highly reflective optical cavity, cavity quantum electrodynamics (QED) strengthens the interaction so much that a single photon can be coherently exchanged between the two.

In the past 15 years, modern fabrication techniques have made that strong coupling regime also accessible to mesoscopic structures. Superconducting qubits and semiconducting quantum dots can be customized to behave like artificial, two-level atoms that interact with microwaves from a transmission-line resonator on the same chip. The approach has been dubbed circuit QED (see Physics Today, November 2004, page 25, and the article by J. Q. You and Franco Nori, November 2005, page 42).

A quantum dot in such a circuit can be configured into the quantum analogue of a transistor. By adjusting the voltage on gate, source, drain, and other electrodes, researchers can controllably pull even a...

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