Experimental Demonstration of a Robust and Scalable Flux Qubit

TL;DR

This paper demonstrates a robust, scalable flux qubit with experimental validation of its quantum properties, noise characteristics, and coherence potential, advancing superconducting qubit technology.

Contribution

It introduces a novel rf-SQUID flux qubit design that is resilient to fabrication variations and provides comprehensive experimental validation and noise analysis.

Findings

Measured persistent current and tunneling energy align with quantum Hamiltonian predictions.

Flux and critical current noise levels are comparable to the best reported devices.

Presented a formula linking flux noise spectral density to qubit coherence time.

Abstract

A novel rf-SQUID flux qubit that is robust against fabrication variations in Josephson junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest lying states, both in the coherent and incoherent regime, are presented. These experimental results are shown to be in agreement with predictions of a quantum mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rf-SQUIDs reported in the literature thusfar, with a $1/f$ flux noise spectral density at $1 $Hz of $1.3^{+0.7}_{-0.5} \mu\Phi_0/\sqrt{\text{Hz}}$. An explicit formula for converting the observed flux noise spectral density into a free induction decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented.