Quantum measurement limits to clock precision

The role of projection noise, an example of quantum measurement noise, in clock design has been well understood for some time. It is based on a particular measurement protocol - Ramsey interferometry. In this paper we discuss more general quantum measurement schemes to estimate the period/frequency of a clock system based on continuous readout of a driven quantum system. The measurement needs to be chosen so as to provide a precise counting signal. We give an example based on a superconducting circuit clock.