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

"The problem is, what makes single nuclear spin so stable -- its weak interaction with its surroundings -- also prevents us from directly manipulating it," Lukin says. "How do you control something that can't interact with anything""

You do it gingerly and indirectly, the Harvard physicists report in Science. 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."

Lukin's co-authors on the Science paper are M.V. Gurudev Dutt, Lilian I. Childress, Liang Jiang, Emre Togan, Jeronimo Maze, and Alexander S. Zibrov, all at Harvard; Fedor Jelezko at Universität Stuttgart; and Phillip R. Hemmer of Texas A&M University. Their work was supported by the National Science Foundation, the Army Research Office's Multi University Research Initiative, and the Packard and Hertz Foundations. Work at Stuttgart was additionally supported by the Deutsche Forschungsgemeinschaft and the European Commission.