ECAMP 15

The full understanding and modeling of few-body systems remains a long-standing challenge in several areas of science, particularly in quantum physics. The ability to create and manipulate dilute gases at ultracold temperatures, composed of particles with kinetic energies E = k$_B$T << 1 mK opened novel opportunities in this respect. The growing availability of quantum gases of ultracold polar molecules (i.e., possessing a permanent electric dipole moment in their own frame) in various laboratories revealed a very peculiar situation in the context of few-body physics: At ultracold energies, two such molecules in their absolute ground level (i.e., in the lowest rovibrational and hyperfine level of their electronic ground state) collide with a universal collisional rate, even if they have no inelastic or reactive energetically allowed channels, so they leave the molecular trap with a short characteristic time. Such a four-body system, which might appear relatively simple at first glance, is not yet fully characterized. Rather than attempting to fully describe this four-body system in order to identify the exact cause of the universal loss rate, one can design protocols where molecules do not reach short distances during their collision.
The goal of our theoretical work is to find ways to suppress inelastic or reactive processes between colliding particles in ultracold quantum gases [1]. Besides microwave coupling recently used for collisional shielding [2,3,4], we propose a method to engineer repulsive long-range interactions between ultracold ground-state molecules using optical fields, thus preventing short-range collisional losses. The process is modeled for a two-photon Raman resonance blue-detuned with respect to excited electronic state. It allows taking advantage of optically driven transitions including insensitivity to polarization and flexibility in the choice of electronic states, while suppressing undesired off-resonant photon scattering which was present in the previously proposed one-photon optical shielding (1-OS) [5]. The proposed protocol, exemplified for $^{23}$Na$^{39}$K, should be applicable to a broad class of polar diatomic molecules as well [6].

[1] Guo, M., Zhu, B., Lu, B., Ye, X., Wang, F., Vexiau, R., ... & Wang, D. (2016). Creation of an ultracold gas of ground-state dipolar na 23 rb 87 molecules. Physical review letters, 116(20), 205303.
[2] Lassablière, L., & Quéméner, G. (2018). Controlling the scattering length of ultracold dipolar molecules. Physical Review Letters,121(16), 163402.
[3] Karman, T., & Hutson, J. M. (2018). Microwave shielding of ultracold polar molecules. Physical review letters, 121(16), 163401.
[4] Schindewolf, A., Bause, R., Chen, X. Y., Duda, M., Karman, T., Bloch, I., & Luo, X. Y. (2022). Evaporation of microwave-shielded polar molecules to quantum degeneracy. Nature, 607(7920), 677-681.
[5] Xie, T., Lepers, M., Vexiau, R., Orbán, A., Dulieu, O., & Bouloufa-Maafa, N. (2020). Optical shielding of destructive chemical reactions between ultracold ground-state NaRb molecules. Physical Review Letters, 125(15), 153202.
[6] Karam, C., Vexiau, R., Bouloufa-Maafa, N., Dulieu, O., Lepers, M., zum Alten Borgloh, M. M., ... & Karpa, L. (2023). Two-photon optical shielding of collisions between ultracold polar molecules. Physical Review Research, 5(3), 033074.