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arXiv:2607.25777

Tunable state-dependent interactions in collisionally stable mixtures of polar molecules

Hubert J. Jóźwiak, Hanwei Yang, Eugen Dizer, Arthur Christianen, Tijs Karman

cond-mat.quant-gasphysics.atom-phphysics.chem-phquant-phSubmitted Jul 28, 2026• Announced Jul 29, 2026• 0 citations• Revised Jul 31, 2026 (v2)
v1 · Jul 28, 2026v2 · Jul 31, 2026v3 · Oct 20, 2026
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Abstract

We propose encoding a pseudo-spin-$1/2$ system in the ground ($v=0$) and first excited ($v=1$) vibrational states of polar molecules. Double microwave shielding simultaneously shields molecules in both states, suppressing two-body losses by orders of magnitude while strictly avoiding three-body recombination. The microwave dressing is state-dependent and results in highly tunable, long-range dipolar Ising exchange ($J_z$), density-density ($V$), and spin-density ($W$) interactions. These interaction length scales readily exceed the typical interparticle spacing, pushing the molecules deep into the strongly interacting regime. In bulk gases, this enables the exploration of itinerant quantum magnetism and quantum droplets with novel anisotropic spin textures; in optical lattices, it naturally realizes extended Hubbard and $t$-$J_z$ models, opening new directions in quantum simulation.

Comments
8 pages, 8 figures

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We propose encoding a pseudo-spin-1/21/21/2 system in the ground (v=0v=0v=0) and first excited (v=1v=1v=1) vibrational states of polar molecules. Double microwave shielding simultaneously shields molecules in both states, suppressing two-body losses by orders of magnitude while strictly avoiding three-body recombination. The microwave dressing is state-dependent and results in highly tunable, long-range dipolar Ising exchange (JzJ_zJz​), density-density (VVV), and spin-density (WWW) interactions. These interaction length scales readily exceed the typical interparticle spacing, pushing the molecules deep into the strongly interacting regime. In bulk gases, this enables the exploration of itinerant quantum magnetism and quantum droplets with novel anisotropic spin textures; in optical lattices, it naturally realizes extended Hubbard and ttt-JzJ_zJz​ models, opening new directions in quantum simulation.
We develop double microwave shielding, which has recently enabled evaporative cooling to the first Bose-Einstein condensate of polar molecules [Bigagli , Nature , 289 (2024)]. Two microwave fields of different frequency and polarization are employed to effectively shield polar molecules from inelastic collisions and three-body recombination. Here, we describe in detail the theory of double microwave shielding. We demonstrate that double microwave shielding effectively suppresses two- and three-body losses. Simultaneously, dipolar interactions and the scattering length can be flexibly tuned, enabling comprehensive control over interactions in ultracold gases of polar molecules. We show that this approach works universally for a wide range of molecules. This opens the door to studying many-body physics with strongly interacting dipolar quantum matter.
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.
We show that microwave shielding, which enables collisionally stable ultracold polar molecules, also leads to genuine nonadditive three-body interactions comparable to the shielding interaction itself. Their magnitude, sign, and geometrical dependence can be flexibly controlled through microwave parameters and orientation. Interaction strengths can reach the ten kilohertz-scale for ultrapolar molecules in typical optical lattices. The nonadditive three-body interactions can stabilize novel many-body phases, and should be accounted for in quantum simulation with microwave shielded molecules.
Abstract Ultracold polar molecular gases provide a powerful platform for exploring quantum many-body physics with strong, long-range, and anisotropic interactions. In this work, we develop an extended Gross-Pitaevskii approach tailored to bosonic dipolar molecules under single-microwave shielding, incorporating their effective interactions and adapting the quantum fluctuation corrections. We benchmark this beyond-mean-field theory against exact path-integral Quantum Monte Carlo simulations. Focusing on the regime of positive scattering lengths, we find excellent agreement across a range of quantum phases, including superfluid, supersolid, and droplet states. We show that elliptic microwave polarization induces anisotropic superfluidity with direction-dependent sound velocities along each spatial axis—an effect absent in atomic dipolar gases. A quasi-one-dimensional theory captures roton softening and predicts roton instabilities tunable via ellipticity. While most experiments rely on double-microwave shielding to reduce losses, we demonstrate that single-shielded molecules already support rich and tunable many-body behavior. Our framework is readily extendable to the double-shielded case. This work establishes a versatile theoretical foundation for ultracold molecular gases and opens the door to future studies with more advanced shielding protocols.
We propose a method to realize stable, tunable two-component quantum mixtures of ultracold polar molecules. First, we show that a pair of polar molecules in two distinct rovibrational states exhibits a repulsive interaction, thereby leading to collisional shielding without requiring any external field. We refer to this as "vibrational shielding". This intercomponent interaction is tunable by an external static electric or a microwave field, with the latter stabilizing both inter- and intracomponent interactions in a two-component bulk mixture. Additionally, we show that two microwave fields can independently tune the interactions of the individual components. Our findings thus open the door to the experimental realization of tunable quantum mixtures using polar molecules, analogous to tunable magnetic Feshbach resonances in two-component atomic quantum gases.
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.