H&C News recently met with Dr Sebastian Weidt, Senior Scientist in the Ion Quantum Technology Group at the University of Sussex, to discuss Quantum Computers and their ability to revolutionise our lives.

##### Dr Sebastian Weidt

Senior Scientist, Department of Physics and Astronomy, University of Sussex

Dr Weidt has a degree in Physics with Management Studies, a PhD in quantum information technology and has previously worked as a management consultant in Berlin. He is currently working on the development of disruptive quantum technologies such as a trapped-ion based quantum computer, a quantum simulation engine and a portable quantum magnetometer and atomic clock at the University of Sussex.

**Last year the University of Sussex unveiled the first ever blueprint to construct a large-scale trapped-ion quantum computer. Please can you tell us what a quantum computer is?**

A quantum computer is a completely new type of computer and works by using strange effects which follow the rules of quantum mechanics. Only very small things, like atoms, show these effects. For example, atoms can be in two places at once. While a conventional computer uses bits as the carrier for information which can be either 0 or 1, a quantum computer uses qubits for example encoded in individual trapped ions (charged atoms) which can be 0, 1 or both at the same time. These qubits are then ‘linked’ together by another strange quantum effect called ‘entanglement’. This provides the quantum computer with the unimaginable computational power required to solve problems on a reasonable timescale even the most powerful classical supercomputer on earth would take a million years to solve.

At the University of Sussex, we focus on constructing exactly this machine, a machine which can solve some of the biggest computational problems. In order to unlock this incredible computational power, we need to move from current small-scale proof of principle quantum computers to a large-scale device with potentially millions of qubits. Understanding how this can be accomplished is crucial if one is serious about building such a machine. To address this point, we unveiled the world’s first blueprint for the construction of a large-scale trapped ion quantum computer last year. The question of ‘can a large-scale quantum computer really be built?’ is then really shifting to ‘When will the first large-scale quantum computer be built?’.

**How did you become involved with the development of quantum computers at the University of Sussex?**

Ever since I did a PhD in this area I knew the time was right to build a quantum computer. I felt the team at Sussex were just the right one to achieve this. The University of Sussex is incredibly supportive of our work and we have gathered an extremely talented and passionate team on this project. This is such an exciting and fascinating field to be involved in. I’m absolutely thrilled to help tackle the incredibly challenging, yet extremely rewarding, task of building a machine which has the potential to change the world.

**How many quantum computers already exist?**

The development of quantum computers has made incredible advances in recent years and we are now in a position where quantum computers are being developed across the globe. This has led to the development of a number of small-scale proof of principle quantum computers such as the one currently in operation at the University of Sussex. The challenge is now to scale these devices to a system size where they can have significant impact and solve problems far beyond the reach of conventional computers.

**How do you and the team go about developing a quantum computer?**

Our goal is to construct a large-scale quantum computer. As such we very much focus on developing the required technologies that will enable us to achieve exactly that instead of focusing on proof-of-principle small-scale implementations using non-scalable technology. As our quantum platform we use trapped ions which act as our qubits. These have some very useful features. For example, trapped ions can be very well isolated from the noisy environment which has already led to trapped ions currently holding the world record for the lowest quantum logic gate error rate. These gates are required to process information and the lower the error the smaller the number of qubits required for a large-scale device. In addition, our apparatus does not need to be cooled down to near absolute zero (-273 degrees C) unlike many other quantum platforms currently being used such as superconducting qubits. This makes scaling to larger system sizes significantly simpler.

We believe this provides a great foundation to build on and have therefore used this platform to develop a truly scalable architecture which we are now constructing. Our architecture is based on individual stand-alone quantum computer modules which are then connected together using electric fields, providing for a much simpler engineering solution compared to for example module connections based on photons. An additional special feature of our approach is that we apply all of our logic operations to process quantum information using microwave and rf radiation as the control fields instead of lasers. In traditional approaches each qubit based on trapped ions requires its own set of control fields. Using a new method we have developed we expect to be able to implement the required quantum logic operations on millions of qubits using only a handful of microwave and rf radiation fields. This is a major reduction in engineering required to construct a large scale quantum computer.

**What kinds of problems are quantum computers very good at solving?**

Quantum computers have the potential to solve some of humanities most difficult computational problems. Problems that would take even the most powerful supercomputer currently available millions of years to solve. This could provide the ability to develop new medicines and materials, break some of the most widely used encryption protocols – for example, those used to protect our bank details – and unravel some of nature’s deepest secrets. We are however only at the beginning of understanding what this completely new and exciting technology will be able to offer.

**‘Superposition’ and ‘entanglement’ are two of the fundamental ingredients needed for a quantum computer, please can you explain what these are and how they work?**

‘Superposition’ is a strange phenomenon in quantum physics which means things can for example be in two places at the same time. That’s like getting in your car and being able to go forward and backwards at the same time. However, it’s only at the very small scale, the atomic scale, that these effects can be observed. Superposition is what allows a quantum computer to have qubits which can represent 0 and 1 at the same time.

‘Entanglement’ is another strange phenomenon in quantum physics which provides a connection between objects. Imagine flipping two coins. Each time you flip the coins you will get two results which are completely independent of each other. For example, the first coin may read heads and the second one may read tails and the following flip may result in both of them reading heads. If these two coins were entangled then the result of flipping the two coins would be correlated, in other words each time the result of flipping the first coin is heads, the second coin will read heads. Somehow the second coin ‘knows’ what the first one is doing. This very counterintuitive effect ‘links’ qubits together in a quantum computer.

**What has been the team’s biggest achievement to date?**

It is difficult to point to only one single achievement however unveiling the first blueprint for the construction of a large-scale trapped-ion quantum computer was most certainly a major milestone and now provides for a great foundation for this work going forward.

**How long do you think it will take the team to build the computer?**

We already have a small-scale quantum computer at the University of Sussex and expect a steady increase in capability going forward. A large-scale device with millions of qubits is >10 years away however it is important to note that we do not need to wait for a large-scale device becoming available to start unlocking some of the incredible power of a quantum computer. We therefore already work with a range of different companies to identify problems a more near-term device with a modest number of qubits could already solve better than any available conventional computer could.

**Google have recently announced its newest 72-qubit quantum computer, called Bristlecone. How important is this?**

The Bristlecone device is the latest device announced by the team at Google working on quantum computing and marks an exciting step towards ‘quantum supremacy’, the point where a quantum computer will outperform the fastest conventional computer at solving a particular problem. It is however important to note that a large-scale quantum computer consisting of millions of qubits is required to unlock the true power of quantum computers and solve some of the hardest currently known computational problems.

**What do you see as the biggest obstacles of building a quantum computer? **

Constructing a quantum computer is extremely challenging, as such there are still many obstacles to be overcome. Nevertheless, now that we have a blueprint for the construction of a large-scale trapped-ion quantum computer we have a good understanding of the route to take to make this machine a reality. It is now important for us to keep attracting the very best talent and continue raising the required level of funds. We are therefore in the process of preparing a spin-out which will help provide the most suitable framework for us to construct a large-scale quantum computer.

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