Today, IQM has introduced a new superconducting-qubit type, the unimon, which unites in a single circuit the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to magnetic noise, and a simple structure consisting only of a single Josephson junction in a resonator. The team achieved fidelities from 99.8% to 99.9% for 13-nanoseconds-long single-qubit gates on three different unimon qubits.
“Because of the higher anharmonicity, or non-linearity, than in transmons, we can operate the unimons faster, leading to fewer errors per operation,” said Eric Hyyppä who is working on his PhD at IQM.
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 aluminum.
“I would like to thank and congratulate Eric and the other team members who worked tirelessly for this major achievement,” said Prof. Möttönen.
The team measured the unimon qubit to have a relatively high anharmonicity while requiring only 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 optimization and major breakthroughs. As next steps, we should optimize the design for even higher noise protection and demonstrate two-qubit gates,” added Prof. Möttönen.
IQM’s commercial quantum computers still use transmon qubits. With transmons, IQM already delivers on-premises quantum computers, for example IQM is building Finland’s first 54-qubit quantum computer as part of a co-innovation project with VTT Technical Research Center of Finland, and an IQM-led consortium Q-Exa is also building a 20-qubit quantum computer in Germany, to be integrated to a supercomputer. The unimon invented now 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. This is a very exciting day for quantum computing!” concluded Prof. Möttönen.