  We propose to use the quantum properties of a superconducting flux qubit in
the construction of a magnetometer with quantum limited sensitivity. The main
advantage of a flux qubit is that its noise is rather low, and 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
SQUID magnetometer. 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.
