Flux qubit as a sensor of magnetic flux

A magnetometer based on the quantum properties of a superconducting flux qubit is proposed. The main advantage of this device is that its sensitivity can be below the so-called "standard quantum limit" ( for an oscillator this is half of the Plank constant). Moreover its transfer functions relative to the measured flux can be made to be about 10mV/Phi(0), which is an order of magnitude more than the best value for a conventional DC SQUIDs with a direct readout. We analyze here the voltage-to-flux, the phase-to-flux transfer functions and the main noise sources. We show that the experimental characteristics of a flux qubit, obtained in recent experiments, allow the use of a flux qubit as magnetometer with energy resolution close to the Planck constant.