  We propose a novel type of composite light-matter magnetometer based on a
transversely driven multi-component Bose-Einstein condensate coupled to two
distinct electromagnetic modes of a linear cavity. Above the critical pump
strength, the change of the population imbalance of the condensate caused by an
external magnetic field entails the change of relative photon number of the two
cavity modes. Monitoring the cavity output fields thus allows for
nondestructive measurement of the magnetic field in real time. We show that the
sensitivity of the proposed magnetometer exhibits Heisenberg-like scaling with
respect to the atom number. For state-of-the-art experimental parameters, we
calculate the lower bound on the sensitivity of such a magnetometer to be of
the order of fT/$\sqrt{\mathrm{Hz}}$--pT/$\sqrt{\mathrm{Hz}}$ for a condensate
of $10^4$ atoms with coherence times of the order of several ms.
