Möttönen discovers refrigerator for quantum supercomputers

CAPTION Mikko Möttönen is the head of the Quantum Computing and Devices research group at Aalto University. The quantum-circuit refrigerator has been fully developed in his QCD Labs. In addition, Möttönen works as a part-time research professor of quantum computing at the University of Jyväskylä. Kuan Yen Tan is a postdoctoral researcher in the Quantum Computing and Devices (QCD) Labs at Aalto University. He conducted most of the measurements on the quantum-circuit refrigerator and is the first author of the publication. CREDIT Mikko Raskinen/Aalto University

Quantum physicist Mikko Möttönen and his team at Aalto University have invented a quantum-circuit refrigerator, which can reduce errors in quantum supercomputing.

The global race towards a functioning quantum supercomputer is on. With future quantum supercomputers, we will be able to solve previously impossible problems and develop, for example, complex medicines, fertilizers, or artificial intelligence.

The research results published today in the scientific journal, Nature Communications, suggest how harmful errors in quantum supercomputing can be removed. This is a new twist towards a functioning quantum computer.

Even a quantum supercomputer needs cooling fins

How quantum supercomputers differ from the computers that we use today is that instead of normal bits, they compute with quantum bits, or qubits. The bits being crunched in your laptop are either zeros or ones, whereas a qubit can exist simultaneously in both states. This versatility of qubits is needed for complex computing, but it also makes them sensitive to external perturbations.

Just like ordinary processors, a quantum supercomputer also needs a cooling mechanism. In the future, thousands or even millions of logical qubits may be simultaneously used in computation, and in order to obtain the correct result, every qubit has to be reset in the beginning of the computation. If the qubits are too hot, they cannot be initialized because they are switching between different states too much. This is the problem Möttönen and his group have developed a solution to.

A refrigerator makes quantum devices more reliable

The nanoscale refrigerator developed by the research group at Aalto University solves a massive challenge: with its help, most electrical quantum devices can be initialized quickly. The devices thus become more powerful and reliable.

"I have worked on this gadget for five years and it finally works!" rejoices Kuan Yen Tan, who works as a postdoctoral researcher in Möttönen's group.

Tan cooled down a qubit-like superconducting resonator utilizing the tunneling of single electrons through a two-nanometer-thick insulator. He gave the electrons slightly too little energy from an external voltage source than what is needed for direct tunneling. Therefore, the electron captures the missing energy required for tunneling from the nearby quantum device, and hence the device loses energy and cools down. The cooling can be switched off by adjusting the external voltage to zero. Then, even the energy available from the quantum device is not enough to push the electron through the insulator.

"Our refrigerator keeps quanta in order," Mikko Möttönen sums up. 

Next, the group plans to cool actual quantum bits in addition to resonators. The researchers also want to lower the minimum temperature achievable with the refrigerator and make its on/off switch super fast.