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arXiv:2503.22253 · Optics Express

Non-resonant inter-species interaction and its effect on the position response function of cold atoms

Anirban Misra, Urbashi Satpathi, Supurna Sinha, Sanjukta Roy, Saptarishi Chaudhuri

Published Mar 27, 2025• 1 citation
arXivHTMLPDF

Abstract

In the context of non-equilibrium statistical physics, the position response of a particle, coupled to a bath, subjected to an external force is a topic of broad interest. A topic of further interest is two distinguishable sets of interacting particles in contact with two distinct baths. Here, we report the experimental evidence of the modification of the position response function (PRF) of an ensemble of cold atoms in a magneto-optical trap when it is placed alongside a dilute cloud of cold atoms of a different species. Our experiment consists of a mass-imbalanced cold atomic mixture of 39 K and 23 Na atoms. We focus on the position response of 39 K atoms when subjected to a sudden displacement in the presence of a cold 23 Na atomic cloud. Notably, we find that, in the underdamped regime of motion, the oscillation frequency of motion of the cold atoms changes as much as 30 % depending on the effective inter-species light-assisted interaction strength. On the other hand, in the overdamped regime, there is a reduction, as high as 10.5 %, in the damping coefficient, depending on the interaction strength. Using a generalized quantum Langevin approach, we develop a theoretical framework, compare it with the experimental results, and find good agreement. Our results are expected to find applications in the domain of mass and charge transport studies in a wide variety of physical conditions, which can be simulated in cold atom experiments.

DOI
10.1364/oe.553636

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In the context of non-equilibrium statistical physics, the position response of a particle, coupled to a bath, subjected to an external force is a topic of broad interest. A topic of further interest is two distinguishable sets of interacting particles in contact with two distinct baths. Here, we report the experimental evidence of the modification of the position response function (PRF) of an ensemble of cold atoms in a magneto-optical trap when it is placed alongside a dilute cloud of cold atoms of a different species. Our experiment consists of a mass-imbalanced cold atomic mixture of 39 K and 23 Na atoms. We focus on the position response of 39 K atoms when subjected to a sudden displacement in the presence of a cold 23 Na atomic cloud. Notably, we find that, in the underdamped regime of motion, the oscillation frequency of motion of the cold atoms changes as much as 30 % depending on the effective inter-species light-assisted interaction strength. On the other hand, in the overdamped regime, there is a reduction, as high as 10.5 %, in the damping coefficient, depending on the interaction strength. Using a generalized quantum Langevin approach, we develop a theoretical framework, compare it with the experimental results, and find good agreement. Our results are expected to find applications in the domain of mass and charge transport studies in a wide variety of physical conditions, which can be simulated in cold atom experiments.
The position response of a particle subjected to a perturbation is of general interest in physics. We study the modification of the position response function of an ensemble of cold atoms in a magneto-optical trap (MOT) in the presence of tunable light-assisted interactions. We subject the cold atoms to an intense laser light tuned near the photoassociation (PA) resonance and observe the position response of the atoms subjected to a sudden displacement. Surprisingly, we observe that the entire cold atomic cloud undergoes collective oscillations. We use a generalized quantum Langevin approach to theoretically analyze the results of the experiments and find good agreement.
We report our experimental measurements and theoretical analysis of the position response function of cold atoms in a magneto-optical trap (MOT) by applying a transient homogeneous magnetic field as a perturbing force. We observe a transition from a damped oscillatory motion to an over-damped relaxation, stemming from a competition between the viscous drag provided by the optical molasses and the restoring force of the MOT. Our observations are in agreement with the predictions of our model based on the Langevin equation. We also study the diffusion of the atomic cloud in the optical molasses and find the measured value of diffusion coefficient matching with the prediction of our theoretical model.
The main objective of this research is to study a particular case of light-matter interaction: the scattering of light in dense atomic samples.The regime of high atomic densities is represented by samples where the distance between neighboring atoms is on the order of the wavelength associated with the atomic transition.In this regime, short-range atomic interactions cannot be neglected, and in these systems, we expect to observe significant modifications in collective effects such as subradiance or superradiance.Overall, the propagation of light through this type of system is not well-known to the scientific community, and the existing models work to describe very specific situations.To pave the way for achieving our overall goal, this work proposes an experimental setup to obtain a dense cloud of 88 Sr, which is the chemical species we work with in our laboratory.For this purpose, atoms previously trapped using a Magneto-Optical Trap are transferred to a Crossed Optical Dipole Trap.During the optimization and characterization of the optical trap, problems were encountered in producing an image with quantitative information of the atomic ensemble and in transferring a large number of atoms in a very small trapping volume.These problems were resolved with the implementation of a Phase Contrast Imaging system and the implementation of an Optical Molasses as an additional cooling step.Under these conditions, we obtained a cloud that can be suitable for conducting studies on collective effects, transitioning from the dilute to the dense regime.The initial experiments, once our atomic ensemble was prepared, involve measuring the coherent transmission of a low-intensity beam through the cloud.For this purpose, incident light near the resonance of a dipolar transition of the type J = 0 ↔ J = 1 of 88 Sr is used, interacting with a homogeneous region of the sample.Simulations of this type of process can be performed using the Coupled Dipole Model, which describes the overall response of light scattering by the sample considering dipole-dipole interactions mediated by light.With this work, we aim to advance the understanding of density effects by comparing the coherent optical response of the dense cloud with the theory based on the Coupled Dipole Model.
Abstract We report the first observation of motional excitation induced by parametric resonance of cold 133^{133}133Cs atoms in a dual-species magneto-optical trap (MOT) simultaneously trapping 133 Cs and 85 Rb atoms. By modulating the cooling laser’s intensity at approximately twice the resonant frequency of the trap, the atoms inside the trap separate into two atomic clouds, oscillating with a π phase difference due to the trap potential’s nonlinearity, resulting in a finite oscillation length. This behavior reflects general nonlinear dynamics and is described as a Hopf bifurcation or limit cycle motion. The measured oscillation amplitude was compared with theoretical calculations derived from simple Doppler cooling theory, changing modulation amplitude, magnetic field gradient, and saturation parameter. In addition, we investigate the noise-induced switching dynamics by measuring the temporal evolution of atomic populations localized in each of the two attractors under resonant symmetry lifting induced by an effective bias field. Notably, this dual-species MOT system is expected to provide a new platform for studying nonlinear and nonequilibrium dynamics between cold rubidium and cesium atomic ensembles. PACS numbers: 37.10.Vz, 37.10.De, 37.10.Gh
Quantum dynamics of impurities in a bath of bosons is a long-standing problem in solid-state, plasma, and atomic physics. Recent experimental and theoretical investigations with ultracold atoms have focused on this problem, studying atomic impurities immersed in an atomic Bose–Einstein condensate (BEC) and for various relative coupling strengths tuned by the Fano−Feshbach resonance technique. Here, we report extensive numerical simulations on a closely related problem: the collision between a bosonic impurity consisting of a few 41 K atoms and a BEC of 87 Rb atoms in a quasi one-dimensional configuration and under a weak harmonic axial confinement. For small values of the inter-species interaction strength (regardless of its sign), we find that the impurity, which starts from outside the BEC, simply causes the BEC cloud to oscillate back and forth, but the frequency of oscillation depends on the interaction strength. For intermediate couplings, after a few cycles of oscillation the impurity is captured by the BEC, and strongly changes its amplitude of oscillation. In the strong interaction regime, if the inter-species interaction is attractive, a local maximum (bright soliton) in the BEC density occurs where the impurity is trapped; if, instead, the inter-species interaction is repulsive, the impurity is not able to enter the BEC cloud and the reflection coefficient is close to one. However, if the initial displacement of the impurity is increased, the impurity is able to penetrate the cloud, leading to the appearance of a moving hole (dark soliton) in the BEC.