  In classical physics, clocks are open dissipative systems driven from thermal
equilibrium and necessarily subject to thermal noise. We describe a quantum
clock driven by entropy reduction through measurement. The mechanism consists
of a superconducting transmon qubit coupled to an open co-planar resonator. The
cavity and qubit are driven by coherent fields and the cavity output is
monitored with homodyne detection. We show that the measurement itself induces
coherent oscillations, with fluctuating period, in the conditional moments. The
clock signal can be extracted from the observed measurement currents and
analysed to determine the noise performance. The model demonstrates a
fundamental principle of clocks at zero temperature: good clocks require high
rates of energy dissipation and consequently entropy generation.
