What are the main materials used as platforms for quantum technologies?

In terms of building quantum computers (which is perhaps the application that people think of most readily), superconducting materials are very important. Most of the work so far has been done using aluminium, because it’s easy to make into devices, and you can also get a nice oxide off it, which is required to make the Josephson junctions that turn a superconducting circuit into a qubit.

Other materials are used as hosts for qubits. Semiconductors like silicon have been researched for a long time, and silicon is obviously a very important material for computing. There are different ways that you can use silicon to build quantum devices. You can do it by implanting impurities like phosphorous into the silicon, but you can also make qubits from silicon quantum dots. The ability to make these devices very reproducibly and very accurately in silicon drives a lot of the research into this area.

Diamond is another potentially important host material, primarily because the defects in diamond can have very nice coherent spin properties that you can use as qubits. So you can start to think about engineering those defects within the diamond to be able to make devices.

A fourth area for materials in quantum computing is ion traps. When trapped ions are used as qubits, they’re floating in free space, held in electromagnetic traps, so the materials element is a little bit more distant. Even so, silicon is important for making the chips on which the electrode structures and circuits are deposited to make suspended circuits of trapped ions. Other materials are becoming important in trapped ion quantum computing as well, primarily in photonics. If you want to be able to integrate devices, there’s lots of materials challenges to address.

What other materials are emerging?

For each of the areas of quantum computing that I’ve mentioned, there are new materials coming on the scene. But if you want to think about the number and richness of these new materials, the areas where people are using defects as qubits are particularly important, because there’s a lot of different materials they could use. There are also some very new ideas being explored, such as topological qubits, where the qubit is protected by the topological state of the system.

For quantum sensing, the main development is in superconductors, which are becoming very popular for single-photon detection. For example, niobium nitride is used for single-photon detectors based on superconducting nanowires. But nitrogen vacancy defects in diamond are also being used for quantum sensing applications, and once you start to include quantum communications in the picture, you open up a whole field of photonics-related semiconducting materials. That includes everything from indium gallium nitride, which can be used to make single-photon sources for quantum communications (and potentially quantum computing as well) to materials used for waveguides – if you have materials that are optically transparent, you can start to build complicated optical circuits in them.

How do we get these materials to reach their potential?

On the research front, you find a very wide range of materials being studied for different things. But the field tends to narrow when you start to build technologies, because exploring and developing new materials is hard work, and it can take years before a new material gets refined to the point where you can build devices out of it.

An example might be defects in materials. For quantum technologies, and particularly quantum computing, you want your materials to be extremely quiet. This tends to mean that you need to minimize the number of defects, because they can cause noise that would affect the coherence of your qubit. This challenge encompasses not just the material’s bulk, but also its surface and the interfaces between different materials, which can be particularly difficult to passivate. That’s a recurring topic that is relevant to most materials being used in quantum technologies.

In addition, if you want to fabricate devices, the processes you use have to be very precise, especially if you’re making microscopic devices. You want very pure materials that have very low inhomogeneity so you can make lots of qubits that are all the same.

There are also challenges related to our theoretical understanding of some of these systems. In these systems, modelling needs to go hand-in-hand with materials development to make sure we fully understand all of the different phenomena at play, and indeed to make better materials that will perform these difficult functionalities more effectively.

If you could communicate just one lesson to quantum technologists on the one hand, and to materials experts on the other, what would it be?

It would be the same message to both groups, and it would be, “Talk to each other.” When you have a research community and an industrial community, there can be a tendency for those communities to talk slightly different languages, and they may come from different backgrounds as well. So dialogue is vital, because people have to know what the relevant concepts are, and they have to be able to benefit from the experience of people who are working in the other camp. A lot of people ask us, “Why would we publish in Materials for Quantum Technology rather than in other journals?” and the answer is that we want to have this conversation between the different communities who are working in quantum technologies. We are trying to do something a little bit different.

If you want to have a quantum technologies industry that’s built on solid foundations and can keep growing and developing into the future, there needs to be communication with the research base. But the research base also needs to understand what the important things are to work on, so they can do research that supports both the existing industry and the next generation of companies.

It’s interesting that you mention industry, because until recently the field of quantum technologies has been dominated by physicists in universities and research labs. What are the challenges of entering a more commercially oriented world?

Once you move out of a research lab, things have to work much more reproducibly. In research, we often report on things that work occasionally, because they reveal new science and a new understanding, and that’s enough to be able to publish a paper. It has to be reproducible, of course. We have to check that it can be done again. But it doesn’t necessarily have to work every single time, because you can get variations in the fabrication of devices that are quite difficult to pin down.

These things become very important when you go into industry. The risk profile has to change, and it might be that the yield of a device is not high enough for it to become a product. In small companies, especially, you have to be very focused on bringing a product or products to market. You have to focus on the things that you know work. It’s a very different style of working, I think, particularly in spin-outs, compared with university research labs and other research labs, in the sense that the risk profile changes a lot. And of course with that comes the possibility that, as I said before, the conversation diverges and people don’t talk to each other quite as much as they should. So I think that’s one of the main risks.

What’s your advice for someone wanting to get into areas of quantum technologies that connect with materials science?

Read widely. That’s always good advice for anyone at the beginning of their career. And it is such a broad and exciting field that getting an overview of what’s going on, finding out what excites you the most and discovering which areas you want to work on is something that you should take the opportunity to do early on.

Materials science has a very strong experimental emphasis, but there’s a lot of theoretical work that goes into it. Some people gravitate more towards experiments. Some people gravitate more towards theory. But there’s a huge range of opportunities for early-stage, early-career researchers to find their niche in terms of what they want to do.

• Jason Smith leads the photonic nanomaterials group at the University of Oxford, UK, and is the founder and director of the spin-out Oxford HighQ, which is developing next-generation chemical and nanoparticle sensors. You can hear more from him in the Physics World Weekly podcast.