A team of researchers from Aalto University, IQM Quantum Computers, and VTT Technical Research Center have developed a new superconducting qubit, the unimon, to improve the precision of quantum computing. The team has made the first quantum logic gates that work 99.9% of the time by using unimons.
This is a big step toward making quantum computers that are commercially viable.
Superconducting qubits are currently the best option for building practical quantum computers among all the available options. However, the qubit designs and methods currently in use don’t offer a high enough level of performance for real-world uses.
The complexity of implementable quantum computations in this noisy intermediate-scale quantum (NISQ) era is mostly limited by errors in single- and two-qubit quantum gates. To be useful, quantum computations must improve in accuracy.
Important Quantum Computing Milestone
The unimon research was conducted under the direction of Professor Mikko Möttönen, who is a joint professor of quantum technology at Aalto University and VTT as well as the co-founder and chief scientist at IQM Quantum Computers.
“Our aim is to build quantum computers which deliver an advantage in solving real-world problems. Our announcement today is an important milestone for IQM, and a significant achievement to build better superconducting quantum computers,”
said Professor Möttönen.
Fewer Errors Per Operation
The unimon, a new type of superconducting qubit developed by Aalto, IQM, and VTT, combines the desired characteristics of increased anharmonicity, a complete insensitivity to DC charge noise, decreased sensitivity to magnetic noise, and a straightforward structure consisting only of a single Josephson junction in a resonator in a single circuit.
On three different unimon qubits, the team achieved fidelities ranging from 99.8% to 99.9% for 13-nanosecond-long single-qubit gates.
“Because of the higher anharmonicity, or non-linearity, than in transmons, we can operate the unimons faster, leading to fewer errors per operation,”
said IQM’s Eric Hyyppä.
The researchers created chips that contained three unimon qubits each in order to experimentally demonstrate the unimon. With the exception of Josephson junctions, where the superconducting leads were made of aluminum, they used niobium as the superconducting material.
Unimon Vs. Transmon Superconducting Qubits
The team found that the unimon qubit could be protected from noise while only requiring a single Josephson junction and having a relatively high anharmonicity. The geometric inductance of the unimon has the potential for greater predictability and yield than conventional fluxonium or quarton qubit superinductors based on junction arrays.
Unimons are incredibly simple, but they have numerous advantages over transmons. Transmon qubits are used by the majority of modern superconducting multi-qubit processors.
The susceptibility of the transition frequency of the transmon to charge noise was exponentially suppressed by adding a shunt capacitor in parallel with a Josephson junction when the transmon was created from the charge qubit.
“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.
To surpass the 99.99% fidelity target for practical quantum advantage with noisy systems and effective quantum error correction, the team is aiming towards further advancements in the design, materials, and gate time of the union.
Reference: Hyyppä, E., Kundu, S., Chan, C.F. et al. Unimon qubit. Nat Commun 13, 6895 (2022)