[Paper Review] Microwave shielding of bosonic NaRb molecules

Junyu Lin, Guanghua Chen+8 more|arXiv (Cornell University)|Apr 17, 2023

Cold Atom Physics and Bose-Einstein Condensates68 references4 citations

TL;DR

This paper demonstrates microwave shielding using a blue-detuned, circularly polarized microwave field to suppress two-body losses in ultracold bosonic 23Na87Rb molecules by two orders of magnitude, reducing the inelastic loss rate coefficient to $3 \times 10^{-12}~\mathrm{cm^3/s}$. The technique enhances elastic collisions and enables efficient evaporative cooling, increasing phase-space density by a factor of 10, paving the way for Bose-Einstein condensation of ultracold polar molecules.

ABSTRACT

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 $3 imes10^{-12}~ m{cm^3/s}$. Meanwhile, the elastic collision rate coefficient is increased to the $10^{-8}~ m{cm^3/s}$ 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.

Motivation & Objective

• To suppress rapid two-body losses in non-reactive bosonic 23Na87Rb molecules, which hinder the formation of quantum degenerate samples.
• To implement microwave shielding via a blue-detuned, circularly polarized microwave field to create a long-range potential barrier that prevents short-range complex formation.
• To enhance elastic collision rates to enable efficient evaporative cooling and increase phase-space density.
• To demonstrate a viable pathway toward achieving a Bose-Einstein condensate of ultracold polar molecules using loss suppression techniques.

Proposed method

• Application of a circularly polarized microwave field detuned below the $J=0 \leftrightarrow J=1$ rotational transition of 23Na87Rb molecules to induce dressed states with effective dipole moments.
• Use of a blue-detuned microwave field to generate a long-range repulsive potential barrier that suppresses molecules from approaching the short-range region where $\mathrm{Na_2Rb_2}$ complexes form.
• Employment of a double-well optical dipole trap with a 346 G magnetic field to confine the molecules and enable controlled microwave coupling.
• Measurement of cross-dimensional rethermalization dynamics to extract the elastic collision rate coefficient $\beta_{\mathrm{el}}$ from the number of collisions per thermalization $N_{\mathrm{col}}$.
• Numerical computation of scattering cross sections using multichannel quantum defect theory and $S$-matrix formalism to model inelastic and elastic processes.
• Validation of collisional dynamics using direct numerical integration of Enskog equations and cross-checked with DSMC simulations to determine $N_{\mathrm{col}}$ and $\beta_{\mathrm{el}}$.

Figure 1: Microwave shielding of 23 Na 87 Rb molecules. (a) Sketch of the experiment setup and the related energy levels. The optical dipole trap is formed by crossing two elliptical shape 1064 nm laser beams. A 346 G magnetic field in the vertical direction always presents. The blue-detuned and cir

Experimental results

Research questions

• RQ1 Can microwave shielding via a blue-detuned, circularly polarized microwave field effectively suppress two-body inelastic losses in ultracold bosonic 23Na87Rb molecules?
• RQ2 To what extent can the elastic collision rate be enhanced relative to inelastic loss rates under microwave shielding?
• RQ3 Is the resulting good-to-bad collision ratio sufficient to enable efficient evaporative cooling in a trapped molecular sample?
• RQ4 What is the quantitative reduction in the inelastic loss rate coefficient $\beta_{\mathrm{in}}$ achieved through microwave shielding?
• RQ5 Can the phase-space density of ultracold 23Na87Rb molecules be increased via evaporative cooling under microwave shielding?

Key findings

• The two-body inelastic loss rate coefficient $\beta_{\mathrm{in}}$ was suppressed to $3.0(3) \times 10^{-12}~\mathrm{cm^3/s}$, representing a two-order-of-magnitude reduction compared to unshielded conditions.
• The elastic collision rate coefficient $\beta_{\mathrm{el}}$ was measured to be at the $10^{-8}~\mathrm{cm^3/s}$ level, indicating a high good-to-bad collision ratio favorable for evaporative cooling.
• Evaporative cooling was successfully demonstrated with an efficiency of 1.7(2), increasing the phase-space density by a factor of 10.
• The number of collisions per rethermalization $N_{\mathrm{col}}$ was found to be above 10 even at large detunings, confirming sufficient collisional thermalization for accurate rate coefficient extraction.
• DSMC simulations confirmed the numerical calculations, showing consistent trends with a 15% lower $N_{\mathrm{col}}$ than direct integration, validating the theoretical model.
• The effective dipole moment $d_{\mathrm{eff}}$ was reduced with increasing detuning $\Delta$, leading to a decrease in $N_{\mathrm{col}}$, but the system retained sufficient collisional activity for cooling.

Figure 2: Loss suppression. (a) The decay of 23 Na 87 Rb number $N$ without (red squares) and with (blue circles) the microwave field. For the latter case, the detuning $\Delta$ is $2\pi\times 8~{}\rm{MHz}$ . The solid curves are for eye guiding. Error bars represent standard deviation of typically