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.
@article{arxiv.cond-mat/0505537,
title = {Efficient readout of flux qubits at degeneracy},
author = {A. Kack and F. K. Wilhelm},
journal= {arXiv preprint arXiv:cond-mat/0505537},
year = {2007}
}