arXiv subjects

Dajun Wang

Publications and source records attributed to Dajun Wang.

At least 19 recordsLinked to original sources

arXiv2026

Rabi-coupling-induced three-component quantum droplet in ultracold Bose gases

We uncover a new mechanism for realizing three-component quantum droplets in ultracold Bose gases, where only one inter-species interaction is attractive. In this scheme, the inter-species attraction leads to a self-bound binary droplet, and the third component joins through Rabi coupling with one component of the binary droplet. We find that a stronger Rabi coupling leads to a larger fraction of the third component, but also destabilizes the entire droplet due to the involvement of more repulsive forces. Such instability can be remedied by a finite detuning between the Rabi-coupled components. We demonstrate these results in realistic Na-Rb mixtures, using both thermodynamic analyses and numerical simulations based on extended Gross-Pitaevskii equations. Our work outlines a general route for stabilizing multi-component droplets by bridging an existing binary droplet with additional components via suitable single-particle fields.

cond-mat.quant-gas↗

arXiv2026

Creation of ultracold heteronuclear p-wave Feshbach molecules

We report the first creation of a bulk sample of ultracold heteronuclear p-wave Feshbach molecules in an optically trapped Bose-Bose mixture of 23Na and 87Rb atoms. Using loss spectroscopy and binding energy measurements, we systematically characterize the interspecies p-wave Feshbach resonances near 284 G. Leveraging this understanding, we use magneto-association to form p-wave NaRb Feshbach molecules, producing both pure samples and mixtures of molecules in different p-wave orbitals. We further measure the molecular lifetime and identify atom-molecule and molecule-molecule collisions as the dominant loss mechanisms. This work establishes a previously unavailable ultracold molecule platform that combines orbital anisotropy with heteronuclear constituents, represents a significant step toward realizing tunable p-wave interactions in Bose-Bose mixtures, and provides a foundation for exploring non-zero angular momentum molecules.

physics.atom-ph↗

arXiv2025

Strongly dipolar molecular Bose-Einstein condensates: From few- to many-body physics

Recent advances in molecular cooling have enabled the realization of strongly dipolar Bose--Einstein condensates (BECs) of molecules, and BECs of many different molecular species may become experimentally accessible in the near future. Here, we explore the unique properties of such BECs and the new insights they may offer into dipolar quantum fluids and many-body physics. We explore which parameter regimes can realistically be achieved using currently available experimental techniques, discuss how to implement these techniques, and outline which molecular species are particularly well suited to explore exotic new states of matter. We further determine how state-of-the-art beyond mean-field theories, originally developed for weakly dipolar magnetic gases, can be pushed to their limits and beyond, and what other long-standing questions in the field of dipolar physics may realistically come within reach using molecular systems.

cond-mat.quant-gas↗

arXiv2025

Bose-Einstein condensate of ultracold sodium-rubidium molecules with tunable dipolar interactions

Realizing Bose-Einstein condensation of polar molecules is a long-standing challenge in ultracold physics and quantum science due to near-universal two-body collisional losses. Here, we report the production of a Bose-Einstein condensate of ground-state sodium-rubidium molecules via high efficiency evaporative cooling, with losses suppressed using the dual microwave shielding technique. The ability to tune the dipolar interaction between these ultracold polar molecules is crucial for producing the condensate and enables exciting prospects for future applications. We explore different regimes of dipolar interactions, realizing both the gas phase and the quantum droplet phase of the molecular condensate. This work opens new avenues for investigating quantum matter with strong dipolar interactions and for quantum simulation of long-range many-body systems.

cond-mat.quant-gas↗

arXiv2025

Probing the hollowing transition of a shell-shaped BEC with collective excitation

