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Journal of Physics B Atomic Molecular and Optical Physics

Loss compensated atom number measurement in an ensemble of cold atoms in free space

Poonam Yadav, N. Singh, Bodhaditya Santra

Published Oct 16, 2026• 0 citations
Publisher

Abstract

Abstract Cold atoms play a crucial role in enabling precision spectroscopy and various quantum technology applications. To fully exploit their potential in emerging technologies, it is essential to characterize atom losses during imaging of atoms. In this work, we systematically investigate atom-number measurement losses in a cold-atom ensemble arising due to light-assisted collisions. We have studied the dynamics of lightassisted collisions by measuring the loss coefficients and the compensated measured atom numbers. We then assess the accuracy of the inferred atom numbers and internalstate readout in the presence of these loss processes. These results present a detailed picture of detection-limited noise and collisional loss in cold atom clouds of quantum sensing and quantum communication applications.

DOI
10.1088/1361-6455/aeb53b

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Abstract Cold atoms play a crucial role in enabling precision spectroscopy and various quantum technology applications. To fully exploit their potential in emerging technologies, it is essential to characterize atom losses during imaging of atoms. In this work, we systematically investigate atom-number measurement losses in a cold-atom ensemble arising due to light-assisted collisions. We have studied the dynamics of lightassisted collisions by measuring the loss coefficients and the compensated measured atom numbers. We then assess the accuracy of the inferred atom numbers and internalstate readout in the presence of these loss processes. These results present a detailed picture of detection-limited noise and collisional loss in cold atom clouds of quantum sensing and quantum communication applications.
Over the last two decades the cold-atom physics has matured from proof-of-principle demonstrations to a versatile platform for precision measurements and study of quantum phenomena. Ultra-cold atomic ensembles have been used both for technological and fundamental science applications. To fully exploit their potential, a precise measurement and control of the atom number in the ensemble is crucial. We report on a precise, minimally destructive measurement technique that can be used to prepare an atomic ensemble with a desired atom number. The measurement relies on the dispersive light-atom interaction; thus it is expected to have a negligible effect on the ensemble temperature and to induce minimal decoherence in the atomic quantum state. As a result, it can be used to perform quantum-enhanced measurements and prepare the atom-number state at the start of an interferometer sequence.
We present the measurements of thermalized collisional rate coefficients for ultra-cold 7Li and 87Rb colliding with room-temperature He, Ne, N2, Ar, Kr, and Xe. In our experiments, a combined flowmeter and dynamic expansion system, a vacuum metrology standard, is used to set a known number density for the room-temperature background gas in the vicinity of the magnetically trapped 7Li or 87Rb clouds. Each collision with a background atom or molecule removes a 7Li or 87Rb atom from its trap, and the change in the atom loss rate with background gas density is used to determine the thermalized loss rate coefficients with fractional standard uncertainties better than 1.6% for 7Li and 2.7% for 87Rb. We find consistency—a degree of equivalence of less than one—between the measurements and recent quantum-scattering calculations of the loss rate coefficients [Kłos and Tiesinga, J. Chem. Phys. 158, 014308 (2023)], with the exception of the loss rate coefficient for both 7Li and 87Rb colliding with Ar. Nevertheless, the agreement between theory and experiment for all other studied systems provides validation that a quantum-based measurement of vacuum pressure using cold atoms also serves as a primary standard for vacuum pressure, which we refer to as the cold-atom vacuum standard.
The noise in absorption imaging of cold atoms significantly impacts measurement accuracy across a range of applications with ultracold atoms. It is crucial to adopt an approach that offers effective denoising capabilities without compromising the unique structure of atom clouds. Here, we introduce, to our knowledge, a novel image enhancement algorithm for cold atomic absorption imaging. The algorithm successfully suppresses background noise, enhancing image contrast significantly. Experimental results showcase that this approach can improve the image's signal-to-noise ratio by about 10 dB and enhance the uncertainty of cold atom number measurements by approximately tenfold, preserving the spatial distribution of the atom clouds. Moreover, the method exhibits exceptional performance and robustness when confronted with fringe noise and multi-component imaging scenarios, offering high stability. Importantly, this method can perform denoising based on a single image (without the need to establish a reference library), and the optimization process is entirely automated, eliminating the need for manual parameter selection. The method is both compatible and practical, making it applicable to various cold atom absorption imaging fields.
Many cold atom experiments rely on precise atom number detection, especially in the context of quantum-enhanced metrology where effects at the single particle level are important. Here, we investigate the limits of atom number counting via resonant fluorescence detection for mesoscopic samples of trapped atoms. We characterize the precision of these fluorescence measurements beginning from the single-atom level up to more than one thousand. By investigating the primary noise sources, we obtain single-atom resolution for atom numbers as high as 1200. This capability is an essential prerequisite for future experiments with highly entangled states of mesoscopic atomic ensembles.
We study light-assisted collisions in an ensemble containing a small number ($\ensuremath{\sim}$3) of cold 87{}^{87}87Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values $\ensuremath{\beta}>{10}^{\ensuremath{-}8}$ cm3{}^{3}3 s${}^{\ensuremath{-}1}$ and varies rapidly with the trap depth and the parameters of the excitation light.