Tunable superconducting flux qubits with long coherence times

TL;DR

This paper demonstrates tunable superconducting flux qubits with significantly improved coherence times, achieving relaxation times around 8 microseconds and echo dephasing times near 4 microseconds, surpassing previous benchmarks.

Contribution

It introduces a design of tunable flux qubits with long coherence times, controlled via asymmetric SQUIDs, and characterizes their performance on a sapphire substrate.

Findings

Relaxation times up to 8 microseconds.

Dephasing times around 4 microseconds.

Tunable frequency range of ±3.5 GHz.

Abstract

In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically $\pm 3.5$ GHz around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain $T_{1}\sim 8 \mu s$. The echo dephasing times are limited by flux noise even at optimal points and reach $T_{2E}\sim 4 \mu s$, almost an order of magnitude longer than state of the art for tunable flux qubits.