Bifluxon: Fluxon-Parity-Protected Superconducting Qubit

TL;DR

This paper introduces a symmetry-protected superconducting qubit called Bifluxon, which significantly suppresses energy decay and dephasing from charge and flux noise, enhancing qubit coherence times.

Contribution

The work demonstrates a new fluxon-parity-protected qubit design that combines a Cooper-pair box with a superinductor, achieving exponential noise suppression and improved coherence.

Findings

Decay time increased up to 100 microseconds with protection

Charge noise sensitivity is reduced compared to traditional charge qubits

Charge-noise-induced dephasing time exceeds 1 microsecond

Abstract

We have developed and characterized a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from charge and flux noise, and dephasing from flux noise. The qubit consists of a Cooper-pair box (CPB) shunted by a superinductor, thus forming a superconducting loop. Provided the offset charge on the CPB island is an odd number of electrons, the qubit potential corresponds to that of a $\cos \phi / 2$ Josephson element, preserving the parity of fluxons in the loop via Aharonov-Casher interference. In this regime, the logical-state wavefunctions reside in disjoint regions of phase space, thereby ensuring the protection against energy decay. By switching the protection on, we observed a ten-fold increase of the decay time, reaching up to $100 \mu \mathrm{s}$. Though the qubit is sensitive to charge noise, the sensitivity is much reduced in comparison with the charge qubit, and the charge-noise-induced dephasing time of the current device exceeds $1 \mu \mathrm{s}$. Implementation of the full dephasing protection can be achieved in the next-generation devices by combining several $\cos \phi / 2$ Josephson elements in a small array.