**Researchers at Chalmers University of Technology in Sweden have demonstrated that small quantum computers can be used to solve problems with real applications.**

In 2019, Google researchers announced that they had reached “quantum supremacy”: a quantum computer solving a task which would be practically impossible for a classical computer. The quantum computer, nicknamed ‘Sycamore’, had 54 qubits, of which 53 were functional.

However, the task solved was of no practical use and was selected for the purposes of putting the quantum computer to the test. An important challenge in quantum challenge is now to find useful problems which can exploit the supremacy of small quantum computers.

“We want to be sure that the quantum computer we are developing can help solve relevant problems early on. Therefore, we work in close collaboration with industrial companies,” said Professor Giulia Ferrini, one of the leaders of the university’s quantum computer project.

The researchers have demonstrated that a quantum computer can contribute towards solving a real logistics problem in the aviation industry. All airlines face scheduling challenges, such as assigning individual aircraft to different routes. This is an optimisation problem – which grows rapidly in complexity as the number of possible routes and aircraft increases – and is thus a good candidate to be solved with a Quantum Approximate Optimisation Algorithm (QAOA).

Ferrini and her colleagues executed the algorithm on a quantum processor with just two qubits, demonstrating that it can solve the aircraft optimisation problem.

“We have shown that we have the ability to map relevant problems onto our quantum processor,” said Professor Jonas Bylander, who was responsible for the experimental design. “We still have a small number of qubits, but they work well. Our plan has been to first make everything work very well on a small scale, before scaling up.”

The theorists on the team simulated solving the same problem for up to 278 aircraft: in practice, this would require a quantum computer with at least 25 qubits. “The results remained good as we scaled up. This suggests that the QAOA algorithm has the potential to solve this type of problem at even larger scales,” said Ferrini.

Surpassing the most powerful classical computers used today will require more complex quantum computers with more qubits, which must be carefully handled in order to prevent decoherence. The Chalmers University team have started scaling up and are now working with five qubits; they hope to reach at least 20 qubits by 2021 while maintaining quality.

The project is part of the Wallenberg Centre for Quantum Technology, an ambitious 12-year project to develop Swedish expertise in quantum technology and to build a quantum computer with at least 100 qubits.