Canceling the cavity length induced phase noise in an optical ring cavity for phase shift measurement and spin squeezing

TL;DR

This paper introduces a novel phase shift measurement method using a high-finesse optical ring cavity that significantly reduces cavity length fluctuation noise, enabling enhanced spin squeezing and quantum measurement precision.

Contribution

The paper presents the first use of an optical ring cavity with two independent beams for noise-reduced phase measurement, achieving a 30 dB noise reduction and a 40-fold improvement in phase sensitivity.

Findings

30 dB reduction in cavity noise up to 30 kHz

Achieved 0.7 mrad phase resolution

Enabled a 40-fold enhancement in spin-squeezing sensitivity

Abstract

We demonstrate a new method of light phase shift measurement using a high-finesse optical ring cavity which exhibits reduced phase noise due to cavity length fluctuations. Two laser beams with a frequency difference of one cavity free spectral range are simultaneously resonant with the cavity, demonstrating noise correlations in the error signals due to the common-mode cavity length fluctuations. The differential error signal shows a 30 dB reduction in cavity noise down to the noise floor in a frequency range up to half the cavity linewidth ($\delta\nu/2 \simeq 30$ kHz). Various noise sources are analyzed and their contributions to the noise floor are evaluated. Additionally, we apply this noise-reduced phase shift measurement scheme in a simulated spin-squeezing experiment where we have achieved a factor of 40 improvement in phase sensitivity with a phase resolution of 0.7 mrad, which may remove one important barrier against attaining highly spin-squeezed states. The demonstrated method is the first reported measurement using an optical ring cavity and two independent beams, a flexible situation. This method can find direct application to non-destructive measurements in quantum systems, such as for the generation of spin-squeezed states in atom interferometers and atomic clocks.