We investigate the hollowing transition of a shell-shaped Bose-Einstein condensate using collective excitations. The shell is created using an immiscible dual-species BEC mixture, with its hollowness controlled by tuning the repulsive interspecies interaction via a Feshbach resonance. Our results reveal two distinct monopole modes in which the two condensates oscillate either in-phase or out-of-phase. The spectrum of the out-of-phase mode exhibits a non-monotonic dependence on the interspecies interaction, providing a clear signature of the topology change from a filled to a hollow condensate. Furthermore, we find that the critical point of the hollowing transition depends strongly on the number ratio of the two species. Our findings provide a detailed understanding of the topology change in shell-shaped quantum gases and pave the way for future study of quantum many-body phenomena in curved spaces.

cond-mat.quant-gas↗

arXiv2024

Quantum state manipulation and cooling of ultracold molecules

An increasingly large variety of molecular species are being cooled down to low energies in recent years, and innovative ideas and powerful techniques continue to emerge to gain ever more precise control of molecular motion. In this brief review we focus our discussions on two widely employed cooling techniques that have brought molecular gases into the quantum regime: association of ultracold atomic gases into quantum gases of molecules and direct laser cooling of molecules. These advances have brought into reality our capability to prepare and manipulate both internal and external states of molecules quantum mechanically, opening the field of cold molecules to a wide range of scientific explorations.

cond-mat.quant-gas↗

arXiv2023

Microwave shielding of bosonic NaRb molecules

Recent years have witnessed tremendous progresses in creating and manipulating ground-state ultracold polar molecules. However, the two-body loss regardless of the chemical reactivities is still a hurdle for many future explorations. Here, we investigate the loss suppression of non-reactive bosonic $^{23}$Na$^{87}$Rb molecules with a circular polarized microwave blue-detuned to the rotational transition. We achieve suppression of the loss by two orders of magnitude with the lowest two-body loss rate coefficient reduced to $3\times10^{-12}~\rm{cm^3/s}$. Meanwhile, the elastic collision rate coefficient is increased to the $10^{-8}~\rm{cm^3/s}$ level. The large good-to-bad collision ratio has allowed us to carry out evaporative cooling of $^{23}$Na$^{87}$Rb with an efficiency of 1.7(2), increasing the phase-space density by a factor of 10. With further improvements, this technique holds great promises for creating a Bose-Einstein condensate of ultracold polar molecules.

physics.atom-ph↗

arXiv2022

Expansion dynamics of a shell-shaped Bose-Einstein condensate

Bose-Einstein condensates (BECs) confined on shell-shaped surfaces have been proposed as a platform for exploring many nontrivial quantum phenomena on curved spaces. However, as the shell-shaped trapping potential generated with the conventional radio frequency dressing method is very sensitive to gravity, so far experimental studies of shell BECs can only be performed in micro-gravity environments. Here, we overcome this difficulty and create a shell BEC in the presence of Earth's gravity with immiscible dual-species BECs of sodium and rubidium atoms. After minimizing the displacement between the centers of mass of the two BECs with a magic-wavelength optical dipole trap, the interspecies repulsive interaction ensures the formation of a closed shell of sodium atoms with its center filled by rubidium atoms. Releasing the double BEC together from the trap, we observe explosion of the filled shell accompanied by energy transfer from the inner BEC to the shell BEC. With the inner BEC removed, we obtain a hollow shell BEC which shows self-interference as a manifestation of implosion. Our results pave an alternative way for investigating many of the intriguing physics offered by shell BECs.

cond-mat.quant-gas↗

arXiv2022

Improved characterization of Feshbach resonances and interaction potentials between $^{23}$Na and $^{87}$Rb atoms

The ultracold mixture of \Na and \Rb atoms has become an important system for investigating physics in Bose-Bose atomic mixtures and for forming ultracold ground-state polar molecules. In this work, we provide an improved characterization of the most commonly used Feshbach resonance near 347.64 G between \Na and \Rb in their absolute ground states. We form Feshbach molecules using this resonance and measure their binding energies by dissociating them via 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 $s$-wave Feshbach resonances for \Na and \Rb in different combinations of Zeeman sublevels of the $F = 1$ 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.

