High-Coherence Fluxonium Qubit

Abstract

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfyingT2>100μs(T2>400μsin one device). Moreover, given the state-of-the-art values of the surface loss tangent and the1/fflux-noise amplitude, the coherence time can be further improved beyond 1 ms. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit—over102here—must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.

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Plain Language Summary

The key characteristic of a superconducting quantum bit (or qubit) is coherence time, which measures how long a qubit can hold information. We report record long coherence—reaching 0.5 ms—in fluxonium, a relatively unexplored superconducting artificial atom with properties desirable for engineering fast, controllable interactions between qubits. Our work significantly expands the toolbox of quantum superconducting circuits, especially in the context of scaling up quantum processors.

A notable feature of fluxonium is that it consists of a superconducting loop interrupted by over 100 Josephson junctions, strips of insulating material a few nanometers thick sandwiched between superconducting layers. This leads to an exceptionally large value of the total inductance of the loop, which makes fluxonium distinct and useful. Having so many junctions per qubit has been generally viewed as a liability for establishing long coherence times. Yet, we conclude that even longer coherence time is likely with our design in the near future by simply upgrading our fabrication procedures to the state of the art. Our experiment thus delivers an important lesson: Complex multijunction circuits can have superior coherence when properly designed.

The reported combination of high coherence and strong anharmonicity of fluxoniums can be readily utilized for improving the fidelity of digital logical operations and constructing analog simulators of strongly interacting quantum spin models.