Silicon-based quantum dots chart path to scalable quantum computation

Ordinarily, an atom barely notices the presence of light. But when it’s placed in a highly reflective optical cavity with a trapped photon, cavity quantum electrodynamics (QED) strengthens the interaction so much that a quantum of energy can be coherently exchanged between the two. In the past decade, modern nanofabrication techniques have made that strong coupling regime accessible to mesoscopic structures as well. Superconducting qubits and semiconducting dots behave like two-level systems that can be manipulated with microwaves from a transmission-line resonator in what’s 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 group led by Jason Petta of Princeton University and one led by Lieven Vandersypen of Delft University of Technology have now independently used the circuit-QED approach to reach the strong coupling limit between a single microwave photon and the spin of a single electron in a double quantum dot made of silicon. The figure shows the Delft implementation, with a square cavity resonator (left) and double dots (circled in white) attached to electrodes (red and purple) and other electrical gates.