Quantum Cocoon

Diamond is emerging as one of the most promising materials for building future quantum computers

By Davide Castelvecchi
From Science News, April 5, 2008

Diamond is cool—even at room temperature. The stiff crystalline structure that makes diamond nature’s hardest material can shield an atom from heat vibrations—not forever, but a lot longer than in other materials.

Physicists have now learned to use that ultimate cocoon quality to store and manipulate information in single atoms at room temperature—feats that in other materials require getting to the neighborhood of absolute zero. Because its atoms can store the notoriously peculiar quantum information, diamond has become a candidate material for use in future quantum computers. Such devices would rely on quantum weirdness to perform certain tasks that would take an ordinary computer till the end of time.

Diamond, specifically artificial diamond, could also find more imminent applications, such as communicating data with unbreakable encryption or even advancing the understanding of quantum theory itself. Powering these applications would require just tiny artificial-diamond chips along with inexpensive tools such as simple lasers.

“The beauty of diamond is that it brings all of this physics to a desktop,” says David Awschalom of the University of California, Santa Barbara (UCSB).

Diamonds can be sharp cutters, but from the point of view of ordinary electronics, they are pretty dull, at least in their purest form. Diamond’s crystal lattice of carbon atoms doesn’t conduct electricity and has virtually no magnetism. There’s no such thing as a 100 percent-pure crystal, though, and diamond’s impurities are in fact Marilyn Monroe beauty marks that make it attractive for physics. “It’s the dirt that gives rise to the unusual properties,” Awschalom said during a recent talk in Boston at the annual meeting of the American Association for the Advancement of Science (AAAS).

Nitrogen is the most common impurity in diamond, where it can replace a carbon atom in the crystal. The most useful nitrogen impurities are those that happen to be next to a vacancy—a gap in the crystal where a carbon would otherwise be. Two of the nitrogen’s electrons stretch their orbits into the vacancy and form a moleculelike structure, even though one of the molecule’s atoms is missing. This virtual molecule, called a nitrogen-vacancy (NV) center, possesses spin, the quantum form of magnetism.

Spins are like microscopic bar magnets and can encode and store information by pointing in different directions. A single unit of information, called a bit, can be, say, a 1 if the spin points up or a 0 if it points down.

Spins can also be simultaneously up and down, and in such cases are said to be in special “quantum states.” Quantum states contain quantum bits of information, or qubits. A quantum computer could perform calculations using the multiple states of qubits, which is essentially like doing several calculations at the same time. That might enable it, for example, to search databases or to find prime factors of whole numbers at speeds unattainable with ordinary computers.

But quantum states are notoriously delicate, and even a small disturbance can result in the complete loss of the information stored in a qubit. Researchers have so far managed to store and manipulate only a handful of qubits in superbly well-controlled systems, such as single ions suspended in an electromagnetic trap or superconducting materials cooled to very low temperatures. In a paper to be published in Science, Awschalom and his collaborators describe how they achieved a similar level of control over NV centers in diamond.

Green with NV

In addition to having a spin, NV centers have a unique way of standing out in the limelight. They have a signature response to light, meaning that they will fluoresce with blue or green light when the rest of the material doesn’t. Typically, they are also few and far between—spaced by micrometers—so that they can be spotted individually using an optical microscope and a sensitive light detector.

Jörg Wrachtrup, now at the University of Stuttgart in Germany, and his collaborators first imaged single NV centers in diamond in 1997. The researchers first tried at cold temperatures, where NV centers were supposedly easier to isolate. That didn’t work. But when the researchers let temperatures go up, they were startled to see the NV centers’ light begin to stand out from a noisy background of scattered light.

In their recent experiments, Awschalom and his team explored for the first time the full extent to which they could manipulate the states of NV centers. The researchers zeroed in on a single NV center. They used a laser pulse to kick the NV center’s electrons down to a known, lowest-energy state, readying it to record a qubit. They then tickled its spin gently using microwave radiation. The spin took different mathematical combinations of three simultaneous directions, thereby simultaneously encoding different information, explains Awschalom’s colleague Adrian Feiguin, part of the Microsoft Corp. research team at UCSB. With a second laser pulse, the researchers also made the NV center fluoresce, so they could measure its state at different times, essentially reading out the information.

At the same time, the NV center also felt the presence of other spins nearby, just like several bar magnets will exert magnetic forces on each other when they’re close together. The other impurities were mostly “dark” nitrogen atoms, meaning that they were not fluorescing because they were not paired with vacancies. In principle, all spins in a small region of a solid can influence one another, and the team needed to test how such a web of interactions would affect the information stored in their NV center qubit.

The team expected that in some cases the NV center would quickly lose its quantum weirdness, and go from its multiple states to a well-defined single state, like any macroscopic object. What the researchers found was that the states of the spins surrounding the NV center in a sense determined the richness of the qubit. Tuning the spins with a magnet enabled the spin to encode more or less information. But in all cases, the qubit worked, keeping the information safe.

According to David DiVincenzo of IBM’s T.J. Watson Research Center in Yorktown Heights, N.Y., Awschalom and colleagues “demonstrate a very high degree of control” over the quantum states of NV centers, comparable to what’s been done with ion traps, the state of the art in quantum information.

