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Diamond "Spin-Bath" Gives Crystal View of Quantum Ripples

For cutting-edge physics, look to a material that cuts glass



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To the lay ear, the term "spin-bath" may sound like an ordeal fit for dirty laundry, but to a physicist, it is the sound of quantum clarity—a clutch of subatomic particles interacting cleanly enough to reveal quantum fluctuations spreading like ripples on a still pond. At a meeting last week of the American Physical Society, researchers described a test run of the most sensitive spin-bath yet, a type of artificial molecule embedded in a small film of synthetic diamond at room temperature.

The U.S. and Dutch team put the spin-bath in a quantum mixture, or superposition, of two spin states. Spin is the property that makes electrons and other subatomic particles act like tiny bar magnets. Although it comes in two forms, up and down, quantum rules allow a particle to enter a superposition of up and down simultaneously.

The result adds to recent demonstrations of the potential for controlling and detecting spin in diamond. Some researchers envision future devices that run on so-called spintronics, or manipulation of spin, instead of electronics, which uses charge.

In effect, the new study demonstrates diamond's potential as a quantum bit or qubit—the basic building block of proposed quantum computers and telecommunications networks—capable of acting as 0 and 1 at the same time.

Physicist Ronald Hanson of the Delft University of Technology in the Netherlands, working with David Awschalom of the University of California, Santa Barbara, and other colleagues, first replaced two adjacent carbon atoms in a diamond film with a lone nitrogen next to a void, or vacancy. This nitrogen-vacancy (NV) center contained a free electron that could be excited into a superposition like that of an atom.

To create the superposition, the researchers fired a laser into the NV center while imposing a magnetic field across the spin-bath. Then they applied a pulse of radio frequency electromagnetic radiation, which in effect turned the quantum state upside down. Like a coin dropped into a pond, the flipping perturbed the randomly oriented spins found in the nuclei of the surrounding nitrogen atoms, analogous to magnetic resonance imaging.

The interaction gradually drained the superposition from the NV center and left it in a definite up or down state, which the team distinguished by whether the NV center emitted a photon or not. Repeating the procedure revealed the evolving quantum state of the central spin. "You can flip the spin and you can watch the information go into the bath," Awschalom says. "It's a very nice, clean physics question that has been very hard to study experimentally."

Next they measured the superposition's duration, or coherence time, which sets the limit on its ability to interact with other qubits. of the superposition by adding a second pulse on a delay that was an exact reversal of the first pulse. For delays of up to six microseconds, the NV center was able to return to at least a semblance of its original state—indicating the superposition was still intact.

"Even at room temperature these spins seem to live essentially forever from the perspective of other similar qubits," which might make them very good at storing information in a quantum memory, says theoretical physicist Sankar Das Sarma of the University of Maryland, College Park. He says the long coherence time probably results from the diamond sample's relative scarcity of carbon 13 isotopes, which have their own spin that would interfere with the NV center.

The spin-bath finding complements other work presented at the conference. Physicist Mikhail Lukin of Harvard University described recent experiments in which his team detected the spin of a single carbon 13 nucleus by its effects on surrounding NV centers—which he says may offer a type of ultraprecise magnetic resonance imaging of single molecules.

Other researchers were testing ways of placing NV centers in precise locations, which is currently challenging, and transferring superpositions from diamond to light and back, to enable qubits to exchange information.

Awschalom is optimistic about the potential of using diamond for quantum technologies. "What's exciting is how well it works under ambient conditions," he says. "There are no showstoppers yet for using diamond for making rudimentary quantum systems."

Rudimentary may be the key word. Das Sarma notes that linking diamond qubits together into a computer may be much harder than using them to store information, given the trial-and-error process for making NV centers. Right now, he says, "no one seems to have a good idea of how you can have many qubits that you can scale up."

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