Tuning interactions between static-field-shielded polar molecules with microwaves

TL;DR

The paper addresses the challenge of achieving tunable, collisionally stable interactions in static-field-shielded polar-molecule gases. It introduces circularly polarized microwave dressing to modify the long-range dipole-dipole potential, enabling control over the s-wave scattering length and dipole length in CaF without sacrificing shielding. A compensation point at a specific Delta/Omega ratio (approximately 1.3) yields d_eff = 0 and a purely repulsive long-range interaction, while increasing Omega reduces two-body losses, resulting in loss rates as low as ~2.4e-13 cm^3/s and potentially multi-second lifetimes at typical densities. This approach significantly expands accessible interaction regimes for dipolar molecular gases and is likely applicable to other ultracold polar molecules beyond CaF.

Abstract

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.

Figures (4)

• Figure 1: Energy-level diagram for pairs of CaF molecules. The highlighted states are labelled by their rotational quantum numbers $(\tilde{n}, \lvert m_n \rvert)$. We apply a near-resonant microwave field with circular polarisation, which couples $(1,0)$ to $(1,1)$.
• Figure 2: Adiabatic potential curves for two CaF molecules at a static electric field of $F=22.5\,\mathrm{kV/cm}$, showing curves only for $L=0$ and 2. (a) No microwave field ($\Omega=0$). The red dashed curve is the potential for s-wave scattering that correlates with $(1,0)$ + $(1,0)$. (b) A weak microwave field ($\Omega=1\,\mathrm{MHz}$, $\Delta/\Omega = 1.3$); other pair states dressed by the microwave field are now close in energy to the incoming state. (c) A stronger microwave field ($\Omega=10\,\mathrm{MHz}$, $\Delta/\Omega=1.3$); the energy separation between the incoming and microwave-dressed pair states is larger. (d) Expanded view of the highlighted curves in (a) and (c). The shallow potential well that exists for $\Omega=0$ disappears at the compensation point $\Delta/\Omega=1.3$.
• Figure 3: Rate coefficients for elastic scattering and total loss, obtained from coupled-channel calculations at the compensation point $\Delta/\Omega = 1.3$, as a function of $\Omega$.
• Figure 4: Characteristics of the two-body system as a function of $\Delta/\Omega$, calculated at $\Omega=60\,\mathrm{MHz}$. (a) The real part $\alpha$ of the s-wave scattering length, and the dipole length $a_\mathrm{d}$. (b) Depth of the long-range potential well for the incoming state with $L=0$. (c) Rate coefficients for elastic scattering and total loss, obtained from coupled-channel calculations.