But the experiment also has broader implications, says Mikhail Lukin of Harvard University. It shows that “diamond qubits can now be used as a test bed for probing fundamental physics.” To physicists, interacting spins are almost an emblem of complexity. Simulations can predict how a few dozen spins will flip each other back and forth, and theories describe the statistical behavior of huge numbers of atoms in macroscopic chunks of a magnetic material. But experimentally, no one has been able to see what happens to a single iron atom in, say, the magnet inside a loudspeaker while music plays. Diamond provides a rare opportunity to see how a single spin interacts with its neighbors, Awschalom says.

This won’t hurt a qubit

Complete control over the states of a qubit is one step toward making diamond viable for quantum computing, physicists say. That path will be long, but encouraging steps have already been made.

Among the most significant was the realization that diamond can keep quantum states undisturbed at room temperature. For example, the spin states of NV centers can last up to a millisecond, Awschalom says, which in the quantum world is an eternity. In one millisecond, a quantum computer would be able to perform thousands of calculations, each involving multiple states at once.

Earlier this decade, a team led by Thomas Kennedy of the Naval Research Laboratory in Washington, D.C., was the first to manipulate a single NV center within diamond, alerting the quantum-computing community to diamond’s potential.

In more recent years, teams led by Awschalom and Wrachtrup performed the first quantum logic operations between two diamond qubits. Logic operations—calculations on bits—are the building blocks of any information processing. In a typical logic operation, a bit can be flipped (from 0 to 1 or vice versa) if a second bit is set to 1, or be left alone if the second bit is set to 0. The two teams performed this simple operation on an NV center using a nearby nitrogen impurity as the second bit. More precisely, they did a more complex version of the operation, involving the quantum states of the two qubits. In the process, the two qubits became a single unit of information by taking up a shared quantum state, which physicists call an entangled state.

Last year, Lukin and his collaborators showed how a single NV center could essentially write information into the nuclei of nearby carbon atoms. While the most common isotope of carbon, carbon-12, has virtually no magnetism, about 1 percent of the carbon in nature is carbon-13. That isotope’s extra neutron endows its nucleus with a spin. A carbon-13 atom’s magnetism is much weaker than that of a nitrogen atom. But Lukin and his team used purified-diamond crystals that had low concentrations of nitrogen, so that the carbon-13 spins would stand out. That way, the researchers could use the NV centers to control the quantum states of several carbon-13 atoms at once, the quantum equivalent of storing information in ordinary RAM.

At a meeting of the American Physical Society in New Orleans in March, Lukin said that carbon-13 nuclei might keep information safe for much longer than even NV centers do, perhaps even for several seconds.

Remote entanglement

Entangling a few qubits is a good step, but a practical quantum computer will need to have dozens or even hundreds of them. With diamond, no one has been able to do that yet; the current record for any type of entangled qubits is eight trapped ions.

A goal more nearly within reach is to entangle two diamond qubits at a distance. Remote entanglement is a crucial requirement for quantum networking, in which a sender and a receiver would share a secret encryption key using sequences of entangled qubits. Any eavesdropper trying to steal the key would destroy the entanglement, and that would let the two legitimate parties know that their communication channel was tapped.

Entangling two diamond qubits is easy in principle, says Lukin. When an NV center emits a photon by fluorescence, and that photon happens to hit another NV center, the two qubits will become entangled. Trouble is, fluorescence photons tend to fly off in random directions. The trick is to somehow guide the photon from one qubit to the other. Lukin, Awschalom, and others are trying various approaches, which they say should soon enable them to entangle pairs of NV centers.

Lukin’s approach, described in the Nov. 15, 2007 Nature, is to turn the photon into a signal traveling on the surface of a metallic nanowire. That would be enough to entangle qubits within the same chip. Awschalom’s team is working on a different technique, described in the Nov. 12, 2007 Applied Physics Letters, in which the qubit is kept inside a tiny cavity. Essentially a hall of mirrors, the cavity traps fluorescence photons of a specific wavelength. By exchanging these photons, two qubits inside the same cavity would then become the optical equivalent of strings vibrating in resonance. Or, an optical fiber could collect photons from the cavity and take them to another destination, possibly far away.

As nano as it gets

Meanwhile, other kinds of impurities will bring more options to the menu. Several labs, including Steven Prawer’s at the University of Melbourne in Australia, are creating designer impurities by shooting atoms or molecules into diamond crystals one at a time. At the recent AAAS meeting, Prawer said that nickel-vacancy centers are especially promising for quantum satellite communication, since they fluoresce with infrared photons that can get through even a cloudy sky.

Atoms sit at the extreme edge of nanotechnology, being themselves much smaller than a nanometer. “That’s about as nano as you’re going to get,” as Awschalom puts it. Computing will probably get to atomic scales eventually, but it’s hard to predict in what form—be it diamond, ion traps, or other candidates. “It’s dangerous to say which technology is more promising,” Awschalom says.

Diamond’s advantage is that it could do logic, storage, and communications on the same chip. But perhaps different technologies will find different applications, Awschalom says.

Kennedy, who has since switched to another candidate technology called quantum dots, agrees. “You have a healthy competition,” he says. “And it’s likely to remain that way for a while.”

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