Research

Ultracold Polar Molecules

Polar molecules can interact via the strong, long-range, and anisotropic electric dipole-dipole interaction (DDI). Because of this, ultracold polar molecules can be used in a broad range of applications including quantum simulation, quantum computation, and quantum metrology. In additoin, ultracold polar molecules also provide a nature platform for investigating chemical reaction at the single partial wave level.

In our lab, the molecule of choice is the bosonic ²³Na⁸⁷Rb, which has a rather large dipole moment of 3.3 Debye. Because of its relatively small rotational constant, it is also quite easy to polarize. At a field of only 5kV/cm, the effective dipole is ~ 2 Debye. This gives us the ability to tune the inter-molecular interactions over a large range.

In their lowest energy state, NaRb molecules are stable against two-body exchange chemical reactions. This provides us the freedom to study ultracold molecular collisions with distinguishable inelastic channels. For instance, by preparing molecules in higher nuclear spins, rotational or vibrational levels and observing the respective losses, we could verify the universal collision model for ultracold molecules in different scenarios.

For the ground state molecule preparation, we plan to follow a 3-steps scheme

1. Prepare ultracold atomic mixtures;
2. Form weakly bound heteronuclear molecules with Feshbach association;
3. Coherent population transfer to the ground state by STIRAP.

Recently, we have successfully created a sample of ground-state NaRb molecules. A dipole moment of > 1 Debye is also demonstrated. For details, please see our paper on Physical Review Letters 116, 205303(2016). The STIRAP path is as shown in this figure.

Heteronuclear quantum droplet

Zhichao Guo, Fan Jia, Lintao Li, Yinfeng Ma, Jeremy M. Hutson, Xiaoling Cui, Dajun Wang, “Lee-Huang-Yang effects in the ultracold mixture of ²³Na and ⁸⁷Rb with attractive interspecies interactions ”, Phys. Rev. Research 3, 033247 (2021)

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 ²³Na and ⁸⁷Rb 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.

The Na-Rb spinor system

We report the observation of coherent heteronuclear spin dynamics driven by interspecies spin-spin interaction in an ultracold spinor mixture, which manifests as periodical and well-correlated spin oscillations between two atomic species. In particular, we investigate the magnetic field dependence of the oscillations and find a resonance behavior which depends on both the linear and quadratic Zeeman effects and the spin-dependent interaction. We also demonstrate a unique knob for controlling the spin dynamics in the spinor mixture with species-dependent vector light shifts.