Meet Unimon: a new superconducting qubit for quantum computers

Scientists from IQM Quantum Computers, Aalto University and VTT Technical Research Centre of Finland have discovered a superconducting qubit, the unimon, to increase the accuracy of quantum computations. The team has reportedly achieved the first quantum logic gates with unimons at 99.9% fidelity — a milestone on the quest to build commercially useful quantum computers. The research was published in the journal Nature Communications.

Superconducting qubits are among the leading approaches to building useful quantum computers. However, the qubit designs and techniques currently used do not yet provide high enough performance for practical applications. In this noisy intermediate-scale quantum (NISQ) era, the complexity of the implementable quantum computations is mostly limited by errors in single- and two-qubit quantum gates. The quantum computations there need to become more accurate to be useful.

Mikko Möttönen, Joint Professor of Quantum Technology at Aalto University, said the researchers aimed to build quantum computers which deliver an advantage in solving real-world problems, adding that the new superconducting qubit is a significant achievement to build better superconducting quantum computers.

The new superconducting-qubit type, the unimon, unites the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to magnetic noise, and a simple structure consisting of a single Josephson junction in a resonator. The researchers achieved fidelities from 99.8% to 99.9% for 13-nanoseconds-long single-qubit gates on three different unimon qubits. Eric Hyyppä, who is working on his PhD at IQM, said that because of the higher anharmonicity, or non-linearity, the researchers could operate the unimons faster, leading to fewer errors per operation.

To experimentally demonstrate the unimon, the scientists designed and fabricated chips, each of which consisted of three unimon qubits. They used niobium as the superconducting material apart from the Josephson junctions, in which the superconducting leads were fabricated using aluminium. The researchers measured the unimon qubit to have a relatively high anharmonicity while requiring a single Josephson junction without any superinductors, and bearing protection against noise. The geometric inductance of the unimon has the potential for higher predictability and yield than the 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 this well gives plenty of room for optimisation and major breakthroughs. As next steps, we should optimise the design for even higher noise protection and demonstrate two-qubit gates,” Möttönen said.

IQM’s commercial quantum computers still use transmon qubits. With transmons, IQM delivers on-premises quantum computers, and is currently building a 54-qubit quantum computer as part of a co-innovation project with VTT Technical Research Centre of Finland, and IQM-led consortium Q-Exa is also building a 20-qubit quantum computer in Germany, to be integrated to a supercomputer. The new unimon is an alternative qubit that may lead to higher accuracy in quantum computations in the future.

“We aim for further improvements in the design, materials and gate time of the unimon to break the 99.99% fidelity target for useful quantum advantage with noisy systems and efficient quantum error correction,” Möttönen said.

Image caption: Artistic impression of a unimon qubit in a quantum processor. Image credit: Aleksandr Kakinen (Graphic: Business Wire)