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May 31, 2007

Single spinning nuclei in diamond offer a stable quantum computing building block

At room temperature, carbon-13 nuclei in diamond create stable, controllable quantum register.
Surmounting several distinct hurdles to quantum computing, physicists at Harvard University have found that individual carbon-13 atoms in a diamond lattice can be manipulated with extraordinary precision to create stable quantum mechanical memory and a small quantum processor, also known as a quantum register, operating at room temperature. The finding brings the futuristic technology of quantum information systems into the realm of solid-state materials under ordinary conditions.

They found that nuclear spins associated with single atoms of carbon-13 -- which make up some 1.1 percent of natural diamond -- can be manipulated via a nearby single electron whose own spin can be controlled with optical and microwave radiation. The excitation of an electron by focusing laser light on a nitrogen vacancy center, a stable defect in a diamond lattice where nitrogen replaces an atom of carbon and develops an electronic spin in its ground state, causes the single electron's spin to act as a very sensitive magnetic probe with extraordinary spatial resolution.

Using the nitrogen center as an intermediary, a single carbon-13 atom's nuclear spin is cooled to near absolute zero, creating in the process a single, isolated quantum bit with a coherence time that approaches seconds. The controlled interaction between the electron and nuclear spins allows the latter to be used as very robust quantum memory.

The Harvard physicists also observed and manipulated coupling between individual nuclear spins, thus demonstrating a way to increase the number of qubits working in the quantum register. Because the electron spin and nuclear spin are controlled independently, the experiments lay the groundwork for development of larger, scalable systems in which such quantum registers are connected via optical photons.

"Beyond specific applications in quantum information science," the authors write, "our measurements show that the electron spin can be used as a sensitive local magnetic probe that allows for a remarkable degree of control over individual nuclear spins."

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