With the advent of toy quantum computers, I’ve been less interested in reporting on the developments of new qubit systems. That doesn’t mean I’ve been ignoring them. Instead, i'm seeing that lots of different types of qubits have deficiencies that are likely to lead to their abandonment at some point. Until I see those overcome, I tend to pay less attention.

Researchers are now reporting that they have overcome one of the major drawbacks in a silicon-doped diamond (SiV-) qubit. The qubit is no longer destroyed so easily and can be manipulated in ways that might make it quite flexible.

Qubits based around a defect in a crystal—in this case, caused by the placement of silicon in an otherwise all-carbon crystal—have been around for a while. But the qubit is way too sensitive to tiny vibrations called phonons. Phonons are basically the crystal’s way of moving heat around, so the amount of energy in a phonon is really tiny and hard to get rid of. Qubits that are readily destroyed by phonons are probably not very useful.

So, solving that problem is a pretty big step.

Qubits that can’t tune out

The problem for these SiV- qubits is that every phonon absorbed or emitted by the qubit changes its internal state and causes it lose the quantum information that it was processing. This process is called decoherence.

To understand how the researchers were able to overcome this problem, we need to know a little bit about how it occurs in the first place. The qubit in question—an extra electron from silicon sitting in a diamond—has four basic states. Two are ground states that are separated by the electron’s spin: the electron can be spin up or spin down, and these have slightly different energies.

The other two are excited states, which also have a spin up and spin down separation, but are at a much higher energy than the ground states. For both the excited states and the ground states, the energy difference between the two spin states is small enough that a sound vibration can flip the spin and change the qubit state.

The problem is that the sound vibrations are nearly unavoidable, because they originate from the surrounding atoms. Worse yet, the vibrations will not stop until the crystal reaches absolute zero, which is impossible.

Cooling crystals has, in fact, been the main solution for other materials. Take your diamond and bathe it in liquid helium. That cools it close to absolute zero and increases the length of time before a phonon hits your qubit and destroys it. For SiV-, this trick doesn’t work that well. The qubit is just way too sensitive to vibrations.

That very sensitivity is what researchers have used to prevent sound from killing their qubits.

Sound waves are a form of strain. A sound wave moves by first displacing some atoms, which strains the crystal locally, and then relaxing it. The applied strain sets other atoms in motion, straining them. The strain and relaxation move through the crystal as a wave.

Tuning qubits out

Strain, however, does not have to be a wave, it can also be a constant: just ask anyone who works for a living. To make their qubit immune to strain, the researchers created a tiny little cantilever sitting just above it. The qubit itself sits just below the surface of a diamond crystal.

By bending the cantilever, the researchers could apply a very local strain to the crystal. The induced strain changes the amount of energy required to flip the spin of qubit. Indeed, the researchers found that they could shift it enough that it became difficult for sound waves to disrupt the qubit's spin. In the end, the researchers were able to increase the qubit lifetime (or more precisely, the coherence time) by a factor of six to around 250ns. That is long enough to start thinking about using qubits based on SiV-.

That, however, is only one benefit. The qubits are solid: they sit in a crystal that is unmoving. But they can be controlled using microwaves, sound, and light. That gives scientists a great deal of freedom in how they couple different qubits together.

You can imagine using cantilevers to stop and start qubit interactions. You can also think about tuning qubits so that when they emit photons, you cannot tell which qubit they came from (this is an important resource for optical quantum computing).

All-in-all, this one demonstration has opened up a number of possibilities for SiV- qubits. That said, there is still a lot to do before this type of qubit catches up with its big brothers and sisters. Nitrogen-doped diamonds have gone beyond this and have been used for demonstrations of simple one- and two-qubit systems, as well as grids of independent qubits. Move outside of crystalline materials, and suddenly you have entire computing systems that are available online.

One architecture to rule them all

Given that companies like IBM have committed themselves to quantum computers based on superconducting rings rather than crystals with defects, you might wonder why I’m excited by this?

It’s quite simple. Yes, we have some small and not very useful demonstrators using other materials. But the evidence that they will scale to the size where they start to be useful is limited. Until an approach finally produces a universal quantum computer of useful size, it is unlikely that any single architecture or approach will be totally abandoned.

Nature Communications, 2018, DOI: s41467-018-04340-3 (About DOIs)