In-situ measurement of vacuum window birefringence by atomic spectroscopy

TL;DR

This paper introduces an in-situ atomic spectroscopy method to precisely measure and characterize birefringence in vacuum windows, enabling improved compensation in quantum optics experiments.

Contribution

The paper presents a novel in-situ technique using microwave spectroscopy on cold atoms to measure birefringence of vacuum windows with high sensitivity.

Findings

Measured birefringence at the level of 10^{-8}

Identified optical axis orientations

Enabled precise birefringence compensation

Abstract

We present an in-situ method to measure the birefringence of a single vacuum window by means of microwave spectroscopy on an ensemble of cold atoms. Stress-induced birefringence can cause an ellipticity in the polarization of an initially linearly-polarized laser beam. The amount of ellipticity can be reconstructed by measuring the differential vector light shift of an atomic hyperfine transition. Measuring the ellipticity as a function of the linear polarization angle allows us to infer the amount of birefringence $\Delta n$ at the level of $10^{-8}$ and identify the orientation of the optical axes. The key benefit of this method is the ability to separately characterize each vacuum window, allowing the birefringence to be precisely compensated in existing vacuum apparatuses.