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arXiv:1908.01759 · Physical Review A

Microwave shielding of ultracold polar molecules with imperfectly circular polarization

Tijs Karman, Jeremy M. Hutson

Published Nov 14, 2019• 38 citations
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Abstract

We investigate the use of microwave radiation to produce a repulsive shield between pairs of ultracold polar molecules and prevent collisional losses that occur when molecular pairs reach short range. We carry out coupled-channels calculations on $\mathrm{RbCs}+\mathrm{RbCs}$ and $\mathrm{CaF}+\mathrm{CaF}$ collisions in microwave fields. We show that effective shielding requires predominantly circular polarization but can still be achieved with elliptical polarization that is around 90% circular.

DOI
10.1103/physreva.100.052704

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We investigate the use of microwave radiation to produce a repulsive shield between pairs of ultracold polar molecules and prevent collisional losses that occur when molecular pairs reach short range. We carry out coupled-channels calculations on RbCs+RbCs\mathrm{RbCs}+\mathrm{RbCs}RbCs+RbCs and CaF+CaF\mathrm{CaF}+\mathrm{CaF}CaF+CaF collisions in microwave fields. We show that effective shielding requires predominantly circular polarization but can still be achieved with elliptical polarization that is around 90% circular.
We propose a method to engineer repulsive long-range interactions between ultracold ground-state molecules using optical fields, thus preventing short-range collisional losses. It maps the microwave coupling recently used for collisional shielding onto a two-photon transition and takes advantage of optical control techniques. In contrast to one-photon optical shielding [Xie et al., Phys. Rev. Lett. 125, 153202 (2020)], this scheme avoids heating of the molecular gas due to photon scattering. The proposed protocol, exemplified for 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K, should be applicable to a large class of polar diatomic molecules.
Harnessing the potential wide-ranging quantum science applications of molecules will require control of their interactions. Here, we used microwave radiation to directly engineer and tune the interaction potentials between ultracold calcium monofluoride (CaF) molecules. By merging two optical tweezers, each containing a single molecule, we probed collisions in three dimensions. The correct combination of microwave frequency and power created an effective repulsive shield, which suppressed the inelastic loss rate by a factor of six, in agreement with theoretical calculations. The demonstrated microwave shielding shows a general route to the creation of long-lived, dense samples of ultracold polar molecules and evaporative cooling.
Double microwave shielding employs σ+σ^{+}σ+- and πππ-polarized microwave fields, tuned close to the lowest rotational transition, to engineer a long-range repulsive barrier between polar molecules. By preventing molecules from reaching the short range, this technique suppresses detrimental two-body losses and recently enabled the realization of molecular Bose-Einstein condensates and self-bound droplets. Yet, the optimal operating regimes of the shielding mechanism remain largely unexplored. Here, by leveraging the underlying universality of the scattering problem, we systematically map the four-dimensional microwave parameter space-spanned by the detunings and intensities of the two fields-to identify configurations that maximize both shielding efficiency and interaction tunability. We define optimal operating regimes as configurations that are strictly free of field-linked bound states while sufficiently suppressing two-body losses to exceed typical lifetimes of ultracold samples. In these regimes, we evaluate the elastic-to-inelastic collision ratios required for efficient evaporative cooling and explore the accessible tuning range of the effective dipolar interactions. Finally, to identify the best platforms for future quantum simulation experiments, we conduct a global survey of candidate molecular species under realistic field constraints. We identify heavy, strongly dipolar molecules as the most promising candidates, demonstrating that they can achieve extreme loss suppression-reaching two-body loss rates as low as 10−1810^{-18}10−18 cm3^{3}3/s-alongside robust interaction tunability using only moderate field strengths.
Microwave shielding of ultracold polar molecules on the transition n=1→2\boldsymbol{n=1 \rightarrow 2}n=1→2
We show that microwave shielding on the rotational transition n=1→2n=1\rightarrow 2n=1→2 can be effective in preventing destructive collisions between ultracold polar molecules. It is slightly less efficient than shielding on the transition 0→10\rightarrow 10→1, but has some important advantages. In particular, it does not produce 2-molecule bound states under the conditions needed for shielding, so it will not enhance 3-body recombination. It thus obviates the need for double-field microwave shielding using a second field of different polarization.
The ability to tune interparticle interactions is one of the main advantages of using ultracold quantum gases for quantum simulation of many-body physics. Current experiments with ultracold polar molecules employ shielding with microwave or static electric fields to prevent destructive collisional losses. The interaction potential of microwave-shielded molecules can be tuned by using microwaves of two different polarisations, while for static-field-shielded molecules the tunability of interactions is more limited and depends on the particular species. In this work, we propose a general method to tune the interactions between static-field-shielded molecules by applying a microwave field. We carry out coupled-channel scattering calculations in a field-dressed basis set to determine loss rate coefficients and scattering lengths. We find that both the s-wave scattering length and the dipole length can be widely tuned by changing the parameters of the microwave field, while maintaining strong suppression of lossy collisions.