Disrupting IBM, Google New superconducting quantum bits enable high-fidelity logic gates

A scientific team from Aalto University Finland, IQM Quantum Computers and VTT Technical Research Center has discovered a new superconducting quantum bit, the unimon; and, the team has implemented the first unimon quantum logic gate with 99.9% fidelity, which is an important milestone in building a commercially useful quantum computer An important milestone.

The research results were published in the journal Nature Communications under the title "The unimon quantum bit" [1].

Compared to transmon, which is used by IBM, Google and others, IQM says that unimon is very simple, but has many advantages over transmon. "Our goal is to further improve unimon's design, materials and pick-through time to achieve the 99.99 percent fidelity goal." Currently transmon has yet to achieve 99.99% fidelity of the double quantum bit gate.

01unimon: gate fidelity reaches 99.9% for the first time

Of all the different approaches to building useful quantum computers, superconducting quantum bits lead the way. However, the quantum bit designs and techniques currently in use do not yet provide high enough performance for practical applications. And in this era of noise-containing intermediate-scale quantum computing (NISQ), the complexity of achievable quantum computing is mostly limited by errors in single- and double-quantum bit gates: quantum computing needs to become more precise to be useful.

The new superconducting quantum bit unimon introduced by Aalto University, IQM and VTT adds desirable features such as non-harmonicity, complete insensitivity to DC charge noise, reduced sensitivity to magnetic noise, and a simple structure consisting of only a single Josephson junction in a resonator, all unified in a single circuit.

Meanwhile, the experimental team achieved a single quantum bit gate lasting 13 nanoseconds with fidelity from 99.8% to 99.9% on three different unimon quantum bits.

02Anharmonicity + geometric inductance: Predictable and easy to produce and operate

Eric Hyyppä, who is working on his PhD, says: "Due to the higher nonwaveability/nonlinearity than transmon quantum bits, we can manipulate unimon faster, thus reducing errors per operation."

To demonstrate unimon in the experiment, the scientists designed and built chips, each consisting of three single-atom quantum bits. In addition to Josephson junctions, they used niobium as a superconducting material, where the superconducting leads were made from aluminum.

The team measured that the unimon quantum bits have relatively high anharmonicity while requiring only one Josephson junction without any superconductivity, which is protective against noise. The geometric inductance of unimon has the potential for higher predictability and yield than the junction array-based superinductance in conventional fluxonium or quarton quantum bits.

unimon quantum bits and their measurement settings

Key properties of unimon quantum bits. a) Predicted (solid line) and measured anharmonicity of five unimon quantum bits under flux bias Φ; b) average energy relaxation time T1 as a function of quantum bit frequency f; c) relaxation time T1, Ramsey coherence time and echo coherence time as a function of flux bias Φ.

Implementation of single quantum bit gates.

03Milestone in IQM superconductivity route to further design optimization

nimon's artistic impression

"Our goal is to build quantum computers that are superior in solving real-world problems. This Unimon proposal is an important milestone for IQM and a major achievement in building a better superconducting quantum computer." Professor Mikko Möttönen, joint professor of quantum technology at Aalto University and VTT, co-founder and chief scientist of the IQM quantum computer, and leader of this research, said [2].

"Unimon is so simple, yet has more advantages than transmon. The fact that the first ever unimon works so well provides a lot of room for optimization and major breakthroughs." Prof. Möttönen added: "As a next step, we should optimize the design for higher noise protection and demonstrate a double bit gate."

"Our goal is to further improve the unimon design, materials, and gate times to break through the 99.99% fidelity target to gain useful quantum advantages in noisy systems and efficient quantum error correction."

Reference links：

[1]https://www.nature.com/articles/s41467-022-34614-w

[2]https://phys.org/news/2022-11-unimon-qubit-boost-quantum-applications.html