If the development of a quantum computer were like motor racing, then we would currently be in the twisty-turny bit that comes before we barrel over the mountain and hit the long, fast straightaway. We know the requirements for quantum computing; we even know systems that kinda-sorta meet these requirements. But no existing quantum computing architecture—that is, how we make quantum bits (qubits) and perform operations on them—is really all that satisfying. If you don't even know which materials are best for building a quantum computer, it makes progress awfully slow.

As a result, a lot of researchers have moved away from constructing proof-of-principle demonstrations of quantum computing, and are now trying to create clever ways to make qubits better behaved. A pair of papers look into the prospects for using impure diamonds as an architecture for quantum computing

Colored diamonds are just better

How do you make a qubit from impure diamond? The electrons in carbon are arranged so that four of its electrons wish they had the company of four other electrons—hence, in diamond, each carbon atom has four carbon atom friends. If we add a nitrogen atom into the mix—making the diamond yellow in color—then there is a problem. Nitrogen has one extra electron, meaning that it only wants an extra three electrons hanging around. The result is a gap in the crystal structure and a spare electron sitting in the gap, looking all sad and dejected.

The nice thing about this electron is that it is protected from the environment by the environment. Diamond is so hard that the motion of the nuclei in the crystal is very restricted. The lack of motion keeps the electromagnetic environment relatively constant in the vicinity of the lone electron. This gives it a wonderfully well defined set of energy levels. It stays in these levels for a relatively long time and has long coherence times.

Even better, the energy levels can be manipulated by a combination of microwave and laser light pulses. This lets qubit states be initialized and read out by sending in and receiving visible light pulses, for which the detection efficiency is high. So, these electrons seem to be the near-perfect qubit.

But wait, it gets better. The electron spin state couples to that of the nearby nitrogen atom's nucleus. This provides the equivalent of a built in memory. The electron's states last a long time compared to most electron states, but they are still too short to make a good memory. The spin states of the nucleus, on the other hand, last for a very long time—MRI relies on long lasting nuclear states to function. If you could transfer the quantum state of the electron to the nucleus, you would have a great memory for a qubit.

And now for the bad news

Unfortunately, all is not rosy. The biggest problem is that it is difficult to make diamonds of good quality while still having just the right number of impurities. You want many nitrogen atoms so that you have many qubits, but you want them far enough apart that you can play with them individually. Optically, this means something like 500nm separation between nitrogen atoms. For the microwave pulses... well, we would be talking about one of the world's largest diamonds.

The second problem was the lack of a really efficient scheme to transfer qubit states from the electron in the vacancy to the nearby nucleus and vise versa. All in all, despite all the excellent properties of diamond qubits, it has been looking just a little too difficult to do quantum computing with them.

Oh, my hero *flutter*

So it turns out that one of the nice things about nitrogen vacancies—their electronic state can be manipulated by visible light—also provides a way to switch some vacancies off. The idea is that a magnetic field is used to split the ground state into two; these two states make up our qubit, and above them are several excited states. Radio frequency pulses can be used to directly manipulate the qubit state by changing the probability of finding it in either of the two ground states.

Alongside the electronic states that we use to set and read the qubit state, there are the excited states, which just sort of hang around. When the qubit is put into one of those states, it stays there for quite a while, and while it's there, it is immune to the laser pulses flying around. And this is what this latest trick makes use of.

The research group that happened to have developed the STED microscope has now turned its lasers on diamonds. They recognized that, if one of the lasers is chosen such that its color will put the nitrogen vacancy into one of the long-lasting invisible states, then nitrogen vacancies outside the beam can be manipulated while those inside are immune.

The beam they used (just as in STED) has a profile that looks like a donut where, the stronger the laser beam, the smaller the hole in the middle becomes. This means that you can choose a laser power such that all the nitrogen vacancies but one (the one that happens to be sitting right at the center of the hole) are switched off. This is exactly what the researchers demonstrated: they could read and write (optically) to just a single qubit, even though many were being hit by the laser doing the reading and writing.

This has been in the cards for at least two years, and I was kind of surprised it took them this long to publish a paper with these results. But it was worth the wait. Why? Because, in a follow up experiment, the researchers also showed that sticking the qubit into this long lasting invisible state preserved its original quantum state. The original state was maintained even in the presence of microwave pulses designed to manipulate the qubit state.

In short, they can address and manipulate a single bit without erasing its neighbors, even in diamonds that are densely packed with nitrogen impurities.

That is the important part of the paper. It's one thing to be able to read and set individual qubits, but it's a different thing entirely to be able to do so in a manner that would allow you to do actual computing. This hints that it should be possible to use light beams to pick out individual nitrogen vacancies and play with them at will with microwave beams.

Would you like to store that quantum state, Madame?

So, you have a nitrogen nucleus that has lovely long coherence times: the perfect memory. But, if you use a direct method to transfer the electron's quantum state to the nuclear state, you have to use a set of radio frequency pulses that last as long as the storage time. That didn't seem all that useful, but a second bit of research shows how you can avoid this problem.

The solution lies in the way the qubit is set up—it is a single ground state that has been split up by a magnetic field. This magnetic field also splits up the nuclear spin states to provide the storage states, but the energy difference between the two electronic states and two nuclear states are very different. This is what makes the transfer time so long. But if you choose your field correctly (in the right circumstances), the split between the energy states will be identical, allowing a quantum state to be pushed from the electronic state to the nuclear state and back again, all with only short pulses of light.

Combined, the two papers create a recipe for quantum memory. The recipe, if you will, is to apply a magnetic field to get large splitting for a nice, clean qubit system. Set your state with a laser pulse. Manipulate the state with microwave fields. Then ramp the magnetic field to a new value and apply a radio frequency pulse to push the state onto the nucleus. Ramp the magnetic field further along to keep the stored state isolated. To recover the qubit, you reverse the process.

All of that is pretty cool.

Physical Review Letters, 2011, DOI: 10.1103/PhysRevLett.107.017601

Nature Physics, 2011, DOI: 10.1038/NPHYS2026