The research team's discoveries may eventually enable clocks smaller than computer chips yet accurate to within a few quadrillionths of a second, or rotational sensors quicker and more tolerant of extreme temperatures than the gyroscopes in smart phones. Before long, an inexpensive chip of diamond may be able to house a quantum computer. The team reports their results inNature Communications.
Nitrogen vacancy centres are some of the most common defects in diamonds. When a nitrogen atom substitutes for a carbon atom in the diamond crystal and pairs with an adjacent vacancy (where a carbon atom is missing altogether), a number of electrons not bonded to the missing carbon atoms are left in the centre.
The electron spin states are well defined and very sensitive to magnetic fields, electric fields, and light, so they can easily be set, adjusted, and read out by lasers.
"The spin states of NV centers are stable across a wide range of temperatures from very hot to very cold", stated Dmitry Budker of Berkeley Lab's Nuclear Science Division, who is also a physics professor at UC Berkeley. Even tiny flecks of diamond costing pennies per gram could be used as sensors because, said Dmitry Budker, "we can control the number of NV centers in the diamond just by irradiating and baking it", that is, annealing it.
The challenge is to keep the information inherent in the spin states of NV centres, once it has been encoded there, from leaking away before measurements can be performed; in NV centres, this requires extending what's called the "coherence" time of the electron spins, the time the spins remain synchronized with each other.
Recently Dmitry Budker worked with Ronald Walsworth of Harvard in a team that included Harvard's Nir Bar-Gill and UC Berkeley postdoc Andrey Jarmola. They extended the coherence time of an ensemble of NV electron spins by more than two orders of magnitude over previous measurements.
"To me, the most exciting aspect of this result is the possibility of studying changes in the way NV centres interact with one another", stated Nir Bar-Gill, the first author of the paper, who will move to Hebrew University in Jerusalem this fall. "This is possible because the coherence times are much longer than the time needed for interactions between NV centres."
Nir Bar-Gill added: "We can now imagine engineering diamond samples to realize quantum computing architectures." The interacting NV centres take the role of bits in quantum computers, called qubits. Whereas a binary digit is either a 1 or a 0, a qubit represents a 1 and a 0 superposed, a state of Schrödinger's-cat-like simultaneity that persists as long as the states are coherent, until a measurement is made that collapses all the entangled qubits at once.
"We used a couple of tricks to get rid of sources of decoherence", stated Dmitry Budker. "One was to use diamond samples specially prepared to be pure carbon-12." Natural diamond includes a small amount of the isotope carbon-13, whose nuclear spin hurries the decoherence of the NV center electron spins. Carbon-12 nuclei are spin zero.
"The other trick was to lower the temperature to the temperature of liquid nitrogen", Dmitry Budker stated. Decoherence was reduced by cooling the samples to 77 degrees Kelvin, below room temperature but still readily accessible.
Working together in Dmitry Budker's lab, members of the team mounted the diamond samples inside a cryostat. A laser beam passing through the diamond, plus a magnetic field, tuned the electron spins of the NV centres and caused them to fluoresce. Their fluorescent brightness was a measure of spin-state coherence.
"Controlling the spin is essential", Dmitry Budker stated, "so we borrowed an idea from nuclear magnetic resonance" - the basis for such familiar procedures as magnetic resonance imaging (MRI) in hospitals.
While different from nuclear spin, electron spin coherence can be extended with similar techniques. Thus, as the spin states of the NV centres in the diamond sample were about to decohere, the experimenters jolted the diamond with a series of up to 10,000 short microwave pulses. The pulses flipped the electron spins as they began to fall out of synchronization with one another, producing "echoes" in which the reversed spins caught up with themselves. Coherence was re-established.
Eventually the researchers achieved spin coherence times lasting over half a second. "Our results really shine for magnetic field sensing and for quantum information", stated Nir Bar-Gill.
Long spin-coherence times add to the advantages diamond already possesses, putting diamond NVs at the forefront of potential candidates for practical quantum computers - a favourite pursuit of the Harvard researchers. What Dmitry Budker's group finds an even hotter prospect is the potential for long coherence times in sensing oscillating magnetic fields, with applications ranging from biophysics to defense.
"Solid-state electronic spin coherence time approaching one second", written by Nir Bar-Gill, Linh M. Pham, Andrey Jarmola, Dmitry Budker, and Ronald L. Walsworth, appears in the 23 April 2013 edition ofNature Communications, on-line at http://www.nature.com/ncomms/journal/v4/n4/full/ncomms2771.html .
This work was supported by the Defense Advanced Research Projects Agency's QuASAR programme, the National Science Foundation, the Israeli Ministry of Defense, and the North Atlantic Treaty Organization's Science for Peace Programme.