Ground-state characterization of Nb charge-phase Josephson qubits

TL;DR

This paper investigates Nb-based Josephson charge-phase qubits, focusing on their ground-state properties, readout via Josephson inductance, and control mechanisms at cryogenic temperatures.

Contribution

It introduces a method for characterizing Nb Josephson qubits using inductive coupling and RF readout, revealing ground-state behavior and control techniques.

Findings

Partial suppression of supercurrent at 4.2 K explained by quantum statistical model

Demonstrated magnetic control of phase and electrostatic control of charge at 20 mK

Fabricated Nb junctions enabling parameter determination at liquid helium temperature

Abstract

We present investigations of Josephson charge-phase qubits inductively coupled to a radio-frequency driven tank-circuit enabling the readout of the states by measuring the Josephson inductance of the qubit. The circuits including junctions with linear dimensions of 60 nm and 80 nm are fabricated from Nb trilayer and allowing the determination of relevant sample parameters at liquid helium temperature. The observed partial suppression of the circulating supercurrent at 4.2 K is explained in the framework of a quantum statistical model. We have probed the ground-state properties of qubit structures with different ratios of the Josephson coupling to Coulomb charging energy at 20 mK, demonstrating both the magnetic control of phase and the electrostatic control of charge on the qubit island.