Until fairly recently, the list of materials with which we might build a quantum computer have been notable because that list has one big exception: silicon. Silicon is, without doubt, an awesome material. Every semiconductor company in the world knows how to build stuff using it. Fabrication processes are so precise that features of just 50 atoms across are possible.

With these advantages, pretty much any time someone makes a new device, the first comment is: well that's very pretty and all, but can you do it in CMOS? CMOS is a silicon-based complementary metal oxide semiconductor, the industry-standard process. If the answer is no, then, unless the product is world-changing (think light emitting diodes and laser diodes), industry interest evaporates faster than spilled vodka.

Now, if a recent theoretical paper is correct, silicon-based quantum computing may be on the verge of making the leap from not-even-on-the-list to technology-to-beat, thanks to a clever new way of thinking about qubit structures.

Why does silicon suck?

In silicon, a quantum bit or qubit is often based around a phosphorous atom. The phosphorous atoms are added at random during the growth of a silicon crystal, and these interlopers happily replace a silicon atom. But they don't quite fit, as the phosphorous has one too many electrons. That electron hangs around in the gap between atoms. At low temperatures, the electron is confined to that gap and, thanks to the confinement, behaves like it's the only electron attached to an atom.

That means that you can use a pair of quantum states of the electron as the one and zero values for a qubit. You can use microwave and laser light to manipulate and read out the state of the qubit. If this sounds familiar, it should be, because it looks a lot like nitrogen vacancy centers in diamond. It all sounds pretty convenient, so why doesn't it work?

The downfall of silicon is that to make a quantum computer, you need to couple qubits together. The easiest way to do that is to place a couple of phosphorous atoms reasonably close to each other. Then the two qubits will interact naturally via the fields that they produce, plus whatever external fields we apply.

Unfortunately, for phosphorous in silicon, the coupling strength between two qubits varies hugely depending on the relative spacing. This is not just the natural fall-off in coupling strength as you move two charges apart (meaning it's not inverse-square law dependence). No, shifting the phosphorous atom by one atomic lattice position (so way less than 1nm) can change the coupling strength by a factor of 10. Put simply, there is no way, currently, to ensure that the phosphorous atoms are located accurately enough to build a reliable network of qubits.

Show silicon the love

The phosphorous atom in silicon has another attractive feature, though. It can also store quantum information in its nuclear state. (Again, like nitrogen vacancy centers in diamond.) Nuclear qubits are often thought of as memory. They are relatively well-shielded from the environment, so the qubit can maintain its state for a long period of time—exactly what you want in a memory.

It turns out that this memory capability may be the saviour of the phosphorous qubit, making silicon-based quantum computing much easier to work with. The researchers behind the new work have suggested using both the electron and nuclear state to encode a single qubit. This, combined with a bit of technical trickery, both protects the qubit from the environment and allows the user to control when qubits talk to each other.

Unfortunately, the explanation is a bit technical. I wanted to avoid this, but we are going to have to discuss spin. Spin is the intrinsic angular momentum of a particle like an electron. No matter what we do in physics, the total angular momentum has to remain the same. In our qubit, the electron and the nucleus both have spin. The only option we have to change the qubit state is through photons. And photons also have angular momentum.

That angular momentum also must be conserved. So, if an electron absorbs a photon, it doesn't just gain energy—it must also absorb the angular momentum. Likewise, if an electron emits a photon, the angular momentum of the photon plus the angular momentum of the electron must remain the same.

So, let's imagine that our qubit is defined by the state of the nucleus and the electron. The electron is spin up and the nucleus is spin down for a 1, while exactly the reverse is true for a 0. To change from a one to a zero, the electron has to give up some angular momentum, and the nucleus has to absorb some angular momentum. So, the net change is zero. But absorbing or emitting a photon always involves a change in angular momentum. So, it is impossible to absorb a single photon and change the qubit state.

That means the qubit is insulated from electromagnetic fields. That's great for maintaining its state but lousy for allowing us to control that state.

Distorting reality for real

To make it work as an actual qubit, the researchers need two things: one is a way to manipulate the qubit state and the other is to link it to neighboring qubits. Both of these can be accomplished by applying a strong electric field.

For this to work, the phosphorous atom that forms the basis of the qubit should be located close to the surface of the silicon. An electrode is placed on top of the silicon over the phosphorous atom. By applying a voltage, the electron is pulled away from the atom a bit. By shifting the electron closer to the surface, the nucleus and the electron create their own electric field in response, called a dipole. This dipole field will affect other qubits, provided that they also have their own dipole. Even better, the dipole also opens the qubit up to having its state changed by a burst of microwave radiation.

Wait, you say, that still involves the absorption or emission of a photon—what happens to the angular momentum? Well, the key is that the qubit has to absorb and emit a photon (or absorb two photons with opposite angular momentum). The dipole is a little antenna that allows both of those to happen at the same time. In the absence of a dipole, it is simply not possible to emit or absorb radiation.

A different way of looking at it is to say that the qubit absorbs a photon from the microwave field and emits a photon into the applied electric field, resulting in no change in angular momentum.

Circuits of qubits

The big advantage is that the field that creates the dipole is under our control. We literally turn on and off the ability of the qubit to communicate with the outside world. Even better, the dipole is just like a little antenna: its strength falls off nice and smoothly, so the precise qubit placement doesn't matter too much.

The researchers think that qubits that are separated by about 150nm can be coupled via this induced dipole. This is really important, because it means that there is enough space between qubits to put in electrodes and other bits and pieces required to control the qubit.

At this stage, though, the idea exists only in the heads of the theorists who wrote the paper. Nevertheless, the concept seems to be pretty robust, and their calculations take all sorts of imperfections into account. So I expect that we will see the first experimental results in a couple of years.

Nature Communications, 2017, DOI: 10.1038/s41467-017-00378-x