The quantum state of a single atomic-sized defect in diamond — known as the nitrogen-vacancy (NV) center — can be manipulated with laser light, providing more unified control and versatility than conventional processes. The discovery opens up the possibility of exploring new solid-state quantum systems. The NV center is a defect in the atomic structure of a diamond where one carbon atom in the diamond lattice is replaced by a nitrogen atom, and an adjacent site in the lattice is vacant. The resulting electronic spin around the defect forms a quantum bit (qubit) — the basic unit of a quantum computer. While the NV center in diamond is a promising qubit that has been studied extensively for the past decade, diamonds are challenging to engineer and grow. Now, scientists at the University of California, Santa Barbara have devised a new method for controlling individual qubits in semiconductors using laser pulses. This all-optical methodology may allow for the exploration of quantum systems in other materials that are more technologically mature, said David Awschalom, director of UCSB’s Center for Spintronics & Quantum Computation, professor of physics and of electrical and computer engineering, and the Peter J. Clarke director of the California NanoSystems Institute. An artist’s rendering of all-optical control of an individual electronic spin within a diamond. This spin is associated with a naturally occurring defect in diamond known as the nitrogen-vacancy center, a promising quantum bit for quantum information processing. In their recently published paper, Yale et al. at UCSB develop techniques to initialize, manipulate and read out the electronic spin of this qubit using only pulses of light. Courtesy of Peter Allen. Conventional processes require this qubit be initialized into a well-defined energy state before interfacing with it. Unlike classical computers, where the basic unit of information is either 0 or 1, qubits can be 0, 1 or any mathematical superposition of both, allowing for more complex operations. The investigators initially tried to find a way to place their qubit into any possible superposition of its state in a single step, said graduate student Christopher Yale. “It turns out that in addition to being able to do that just by adjusting the laser light interacting with our spin, we discovered that we could generate coherent rotations of that spin state and read out its state relative to any other state of our choosing using only optical processes.” The all-optical control allows for greater versatility in manipulating the NV center over disparate conventional methods that use microwave fields and exploit defect-specific properties. It also has the potential to be more scalable, said physics graduate student David Christle. “If you have an array of these qubits in order, and if you’re applying conventional microwave fields, it becomes difficult to talk to one of them without talking to the others,” Christle said. “In principle, with our technique in an idealized optical system, you would be able focus the light down onto a single qubit and only talk to it.” From left, physics graduate students Christopher Yale, David Christle, Bob Buckley and F. Joseph Heremans behind an optical table used for their study. Courtesy of UCSB. Although practical quantum computers are still years away, the research opens up new paths toward their eventual creation, the researchers say. They anticipate that these devices will be capable of performing sophisticated calculations and functions more efficiently than today’s computers can — leading to advances in fields as diverse as encryption and quantum simulation. Additional theoretical work was performed at the University of Konstanz in Germany. The study appeared in the Proceedings of the National Academy of the Sciences (doi: 10.1073/pnas.1305920110). For more information, visit: www.ucsb.edu