The unimon, a new qubit to boost quantum computers for useful applications

A group of scientists from Aalto University, IQM Quantum Computers and VTT Technical Research Center have discovered a new superconducting qubit, the unimon, to increase the precision of quantum calculations. The team achieved the first quantum logic gates with 99.9% fidelity unimons, a major step in the quest to build commercially useful quantum computers. This research has just been published in the journal Nature Communication.

Of all the different approaches to building useful quantum computers, superconducting qubits come out on top. However, currently used qubit designs and techniques do not yet provide high enough performance for practical applications. In this noisy intermediate-scale quantum (NISQ) era, the complexity of implementable quantum computations is mostly limited by errors in the one- and two-qubit quantum gates. Quantum calculations must become more precise to be useful.

“Our goal is to build quantum computers that provide an edge in solving real-world problems. Our announcement today is a milestone for IQM and a significant achievement in building better superconducting quantum computers,” said Professor Mikko Möttönen, co-professor of Quantum Technology at Aalto University and VTT, as well as a co-founder and chief scientist at IQM Quantum Computers, who led the research.

Today, Aalto, IQM and VTT have introduced a new type of superconducting qubit, the unimon, which brings together in a single circuit the desired properties of increased anharmonicity, total insensitivity to DC load noise, reduced to magnetic noise and a simple structure consisting of only a single Josephson junction in a resonator. The team achieved fidelities of 99.8% to 99.9% for 13-nanosecond single-qubit gates on three different unimon qubits.

“Due to the higher anharmonicity or nonlinearity than in transmons, we can run unimons faster, resulting in fewer errors per operation,” said Eric Hyyppä, who is working on his Ph.D. . at the IQM.

To experimentally demonstrate unimon, scientists designed and fabricated chips, each consisting of three unimon qubits. They used niobium as the superconducting material except for the Josephson junctions, in which the superconducting conductors were made using aluminum.

The team measured that the unimon qubit had relatively high anharmonicity while requiring only a single Josephson junction with no superinductors and wearing noise shielding. Geometric unimon inductance has the potential for higher predictability and efficiency than junction array-based superinductors in conventional fluxonium or quarton qubits.

“Unimons are so simple and yet have many advantages over transmons. The fact that the very first unimon ever made worked so well leaves a lot of room for optimization and major advancements. In the next steps, we should optimize the design for even higher noise shielding and demonstrate two-qubit gates,” Prof. Möttönen added.

“We are aiming for further improvements in unimon’s design, materials and gate time to exceed the 99.99% fidelity goal for a useful quantum advantage with noisy systems and efficient quantum error correction. It’s a very exciting day for quantum computing,” Prof. Möttonen.