Continuous quantum clock with high precision and long recurrence time

Continuous clocks, i.e. the clocks that measure time in a continuous manner, are regarded as an essential component of sensing technology. Precision and recurrence time are two basic features of continuous clocks. In this paper, in the framework of quantum estimation theory various models for continuous quantum clocks are proposed, where all tools of quantum estimation theory are employed to seek the characteristics of clocks with high precision and long recurrence time. Then, in a resource-based approach, the performance of the proposed models is compared. It is shown that quantum clocks based on $n$ two-qubits system not only can have better precision than quantum clocks based on $2n$ one-qubit system but also support long recurrence time. Finally, it is shown that while employing the number of $n$ entangled qubits improves the precision of clocks by a factor of $1/\sqrt n$, it inevitably worsens the recurrence time of the clock.