The quantum Allan variance

TL;DR

This paper introduces a comprehensive method to determine the fundamental limits of atomic clock stability, accounting for entanglement, measurement, and feedback, thus advancing precision timekeeping.

Contribution

It provides a rigorous, general approach to compute the ultimate Allan variance bound for atomic clocks with arbitrary entanglement and measurement strategies.

Findings

Derives explicit bounds on atomic clock stability.

Incorporates effects of entanglement and feedback.

Highlights computational challenges for large systems.

Abstract

The instability of an atomic clock is characterized by the Allan variance, a measure widely used to describe the noise of frequency standards. We provide an explicit method to find the ultimate bound on the Allan variance of an atomic clock in the most general scenario where N atoms are prepared in an arbitrarily entangled state and arbitrary measurement and feedback are allowed, including those exploiting coherences between succeeding interrogation steps. While the method is rigorous and general, it becomes numerically challenging for large N and long averaging times.