Continuous real-time tracking of a quantum phase below the standard quantum limit

Presenting Author: Athreya Shankar, University of Colorado JILA
Contributing Author(s): Graham P. Greve, Baochen Wu, James K. Thompson, Murray Holland

We propose a scheme for continuously measuring an evolving quantum phase with precision beyond the standard quantum limit of $\Delta \phi_\text{SQL} = 1/\sqrt{N}$ radians, where $N$ is the number of pseudospins. Quantum non-demolition measurements of a lossy cavity mode interacting with an atomic ensemble are used to directly probe the phase of the collective atomic spin without converting it into a population difference. Unlike traditional Ramsey measurement sequences, our scheme allows for real-time tracking of time-varying signals. As a bonus, spin-squeezed states develop naturally, providing real-time phase estimation significantly more precise than $\Delta \phi_\text{SQL}$.

Read this article online: https://arxiv.org/abs/1809.08216

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