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arXiv:2501.15546 · Physical Review Research

Dispersive measurement of spin shot noise in a Bose-Einstein condensate

Kosuke Shibata, Naota Sekiguchi, Junnosuke Takai, Takuya Hirano

Published Sep 19, 2025• 1 citation
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Abstract

We report dispersive spin shot noise measurement of a Bose-Einstein condensate (BEC). While dispersive probing has been used for quantum spin noise measurement of thermal and cold gases for decades, confirmative measurement of shot noise of real spin, i.e., the linear dependence of the spin variance on the number of atoms in a BEC, has been lacking. Here, we demonstrate precise spin noise measurement of a BEC of rubidium atoms at the spin shot noise level by polarization rotation using a two-color probe at optimal detunings, with power balance stabilization to suppress probe-induced excess spin noise. This work opens the possibility for the unexplored study of quantum spin fluctuations in multicomponent or spinor BECs and offers an approach to improve spin measurement precision, which is relevant to atomic spin-based sensors.

DOI
10.1103/md2j-t2qq

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Abstract We demonstrate how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the parameter dependence of the transition to a Bose–Einstein condensate (BEC). In this way shot-to-shot fluctuations in initial conditions are detected and the information extracted per experimental realization is increased. We furthermore calibrate in situ images from dispersive probing of a BEC with corresponding absorption images in time-of-flight. This allows for the determination of the transition point in a single experimental realization by applying multiple dispersive measurements. Finally, we explore the continuous probing of several consecutive phase transition crossings using the periodic addition of a focused ‘dimple’ potential.
We report a continuous measurement of the spin state of a spinor BEC. A far off-resonant linearly polarised probe beam illuminates a sodium spinor condensate in F=1. The beam is Faraday rotated in proportion to the spin component along the beam propagation direction, and the rotation measured at the shot-noise limit with an autobalanced polarimeter. The measurement is minimally-destructive and continuous. As the condensate spin Larmor precesses in a weak magnetic bias field, the Faraday signal oscillates at the Larmor frequency, typically of order 50 kHz. We investigate this novel measurement for monitoring spin-mixing and decoherence processes in real time.