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 single electron into the dot and store it there. The up-or-down spin of the electron makes it a natural qubit that couples to the surrounding crystal lattice with coherence-preserving weakness (see Physics Today, March 2006, page 16).
Two groups—one led by Jason Petta of Princeton University1 and the other led by Lieven Vandersypen of Delft University of Technology2—have now independently demonstrated strong coupling between a single microwave photon and the spin of a single electron placed in a double quantum dot made of silicon. The Vandersypen group’s integrated device is shown in figure 1.

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