Revisiting the multi-mode rhombus circuit as a biased-noise qubit

TL;DR

This study revisits a Josephson junction interferometer as a biased-noise qubit, demonstrating improved coherence times and probing qubit transitions over GHz frequencies to optimize performance.

Contribution

It introduces a modified rhombus qubit with energy alteration of junctions, enabling direct GHz transition measurements and revealing biased-noise qubit advantages.

Findings

Measured $T_1$ relaxation times of approximately 500 μs and 27 μs in different regimes.

Achieved a Ramsey dephasing time of about 90 ns in the biased-noise regime.

Identified flux noise and quasiparticle tunneling as primary relaxation limits at low frequencies.

Abstract

In this work, we revisit the idea of using an interferometer of pairs of Josephson junctions as a protected rhombus qubit. Unlike in the original proposal, where the qubit states are encoded into odd and even parity charge states, here, we intentionally alter the energy of one of the junctions to investigate the soft version of the rhombus qubit. This approach allows us to directly probe the qubit transitions over several GHz and reduce the potential drawbacks of the interferometer-based protection. Away from a half flux quantum external field, the large shunting capacitors of the circuit ensure localized qubit states in different phase valleys, leading to a biased-noise qubit. In the realized circuit, we measure an average $T_1\approx500\,\mu$s relaxation time in the biased-noise regime (with a Ramsey dephasing time of $T^{R}_\varphi\approx90\,$ns), while an average $T_1\approx27\,\mu$s relaxation time at frustration (with $T^{R}_\varphi\approx670\,$ns). Our loss analysis on this multi-mode circuit indicates that at low frequencies, flux noise and quasiparticle tunneling limit the relaxation times, pointing toward the presence of an optimal operating regime of around a few GHz.