Diamonds offer promise for quantum computing breakthrough
Diamonds may offer a breakthrough for quantum computing, according to a study in Nature Photonics. The nitrogen-vacancy (NV) centres in diamonds are said to have excellent characteristics for qubits, or quantum bits. But diamonds are impure, changing the properties of NV centres and making them difficult to locate. To address this problem, scientists grew their own diamonds and created a 3D grid with predominantly single NV centres, meaning data can be stored, read and manipulated with great accuracy.

One of the more exciting things happening in physics at the moment is the development of quantum computers that may actually be able to do something useful. But, as we get closer to that glorious day, one of the big questions remaining is: what type of basic technology should be used as the basis for quantum computers? We know from our experience with classical computers that whatever technology wins is going to dominate for a long time. For instance, even though silicon-based integrated circuits were known to be worse than their germanium-based equivalent, silicon won for engineering reasons. Ever since, other semiconductor materials have remained niche, despite continued development. And they'll likely stay niche until silicon is tapped out.

The decision on silicon is ancient history, and the consequences are set. But with quantum computers, we haven't reached such a decision point yet. As such, a bazillion ideas are competing, and we get to sit on the sidelines and cheer the players on.

One approach, which has been quietly awaiting its next moment, is nitrogen-vacancy centers (NV- centers) in diamond. Although there have been a lot of exciting developments in terms of showing that NV- centers have excellent characteristics for qubits (a unit of quantum information), no one seems to be able to find a practical way to link anything more than a couple of qubits. Those days may be coming to an end, though, according to a recent study in Nature Photonics, the results of which could shift NV- centers off the bench, back onto the playing field.

Before we get to the good stuff, let's go through what an NV- center is and why it makes such excellent qubits. A diamond is supposed to be pure carbon. Each carbon atom is attached to four other carbon atoms. The structure is very rigid, conducts heat very well, and conducts electricity not at all. In that sense, if a qubit were contained in a diamond crystal, it would be fairly well isolated from the surrounding environment, which is exactly what we need to protect the data stored in each qubit.

Diamonds are not pure, though, which is both a blessing and a curse. Their impurities are a blessing because they form the basis for a diamond qubit, and they are a curse because they ruin some of the excellent properties that diamond has.

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Count your blessings

Our qubits are based on nitrogen impurities. Unlike carbon, nitrogen only likes to be connected to three other atoms. So, when nitrogen is surrounded by carbon, say, in a diamond, it buys drinks for three carbons and leaves the fourth hanging.

An electron from that fourth carbon atom is left unloved: its parent carbon atom doesn't want it, and its neighboring nitrogen atom really doesn't want it. As a result, it spends its time hanging around in the void between them, creating a kind of pseudo-atom with its own structure. The electron has spin states that can be manipulated with magnetic fields (these are used to store quantum states). And, to make life even easier for the rest of us, these spin states are connected to energetic states that emit and absorb light.

What does all that mean? Firstly, we can store and read information optically, and we can do it with a high degree of certainty, thus reducing noise. Second, because the information is stored in spin states, we can use magnetic fields to perform operations and to control external influences. Together, that means that information can be stored, read, and manipulated with a high degree of accuracy.

Enumerate your curses

All the properties above make diamond seem like a shoe-in for quantum computing, right? Well, no. Firstly, nitrogen isn't the only impurity found in diamond. And those other impure elements, as well as other isotopes—for instance, an isotope of carbon that has an extra neutron in the nucleus—have different electronic and magnetic properties. This means that any of these other impurities will slightly change the properties of any NV- centers that are nearby.

For instance, a nearby carbon atom with an extra neutron is known to cause a qubit to lose information faster than normal. So all NV- centers are unique because each is surrounded by randomly placed impurities, which all have different effects, some of which are negligible and some not. We love unique people, but qubits should all be cut from the same cloth.

All of that might be overcome if it weren't for the worst problem: no one knows where the damn NV- centers are. Just as other impurities are randomly distributed through the diamond crystal, so too are the nitrogen atoms.

So, for quantum computing purposes, the first task is to find a few NV- centers that seem to be on their own and not too disrupted by other impurities. Researchers can then nominate them as qubits, deposit electrodes around them to apply magnetic fields, and link the qubits optically using lenses. You can see why this won't work for more than a few qubits. Imagine being asked to make a modern computer chip from randomly placed transistors that have no wiring between them.

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Grids of NV- centers

It is this latter problem of finding NV- centers that the current research paper has focused on (the problem of other impurities can be solved by controlling the growth better). The trick that they use involves the following: grow a layer of diamond that has a bit of nitrogen in it. The nitrogen will be randomly located throughout the crystal. The vast majority of nitrogen atoms, however, do not result in NV- centers, because nitrogen and carbon can combine in ways that do not result in an NV-. So at this point the crystal is fairly useless. Then, at regular intervals, the researchers give the nitrogen a series of short, sharp kicks with a bright laser. If the laser is sufficiently intense, it modifies the way the nitrogen is bound to carbon, creating an NV- center.

The nice thing about this is that it requires the simultaneous absorption of many photons, so it only happens at the most intense location of the laser beam. That means that the NV- centers occur very close to the center of the focal point of the laser beam, thus pinpointing where NV- centers form.

The processing also melts the crystal locally and shuffles the atoms about. To repair all the damage to the crystal structure, the researchers cook the diamond gently at high temperature. This allows the carbon atoms to move about and reform their crystal structure.

To demonstrate that their plan worked, the researchers used a grid to place NV- centers in a diamond layer. For each row in the grid, they turned the laser power up. Below a critical laser power, they melted the diamond without creating NV- centers. But, above a critical power, they created single NV- centers.

The researchers now have the ability to create a 3D grid that predominantly has single NV- centers within about 100nm of the grid points. The result isn't perfect: somewhere between 10 and 30 percent of the grid points have more than one NV- center. And, perhaps more critically, the NV- centers don't really have identical properties. It may be possible that they can be tuned using external magnetic fields, but I think that the researchers will still have to select specific grid locations to use as active qubits.

Still, what the researchers have done is easier to deal with than finding and linking up randomly located qubits. It means that one can deposit electrodes in a regular array, and one can imagine lens arrays with optical fibers and switches or integrated waveguides that couple qubits. Yes, the useless grid points—locations where more than one NV- center or the NV- has poor properties—represent a problem, but one that is predictable and can be worked around.

Not only that, quantum computing isn't the only use for NV- centers; they also make excellent magnetic field sensors. These are so small, though, that you could imagine creating an array of them (like the sensor in a digital camera) to image magnetic fields.

Nature Photonics, 2016, DOI: 10.1038/nphoton.2016.234 (About DOIs).