  Bloch oscillations are a hallmark of coherent wave dynamics in periodic
potentials. They occur as the response of quantum mechanical particles in a
lattice if a weak force is applied. In optical lattices with their perfect
periodic structure they can be readily observed and employed as a quantum
mechanical force sensor, for example, for precise measurements of the
gravitational acceleration. However, the destructive character of the
measurement process in previous experimental implementations poses serious
limitations for the precision of such measurements. In this article we show
that the use of an optical cavity operating in the regime of strong cooperative
coupling allows one to directly monitor Bloch oscillations of a cloud of cold
atoms in the light leaking out of the cavity. Hence, with a single atomic
sample the Bloch oscillation dynamics can be mapped out, while in previous
experiments, each data point required the preparation of a new atom cloud. The
use of a cavity-based monitor should greatly improve the precision of Bloch
oscillation measurements for metrological purposes.
