Session Z03: Focus Session: Frontiers of Manipulating Ultracold Molecules

Quantum-state manipulation and magic trapping of molecules for metrology

Ultracold atomic and molecular technologies are transforming our ability to perform high-precision spectroscopy and apply it to time and frequency metrology. Many of the highest-performing clocks are based on laser-cooled atoms trapped in optical lattices. These clocks can be used to research fundamental questions, for example, to improve our understanding of gravity and general relativity. Here I will discuss using pure vibrational energy states of lattice-trapped ultracold diatomic molecules as a reference for clocks. Molecules have more internal quantum states compared to atoms and therefore are relatively challenging to control. On the other hand, they offer a large number of prospective clock transitions based on different types of quantum states, and can help us probe complementary aspects of new physical interactions. I will cover the current precision limit of molecular metrology as well as possible paths forward.   

You shall not pass! Shielding and control of ultracold collisions

By virtue of moving slowly, molecules in microkelvin gases are extraordinarily sensitive to their long-range interactions.  This sensitivity can be exploited to control the way in which the molecules interact and collide, by subjecting them to electric or microwave fields.  This control includes shielding that can prevent the molecules from reacting; and “field-linked” resonances that can bind pairs of molecules non-chemically.   I discuss some of the background concepts that make this manipulation possible, with reference to its recent realization in several experiments.  The ultracold community is tired of hearing me talk about this, but if it’s new to you, you might learn something interesting!

    

Microwave Shielding of Bosonic Ground State Molecules

We report on the collisional stabilization of ultracold sodium-cesium (NaCs) molecules via microwave shielding. Through the use of a blue-detuned, circularly polarized microwave field, we prepare NaCs molecules in a dressed state that suppresses collisional losses by over two orders of magnitude.  

For samples of 3 × 10⁴ molecules with a peak density of 1 × 10¹² cm^-3, we enhance the lifetime to beyond 1 s. We study the elastic collisional properties of the dressed gas using cross-dimensional thermalization. We observe an enhancement of elastic collision rates as a result of dipolar interactions. At the same time, the rates of cross-dimensional thermalization are slower than initially expected, which we attribute to the anisotropic character of dipolar interactions. Finally, we demonstrate evaporative cooling of our gas, increasing its phase-space density by a factor of 20. 

With these advances, bosonic NaCs molecules are becoming a promising platform for quantum many-body physics, quantum simulation, and quantum computing.    

Magnetic tuning of electric dipole moments and dipolar interactions of alkali-dimer molecules

We show that the electric dipole moments (EDMs) of ground-state alkali-dimer molecules such as KRb(X¹Σ) can be efficiently tuned over a wide dynamic range (including zero) with an external dc magnetic field even at zero external dc electric field. The magnetic field tunability is enabled by the electric quadrupole interaction, which couples hyperfine sublevels within the first excited rotational states at low magnetic fields. The admixture generates non-zero transition dipole moments with hyperfine sublevels in the ground rotational states, which are otherwise suppressed at large magnetic fields. The tunability of the EDMs via magnetic switching opens up new opportunities for Hamiltonian engineering and interaction control of lattice-confined ultracold polar molecules.   

Creating dipolar molecules from Bose polarons

We report creation of dipolar ^{23}Na^{40}K molecules in their absolute ground state directly from Bose polarons, dressed K impurities immersed in a Na Bose-Einstein condensate. In contrast to Feshbach molecules, Bose polarons at negative scattering length are long-lived in the presence of the BEC. We demonstrate direct photoassociation from Bose polarons to electronically excited molecular states, dark resonance spectroscopy of the absolute ground state molecular state, and finally stimulated rapid adiabatic passage (STIRAP) of Bose polarons into the molecular ground state, all in the presence of the Na BEC. This leads to a dense molecular gas at temperatures on the order of 70 nK immersed inside a $T/T_c=0.1$ BEC.   

Coherent control of cold collisions: role of resonance

Coherent control of collisions uses superpositions of internal states of molecules (or atoms) to create interference effects that can be controlled by varying the relative phase between the states. Unlike current method to control ultracold collisions, this technique is a free-field method, avoiding large perturbations of molecular energy levels. Coherent control is usually limited by cancellation of different partial wave contributions. As shown in one of our previous works, this issue can be overcome for ultracold resonant exchange (spin, charge or excitation exchange) where only a single partial wave is involved in both the incident and final collision channels. For exothermic processes, an isolated resonance provides a solution. We will show that an improvement of control over multiple final channels can be achieved around an isolated resonance for hyperfine de-excitation in Rb-Rb scatterings.

 

    In addition, we will consider collisions that involve a high density of resonances and reach a statistical regime. Although a large extent of control can be achieved when resonances are resolved, energy averaging and the overlap of multiple resonances impacts control.