Gradiometric flux qubits with tunable gap

TL;DR

This paper demonstrates tunable flux qubits with adjustable energy gaps using a dc SQUID, enabling resonance tuning with other circuits and maintaining optimal coherence at the symmetry point.

Contribution

It introduces a method to tune the qubit gap in gradiometric flux qubits via a dc SQUID, validated by spectroscopic measurements and model calculations.

Findings

Qubit gap tunable from zero to over 10 GHz

Good agreement between experimental data and theoretical models

Effective biasing near the symmetry point with flux trapping

Abstract

For gradiometric three-Josephson-junction flux qubits, we perform a systematic study on the tuning of the minimal transition frequency, the so-called qubit gap. By replacing one of the qubit's Josephson junctions by a dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be tuned in situ by a control flux threading the SQUID loop. We present spectroscopic measurements demonstrating a well-defined controllability of the qubit gap between zero and more than 10 GHz. In the future, this enables one to tune the qubit into and out of resonance with other superconducting quantum circuits, while operating the qubit at its symmetry point with optimal dephasing properties. The experimental data agree very well with model calculations based on the full qubit Hamiltonian. From a numerical fit, we determine the Josephson coupling and the charging energies of the qubit junctions. The derived values agree well with those measured for other junctions fabricated on the same chip. We also demonstrate the biasing of gradiometric flux qubits near the symmetry point by trapping an odd number of flux quanta in the gradiometer loop. In this way, we study the effect of the significant kinetic inductance, thereby obtaining valuable information for the qubit design.