An optical switch that allows one photon to control the quantum state of another could yield a quantum computer that offers stability and control. Quantum particles have an odd property that enables them to be in “superposition”; i.e., they can be in two states at the same time. For instance, if a single photon were fired at a barrier with two slits in it, it would pass through both of them. Bits in ordinary computers are represented as either 0 or 1, but a bit made from a quantum particle – a qubit – could represent both 0 and 1 at the same time. For this reason, a string of only 16 qubits could represent 64,000 numbers simultaneously. One difficulty in building quantum computers, however, is that superpositions of states can be very fragile. Any environmental interactions could cause a subatomic particle to snap into just one of its possible states. Although photons are much more resistant to outside influences than subatomic particles, they also are much more difficult to control. “We have long been interested in the question if and how it is possible to make single photons interact with one another, since this would enable many new quantum devices to be used for secure quantum communication, photonic quantum circuits and, ultimately, quantum computers,” said Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT. In the setup for vacuum-induced transparency, an atomic gas sensor is trapped between two high-quality mirrors (top and bottom), forming an optical resonator. The absorption of a probe beam incident onto the atoms from the side can be reduced by the vacuum field of the resonator if it is tuned to an atomic transition. Courtesy of Haruka Tanji, Harvard/MIT. To achieve this, MIT and Harvard University scientists developed an optical switch consisting of a small cluster of cesium atoms suspended between two tiny mirrors in a vacuum cavity. When a photon enters the cavity, it bounces back and forth between the mirrors, delaying its emission on the other side. If another photon has already struck the cesium atoms, then each pass through them delays the second photon even more. Although a delay by a single pass through the atom would be imperceptible, the mirror-lined cavity enabled the scientists to pass the photon many times through the atoms – in their case, 40,000 times. Once it has emerged from the cavity, the second photon has two possible states – delayed or extra-delayed – depending upon whether another photon preceded it. The cavity thus serves as a quantum switch, the fundamental building block of a quantum computer. The work was described in the Sept. 2 issue of Science (doi: 10.1126/science.1208066). Currently, the team says, the extra delay is not quite long enough to distinguish the delayed photons from the extra-delayed ones. “We hope that this work will enable new photonic quantum devices, such as a single-photon transistor, and, ultimately, photonic quantum gates,” Vuletic said. “Together with quantum memories for light and single photons that have already been demonstrated, this could open the way for quantum computing with photons, at least on the level of a few qubits.” The scientists now hope to make the observed effects large enough to enable quantum gates between photons. This would require improved resonators with a smaller mode area, enabling even stronger coupling between a single photon and the atoms, and an increase in the optical depth, Vuletic said.