17 June |  Pauline Alvarez

Will diamond enable the scalability and reliability that is necessary to build a Quantum Computer?

That is the $1M question right now. Or should I say $18B as the Market is expected to reach by 2024, according to ResearchAndMarket?

Researchers around the world are investing a lot of time, effort and money to find the answer to this thorny question. Some are placing bets on Superconducting qubits, others are going down the Trapped Ion route, and quite a few are investigating Quantum dots, Topological qubits & Photonic qubits (check out our previous post on Harnessing Quantum for a quick review of each platform)… And let’s not forget Diamond!

Diamond is attracting its fair of attention and has found some strong supporters amongst research groups as well as large companies. Why is that and what is so “hard” about making Diamond qubits? Those are the questions we will go through today.

First things first, what makes a qubit a good qubit?

The challenges that the community is facing to build a quantum computer are manifold. On top of the basic properties that quantum memories must exhibit to simply deserve the name of qubits, i.e. superposition, entanglement and interference (view a short explanation of qubits properties), they should also have a long enough coherence time to be able to run any computation and be easy to fabricate in large numbers, and in a reproducible manner so that we can build systems of multiple qubits and thereby unlock the potential of running complex simulations and modelling that are nowadays impossible, even with the most powerful supercomputers. And if that’s not too much to ask, ideally, we’d want them to require low power to operate as well.

And what’s so interesting about Diamond?

As you can imagine, it is a bit of a headache to find a technology that ticks all the boxes. And so far, none actually does. So no, we don’t really expect Diamond to outshine all currently existing platforms, but it does have a lot to offer and that’s notably thanks to its unique crystal structure.

As a matter of fact, diamond qubits are essentially defects within an almost perfect crystal structure. First of all there are Nitrogen Vacancies, commonly referred to as NV centres, and artificially created by “knocking off” two neighbour Carbon atoms and only substituting one of them with Nitrogen, thereby leaving behind an unpaired electron (as Nitrogen is trivalent and Carbon is tetravalent) and a vacancy. This unpaired electron transfers to the nearby vacancy, and can then be manipulated, i.e. put into an excited state corresponding to level | 1>, or dropped into a lower energy state corresponding to level | 0>, or a superposition of both, therefore forming a qubit to store information and perform logic operations. And then, there are other types of defects, such as the naturally occurring isotopes ¹³C, which can also act as qubits.

Webinar: Key advancements in engineering diamond-based NV centres.

What’s common between all of them and particularly interesting here is that the almost perfect diamond lattice that surrounds those qubits actually provide them with great isolation from external interactions that would otherwise cause them to decohere and lose their quantum states, as they have an irritating tendency to do. The level of isolation, notably associated with the exceptional stiffness of the diamond bond resulting in a very low level of lattice vibration, is such that it enables operation of diamond qubits at room temperature, thereby considerably reducing power consumption in comparison with superconducting qubits or quantum dots which also show good stability but require cooling to cryogenic temperatures.

New records of coherence time have recently been achieved by a team of researchers at Delft University, with entangled qubits retaining their quantum state for up to 10s, thereby placing NV centres as second best qubits for stability, just behind Trapped Ions. But where Trapped Ions require many lasers to operate, the relative simplicity of diamond qubits and their solid-state nature represent a significant advantage for integration into devices.

Finally, NV centres have the very attractive property of being able to couple with light: they can absorb and emit very well-defined wavelengths which is the very principle on which we rely to address those qubits. And as the quantum information is encoded into photons, it opens the exciting opportunity of using existing datacom infrastructures to transport data over long distances.

Seems promising… but how do you get the light out of diamond?

Fair point indeed. The total internal reflection which gives diamonds their shining appearance makes it particularly difficult to extract light from it. And yet, we do need to retrieve the photons from those NV centres as they carry the quantum information. That is the reason why the NV centres need to be very close to the surface of the crystal. One way to form those NV centres consists of doping diamond with nitrogen, by implantation. But it turns out to be very difficult to directly implant Nitrogen just below the surface.

So, at Oxford Instruments, we’ve developed a process to thin down diamond and bring those NV centres closer to the newly formed surface, while keeping them intact. Our RIE process carried out on the Oxford Instruments PlasmaPro 100 Cobra, allows to rapidly etch through the bulk material while ensuring a controlled and residue/contamination free surface, with very low roughness (typically below 3A) to limit noise and optical losses, and oxygen terminated to provide additional protection to the shallow qubits.

To further facilitate light extraction from the NV centres, we also propose solutions to directly nano pattern the diamond surface into photonic crystals, or plasma etch III-V materials (bonded to the diamond substrate) into photonic crystals.

Finally, we do understand that diamond-based quantum technologies, as promising as they may be, are not perfect. And as such, longer-term useful devices are likely to be hybrid structures, incorporating a variety of qubit types, each performing a specific activity for a tailored application.

At Oxford Instruments, we also provide solutions for superconducting qubits and photonic qubits. These include PEALD of NbN and TiN thin films with respective critical temperatures as high as 12.9K and 3K, for interconnects and resonators, PECVD of low loss SiNx for waveguides and Bosch or cryo etch of optical components, with very smooth sidewalls, low etch rates, high selectivity and notch control.

Find out more about our portfolio of Quantum solutions.