Efficient readout of flux qubits at degeneracy

TL;DR

This paper introduces a superconducting circuit with a flux qubit and a charge transistor for efficient measurement at the flux degeneracy point, leveraging conjugate variables for high-fidelity detection.

Contribution

It presents a novel superconducting circuit design that enables effective readout of flux qubits at degeneracy using a conjugate variable detector.

Findings

The device can detect flux qubit states near degeneracy with high sensitivity.

The coupling preserves excitation number, enabling clear measurement outcomes.

Practical evaluation shows the circuit's effectiveness for quantum measurements.

Abstract

We present a superconducting circuit consisting of a flux qubit and a single-charge transistor serving as a detector. As flux and charge are conjugate, the transistor can detect states of the qubit close to the flux degeneracy point, when the eigenstates are quantum superpositions of fluxes. The coupling has a flip-flop symmetry conserving the total number of excitations, and so the measurement outcome results in the absence or presence of an incoherent tunneling cycle. We evaluate the performance of a practical device and show that it is an attractive tool for measuring at the degeneracy point.