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Frontiers of Physics

Observation of an inelastic feshbach resonance between individual 87Rb and 39K atoms in optical tweezers

Kedi Wei, S B Li, Ruike Wu, Y H Liu, Tao Chen, Bo Yan

Published Jan 1, 2026• 0 citations
Publisher

Abstract

An important route toward the preparation of cold molecules is magnetically tunable Feshbach resonances. Here we investigate an inelastic Feshbach resonance between a single pair of 87Rb and 39K atoms confined in an optical tweezer, with both atoms prepared in the |F=1,mF=−1⟩ state. By developing an in-situ imaging scheme for 39K, we directly measure magnetic-field-dependent collisional loss without separating the atoms after the interaction. For weak confinement, the resonance position measured in our experiment differs from the bulk-gas experiments. In contrast, we observe a significant shift of the resonance toward higher magnetic fields as the tweezer trap depth increases. Coupled-channel calculations accounting for finite temperature effects fail to explain the magnitude of this shift, suggesting that it originates from differential optical responses of the open-channel atom pair and the weakly bound molecular state under strong confinement. Our results indicate that optical tweezer light can significantly modify Feshbach resonance properties, opening a pathway toward optical control of atom–atom interactions and weakly bound molecular states in dual-species tweezer platforms.

DOI
10.15302/frontphys.2026.112202

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An important route toward the preparation of cold molecules is magnetically tunable Feshbach resonances. Here we investigate an inelastic Feshbach resonance between a single pair of 87Rb and 39K atoms confined in an optical tweezer, with both atoms prepared in the |F=1,mF=−1⟩ state. By developing an in-situ imaging scheme for 39K, we directly measure magnetic-field-dependent collisional loss without separating the atoms after the interaction. For weak confinement, the resonance position measured in our experiment differs from the bulk-gas experiments. In contrast, we observe a significant shift of the resonance toward higher magnetic fields as the tweezer trap depth increases. Coupled-channel calculations accounting for finite temperature effects fail to explain the magnitude of this shift, suggesting that it originates from differential optical responses of the open-channel atom pair and the weakly bound molecular state under strong confinement. Our results indicate that optical tweezer light can significantly modify Feshbach resonance properties, opening a pathway toward optical control of atom–atom interactions and weakly bound molecular states in dual-species tweezer platforms.
Abstract We prepare pairs of 133Cs atoms in a single optical tweezer and perform Feshbach spectroscopy for collisions of atoms in the states (f = 3, m f = ±3). We detect enhancements in pair loss using a detection scheme where the optical tweezers are repeatedly subdivided. For atoms in the state (3, −3), we identify resonant features by performing inelastic loss spectroscopy. We carry out coupled-channel scattering calculations and show that at typical experimental temperatures the loss features are mostly centred on zeroes in the scattering length, rather than resonance centres. We measure the number of atoms remaining after a collision, elucidating how the different loss processes are influenced by the tweezer depth. These measurements probe the energy released during an inelastic collision, and thus give information on the states of the collision products. We also identify resonances with atom pairs prepared in the absolute ground state (f = 3, m f = 3), where two-body radiative loss is engineered by an excitation laser blue-detuned from the Cs D2 line. These results demonstrate optical tweezers to be a versatile tool to study two-body collisions with number-resolved detection sensitivity.
Tunable Feshbach resonances and interaction potentials between 23^{23}23Na and 87^{87}87Rb atoms
We measure the binding energies of weakly bound Feshbach molecules formed between \Na and \Rb atoms in their lowest hyperfine Zeeman levels. We form molecules at the Feshbach resonance near 347.64 G and dissociate them by magnetic field modulation. We use the binding energies to refine the singlet and triplet potential energy curves, using coupled-channel bound-state calculations. We then use coupled-channel scattering calculations on the resulting potentials to produce a high-precision mapping between magnetic field and scattering length. We also observe 10 additional sss-wave Feshbach resonances for \Na and \Rb in different combinations of Zeeman sublevels of the F=1F = 1F=1 hyperfine states. Some of the resonances show 2-body inelastic decay due to spin exchange. We compare the resonance properties with coupled-channel scattering calculations that full take account of inelastic properties.