cond-mat.quant-gas↗

arXiv2022

Lee-Huang-Yang effects in the ultracold mixture of $^{23}$Na and $^{87}$Rb with attractive interspecies interactions

The beyond-mean-field Lee-Huang-Yang (LHY) correction is ubiquitous in dilute ultracold quantum gases. However, its effects are often elusive due to the typically much larger influence of the mean-field energy. In this work, we study an ultracold mixture of $^{23}$Na and $^{87}$Rb with tunable attractive interspecies interactions. The LHY effects manifest in the formation of self-bound quantum liquid droplets and the expansion dynamics of the gas-phase sample. A liquid-to-gas phase diagram is obtained by measuring the critical atom numbers below which the self-bound behavior disappears. In stark contrast to trapped gas-phase condensates, the gas-phase mixture formed following the liquid-to-gas phase transition shows an anomalous expansion featuring a larger release energy for increasing mean-field attractions.

cond-mat.quant-gas↗

arXiv2021

Seconds-scale coherence on nuclear spin transitions of ultracold polar molecules in 3D optical lattices

Ultracold polar molecules (UPMs) are emerging as a novel and powerful platform for fundamental applications in quantum science. Here, we report characterization of the coherence between nuclear spin levels of ultracold ground-state sodium-rubidium molecules loaded into a 3D optical lattice with a nearly photon scattering limited trapping lifetime of 9(1) seconds. After identifying and compensating the main sources of decoherence, we achieve a maximum nuclear spin coherence time of $T_2^* = 3.3(6)$~s with two-photon Ramsey spectroscopy. Furthermore, based on the understanding of the main factor limiting the coherence of the two-photon Rabi transition, we obtain a Rabi lineshape with linewidth below 0.8 Hz. The simultaneous realization of long lifetime and coherence time, and ultra-high spectroscopic resolution in our system unveils the great potentials of UPMs in quantum simulation, computation, and metrology.

physics.atom-ph↗

arXiv2021

Characterization of the lowest excited-state ro-vibrational level of $^{23}$Na$^{87}$Rb

Starting from an ultracold sample of ground-state $^{23}$Na$^{87}$Rb molecules, we investigate the lowest ro-vibrational level of the $b^3Π$ state with high resolution laser spectroscopy. This electronic spin-forbidden $X^1Σ^+ \leftrightarrow b^3Π$ transition features a nearly diagonal Franck-Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the $a^3Σ^+$ state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned to near this transition.

physics.atom-ph↗

arXiv2021

Laser Control of Singlet-Pairing Process in an Ultracold Spinor Mixture

In the mixture of ultracold spin-1 atoms of two different species A and B (e.g., $^{23}$Na (A) and $^{87}$Rb (B)), inter-species singlet-pairing process ${\rm A}_{+1}+{\rm B}_{-1}\rightleftharpoons {\rm A}_{-1}+{\rm B}_{+1}$, can be induced by the spin-dependent inter-atomic interaction, where subscript $\pm 1$ denotes the magnetic quantum number. Nevertheless, one cannot isolate this process from other spin-changing processes by tuning the bias real magnetic field. As a result, so far the singlet-pairing process have not been clearly observed in the experiments, and the measurement of the corresponding interaction strength becomes difficult. In this work we propose to control the singlet-pairing process via combining the real magnetic field and a laser-induced species-dependent synthetic magnetic field. With our approach one can significantly enhance this process and simultaneously supperess all other spin-changing processes. We illustrate our approach for both a confined two-atom system and a binary mixture of spinor Bose-Einstein condensates. Our control scheme is helpful for the precise measurement of the weakly singlet-pairing interaction strength and the entanglement generation of two different atoms.

cond-mat.quant-gas↗

arXiv2020

Observation of resonant dipolar collisions in ultracold $^{23}$Na$^{87}$Rb rotational mixtures

