Even with the world’s largest super computers, there are limitations to what we can ever hope to be able to calculate. This may sound odd, as computers become more and more advanced and are experienced as faster and faster. But there are several problems, specifically from the world of physics, chemistry and medicine, which are so complex that something completely different is needed to tackle these problems.

Quantum computers – which are computers utilizing the laws of quantum physics – are turning the tables on what is possible to solve. By utilizing some of the extraordinary properties of quantum mechanics that Niels Bohr and Albert Einstein discussed 100 years ago, it now becomes possible to solve problems that we would never be able to solve with an ordinary computer.

If the quantum computer is so powerful, why did we not build it yet? To achieve the quantum mechanical advantages you have to shield your quantum computer from even the slightest disruption or noise from the surroundings that will otherwise destroy the delicate dance that the quantum particles are performing. So today the race to build a large quantum computer is largely concentrated on getting the quantum particles to conduct a series of steps, so-called ’gates’, without being interrupted.

To exploit quantum mechanics to its fullest, it is important that all the qubits are moving in coordination, i.e. that they are ‘dancing to the same tune’ as one may put it. If just a single qubit is not feeling the rhythm, it can cause a domino effect on the other qubits, and soon all the bits have no sense of coordination. The enhancement of this `sense of rhythm’ and the type of complex ‘dance moves’ the qubits can perform is currently one of the most active fields in quantum computing research.

Development of the ’diamond gate’ increases complexity and minimizes noise

Researchers from Aarhus University and Massachusetts Institute of Technology (MIT) in the USA have developed a new gate, the ’diamond gate’, that increases the complexity of this quantum mechanical dance without the noise causing problems. Previously, to stay in the ’dance’ analogy, you could only do a couples dance without stepping over each other’s feet. However, now the researchers have added a four-person-dance to the repertoire. It is easy to imagine that this creates an opportunity for much more complicated dance shows, which in computer language means larger, more powerful and complex calculations.

”With the type of quantum computer that we have today, each individual dance step is incredibly valuable, so we need as much power as possible in each single movement. Our new gate really contributes with an increased complexity, in just one single step”, says Associate Professor Morten Kjærgaard from Massachusetts Institute of Technology (MIT) in the USA.

The new ’gate’ consists of four superconducting coupled qubits arranged in a diamond-shaped architecture, hence its name, the ’diamond gate’. The new thing about the diamond gate is that it manipulates all four qubits simultaneously; until now, one would typically only manipulate one or two at a time.

The study is a result of a collaboration between researchers from Aarhus University and Massachusetts Institute of Technology (MIT) in the USA. The study is led by Associate Professor Nikolaj T. Zinner from the Department of Physics and Astronomy and deputy director at the Aarhus Institute of Advanced Studies (AIAS) at Aarhus University, and has been published in the scientific journal npj Quantum Information.

One step closer to an application of the quantum computer

The first areas to find practical application of the quantum computer will presumably be in the chemical and medical industries. Here they can use the so-called ’variational quantum algorithms’ to understand the structure of molecules. This is exactly the topic of on-going research conducted in Nikolaj T. Zinner’s research group.

The first results that have not yet been peer-reviewed indicate that the new diamond gate can be applied in these algorithms, and that the results can compete with the other gates currently available on the marked. The next step will therefore be to build a ‘physical’ computer chip, which implements the diamond gate and demonstrates its capabilities in practice.

The scientific articles

Scientific article about the new gate, the ’diamond gate’:

N. J. S. Loft, M. Kjaergaard, L. B. Kristensen, C. K. Andersen, T. W. Larsen, S. Gustavsson, W. D. Oliver, N. T. Zinner: ”Quantum interference device for controlled two-qubit operations”, npj Quantum Information 6:47 (2020).
Link: https://www.nature.com/articles/s41534-020-0275-3.pdf

Preprint of a scientific article discussing how certain quantum algorithms can be improved, e.g. with the application of the new gate:

S. E. Rasmussen, N. J. S. Loft, T. Bækkegaard, M. Kues, N. T. Zinner:  ”Single-qubit rotations in parameterized quantum circuits”, arXiv pre-print 2005:13548 (2020).
Link: https://arxiv.org/pdf/2005.13548.pdf

Popular scientific article

Popular scientific article discussing the opportunities and limitations of current quantum computers (in Danish only):

N. J. S. Loft, E. Bahnsen, N. T. Zinner: “Hvad kan vi regne på en kvantecomputer i dag?”, Aktuel Naturvidenskab 2 (2020).
Links: https://aktuelnaturvidenskab.dk/find-artikel/nyeste-numre/2-2020/kvantecomputer/ og http://reader.livedition.dk/aarhusuniversitet/2263/html5

Contact

Associate Professor Nikolaj Thomas Zinner, AIAS deputy director
zinner@aias.au.dk

Aarhus University
Department of Physics and Astronomy and
Aarhus Institute of Advanced Studies, AIAS
Høegh-Guldbergs Gade 6B 
DK-8000 Aarhus C