We have successfully prepared an optically trapped ultracold mixture of 23{}^{23}23Na and 87{}^{87}87Rb atoms and studied their interspecies Feshbach resonances. Using two different spin combinations, several sss- and ppp-wave resonances are identified by observing high inelastic losses and temperature increases for both species near resonant magnetic field values. The two sss-wave resonances observed below 500 G between atoms in their lowest-energy levels are possible candidates for Feshbach molecule association. Our results are well characterized by a coupled-channel model which also refines the ground-state interaction potentials between 23{}^{23}23Na and 87{}^{87}87Rb. This work opens up the prospect for preparing ultracold ensembles of ground-state bosonic NaRb molecules which are chemically stable and can provide strong dipolar interactions.
We have developed a robust interrogation system using frequency modulation spectroscopy to measure the quantum state-dependent phase shift incurred on an off-resonant optical probe when transmitted by an atomic medium. Recently, our focus has been on extending this technique for the detection of Feshbach resonances in 87Rb atoms. Feshbach resonance is a mechanism which allows the atomic interaction strength to be precisely tuned via an external magnetic field. To access a Feshbach resonance atoms must be independently prepared in certain internal states, during which we utilize programmable optical tweezers to perform precise spatial micro-manipulation of the ensemble in laser “test-tubes.” We use our dispersive probing system to identify the resonant magnetic field value in a sample with a dense “ball” geometry. An important design consideration for such a probing scheme is the three-dimensional mode-matching at the interface between light and the atomic sample when coupled by the dispersive interaction. We discuss challenges which dealing with this new geometry compared to the previously used prolate geometry, and consider the possibility of dipole-dipole interactions in our sample leading to cooperative light scattering processes.
This thesis reports on the first observation of optical tuning of interactions in an ultracold atomic gas. To control the elastic interactions, we use optically induced scattering resonances. They are realized by coupling pairs of atoms to a bound molecular state with the help of laser light. In comparison to the well-known magnetic Feshbach resonances, these optical Feshbach resonances allow a very fast switching of interactions in a spatially resolved way. This is because laser light can be switched on and off much faster than magnetic fields and because the light intensity can be spatially structured. Our experiments are performed with a Bose-Einstein condensate (BEC) of Rb atoms, which we produce in our new BEC setup. This apparatus has been constructed in the framework of this thesis and offers excellent optical access to the ultracold atom cloud. To investigate the tunability of atomic interactions with single-frequency laser light, we optically couple pairs of free atoms to an electronically excited molecular state. The coupling is achieved with a laser that is tuned close to the respective free-bound photoassociation transition. By changing the laser detuning we are able to adjust the atomic scattering length to any value between 10 a0 and 190 a0, where a0 denotes the Bohr radius. The atomic scattering length is a single quantity that fully describes ultracold elastic interactions. Since laser induced atom losses restrict the duration of our experiments, Bragg spectroscopy is employed as a fast method to measure the scattering length. We also demonstrate that optical Feshbach resonances can be realized with a two-color Raman coupling scheme. To implement this scheme, a second laser is added that couples the excited molecular state to a weakly bound molecular level in a ground state potential. Tuning the second laser results in a modification of the atomic scattering length similar to the one observed with a single laser. In addition, this scheme allows to control the width of the resonance via the detuning of the first laser. For the one-color and two-color coupling schemes, we investigate how the elastic scattering properties depend on the different available parameters, i.e. the laser detunings and intensities. Our experimental findings can successfully be described with an available theoretical model, which treats ultracold collisions in the presence of laser light. Optical Feshbach resonances have the possibility of becoming a valuable tool for fast switching of interactions. This is of special interest in quantum computation schemes with neutral atoms, where interactions between different atomic qubits need to be controlled.
We use laser light near resonant with a molecular bound-to-bound transition to control a magnetic Feshbach resonance in ultracold Fermi gases of 40{}^{40}40K atoms. The spectrum of excited molecular states is measured by applying a laser field that couples the ground Feshbach molecular state to electronically excited molecular states. Nine strong bound-to-bound resonances are observed below the 2P1/2+2S1/2{}^{2}{P}_{1/2}+{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}2P1/2​+2S1/2​ threshold. We use radio-frequency spectroscopy to characterize the laser-dressed bound state near a specific bound-to-bound resonance and show clearly the shift of the magnetic Feshbach resonance using light. The demonstrated technology could be used to modify interatomic interactions with high spatial and temporal resolutions in the crossover regime from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid.