Simultaneous Unbalanced Shared Local Oscillator Heterodyne Interferometry (SUSHI) for high SNR, minimally destructive dispersive detection of time-dependent atomic spins

TL;DR

SUSHI is a novel interferometry technique enabling high SNR, minimally destructive atomic spin detection by actively canceling technical noise, achieving near quantum-limited sensitivity over a broad bandwidth.

Contribution

The paper introduces SUSHI, a new heterodyne interferometry method that actively suppresses technical noise without signal balancing or piezo mirrors, enabling ultra-low noise atomic measurements.

Findings

Achieved 51 nrad/√Hz sensitivity over 60 Hz to 8 kHz bandwidth.

Maintained within 3 dB of the standard quantum limit at probe powers as low as 650 pW.

Demonstrated effective noise rejection without phase bias or signal balancing.

Abstract

We demonstrate "Simultaneous Unbalanced Shared Local Oscillator Heterodyne Interferometry (SUSHI)," a new method for minimally destructive, high SNR dispersive detection of atomic spins. In SUSHI a dual-frequency probe laser interacts with atoms in one arm of a Mach-Zehnder interferometer, then beats against a bright local oscillator beam traversing the other arm, resulting in two simultaneous, independent heterodyne measurements of the atom-induced phase shift. Measurement noise due to mechanical disturbances of beam paths is strongly rejected by the technique of \emph{active subtraction} in which anti-noise is actively written onto the local oscillator beam via an optical phase-locked-loop. In SUSHI, technical noise due to phase, amplitude, and frequency fluctuations of the various laser fields is strongly rejected (i) for any mean phase bias between the interferometer arms, (ii) without the use of piezo actuated mirrors, and (iii) without signal balancing. We experimentally demonstrate an ultra-low technical noise limited sensitivity of 51 nrad$/\sqrt{\R{Hz}}$ over a measurement bandwidth of 60 Hz to 8 kHz using a 230 $\mu$W probe, and stay within $\sim$3 dB of the standard quantum limit as probe power is reduced by more than 5 orders of magnitude to as low as 650 pW. SUSHI is therefore well suited to performing QND measurements for preparing spin squeezed states and for high SNR, truly continuous observations of ground-state Rabi flopping in cold atom ensembles.