We report the investigation on dipolar collisions in rotational state mixtures of ultracold bosonic $^{23}$Na$^{87}$Rb molecules. The large resonant dipole-dipole interaction between molecules in rotational states of opposite parities brings about significant modifications to their collisions, even when an electric field is not present. In this work, this effect is revealed by measuring the dramatically enhanced two-body loss rate constants in the mixtures. In addition, the dipolar interaction strength can be tuned by preparing the NaRb mixture in different rotational levels with microwave spectroscopy. When the rotational level combination is not of the lowest energy, contributions from hyperfine changing collisions are also observed. Our measured loss rate constants are in good agreement with a quantum close-coupling calculation which we also present in full detail.

physics.atom-ph↗

arXiv2020

Anisotropic Polarizability of Ultracold Ground-state $^{23}$Na$^{87}$Rb Molecules

We report measurements of the ac polarizabilities of ultracold ground-state $^{23}\rm{Na}^{87}\rm{Rb}$ molecules. While the polarizability of the ground rotational state $J = 0$ is isotropic, that of the first excited rotational state $J = 1$ is anisotropic and depends strongly on the light polarization angle. We obtain both polarizabilities precisely by combining trap oscillation frequency measurement and high resolution rotational spectroscopy driven by microwave. With the optimized light polarization angle and intensity combination, the nonuniformity of the differential ac Stark shift between the two rotational states is minimized and the rotational coherence time is observed to be the longest.

physics.atom-ph↗

arXiv2020

Detection of NaRb Feshbach molecule by photodissociation

We demonstrate detection of NaRb Feshbach molecules at high magnetic field by combining molecular photodissociation and absorption imaging of the photofragments. The photodissociation process is carried out via a spectroscopically selected hyperfine Zeeman level correlated with the Na ($3P_{3/2}$) + Rb ($5S_{1/2}$) asymptote which, following spontaneous emission and optical pumping, leads to ground-state atoms in a single level with near unity probability. Subsequent to the dissociation, the number of molecules is obtained by detecting the resultant $^{23}$Na and $^{87}$Rb atoms. We have also studied the heating effect caused by the photodissociation process and optimized the detection protocol for extracting the temperature of the molecular cloud. This method enables the $in~situ$ detection of fast time scale collision dynamics between NaRb Feshbach molecules and will be a valuable capability in studying few-body physics involving molecules.

physics.atom-ph↗

arXiv2020

Manipulation of heteronuclear spin dynamics with microwave and vector light shift

We report the observation and manipulation of heteronuclear spin dynamics in a spin-1 mixture of ultracold $^{87}$Rb and $^{23}$Na atoms. The dynamics is driven by the interspecies spin-dependent interaction and shows a pronounced dependence on magnetic fields with influences from both linear and quadratic Zeeman shifts. Similar to the well-studied homonuclear cases, the interspecies spin dynamics can be controlled by tuning the quadratic Zeeman shift with far-detuned microwave fields. In addition, we successfully realize spin dynamics control with vector light shifts which act as a species-selective effective magnetic field on $^{87}$Rb atoms. Both methods show negligible loss of atoms thus will be powerful techniques for investigating spin dynamics with fast temporal and high spatial resolutions.

cond-mat.quant-gas↗

arXiv2019

Observation of resonant scattering between ultracold heteronuclear Feshbach molecules

We report the observation of a dimer-dimer inelastic collision resonance for ultracold Feshbach molecules made of bosonic sodium and rubidium atoms. This resonance, which we attribute to the crossing of the dimer-dimer threshold with a heteronuclear tetramer state, manifests itself as a pronounced inelastic loss peak of dimers when the interspecies scattering length between the constituent atoms is tuned. Near this resonance, a strong modification of the temperature dependence of the dimer-dimer scattering is observed. Our result provides insight into the heteronuclear four-body system consisting of heavy and light bosons and offers the possibility of investigating ultracold molecules with tunable interactions.

cond-mat.quant-gas↗