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.

The electron’s up-or-down spin makes it a natural qubit, and it couples to the surrounding lattice with coherence-preserving weakness (see Physics Today, March 2006, page 16 ). But the spin’s magnetic dipole is so weak that it makes grabbing hold of a photon practically impossible, even in a cavity. The two groups turned to a stronger handle—the electron’s charge, whose electric dipole interaction with the photon’s electric field is five orders of magnitude larger than the spin’s magnetic dipole interaction with the photon’s magnetic field. By adding a magnetic gradient across the double quantum dot, the groups hybridized the electron’s spin and charge, which enabled the spin degree of freedom to strongly couple to the photon’s electric field.

At the moment, spin qubits communicate with each other only when their wavefunctions overlap. Both groups envision an eventual quantum computer in which an array of qubits embedded in a silicon chip can communicate, become entangled, and perform operations through a photon intermediary. (N. Samkharadze et al., Science, in press; X. Mi et al., Nature, in press.)