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arXiv:1311.0429 · Physical Review A

Evaporative cooling of reactive polar molecules confined in a two-dimensional geometry

Bihui Zhu, Goulven Quéméner, Ana María Rey, Murray Holland

Published Dec 10, 2013• 26 citations
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Abstract

Recent experimental developments in the loading of ultracold KRb molecules into quasi-two-dimensional traps, combined with the ability to tune the ratio between elastic and loss (inelastic, reactive) collisions through application of an external electric field, are opening the door to achieving efficient evaporative cooling of reactive polar molecules. In this paper, we use Monte Carlo simulations and semianalytic models to study theoretically the experimental parameter regimes in which evaporative cooling is feasible under current trapping conditions. We investigate the effect of the anisotropic character of dipole-dipole collisions and reduced dimensionality on evaporative cooling. We also present an analysis of the experimentally relevant antievaporation effects that are induced by chemical reactions that take place when more than one axial vibrational state is populated.

DOI
10.1103/physreva.88.063405

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Recent experimental developments in the loading of ultracold KRb molecules into quasi-two-dimensional traps, combined with the ability to tune the ratio between elastic and loss (inelastic, reactive) collisions through application of an external electric field, are opening the door to achieving efficient evaporative cooling of reactive polar molecules. In this paper, we use Monte Carlo simulations and semianalytic models to study theoretically the experimental parameter regimes in which evaporative cooling is feasible under current trapping conditions. We investigate the effect of the anisotropic character of dipole-dipole collisions and reduced dimensionality on evaporative cooling. We also present an analysis of the experimentally relevant antievaporation effects that are induced by chemical reactions that take place when more than one axial vibrational state is populated.
We consider ultracold collisions of polar molecules confined in a one-dimensional optical lattice. Using a quantum scattering formalism and a frame transformation method, we calculate elastic and chemical quenching rate constants for fermionic molecules. Taking 40K^{40}\mathrm{K}40K87Rb^{87}\mathrm{Rb}87Rb molecules as a prototype, we find that the rate of quenching collisions is enhanced at zero electric field as the confinement is increased but that this rate is suppressed when the electric field is turned on. For molecules with 500 nK of collision energy, for realistic molecular densities, and for achievable experimental electric fields and trap confinements, we predict lifetimes for KRb molecules to be 111 s. We find a ratio of elastic to quenching collision rates of about 100100100, which may be sufficient to achieve efficient evaporative cooling of polar KRb molecules.
Ultracold polar molecules possess long-range, anisotropic, and tunable dipolar interactions, providing unique opportunities to probe novel quantum phenomena. However, experimental progress has been hindered by excessive two-body loss, which also limits further cooling via evaporation. Recent work shows the loss can be mitigated by confining molecules in a two-dimensional geometry. However, a general approach for tuning molecular interactions in a full three-dimensional (3D) stable system has been lacking. Here, we demonstrate the use of an electric field-induced shielding resonance to suppress the reactive loss by a factor of 30 while preserving elastic, long-range dipolar interactions in a bulk gas of ultracold 40K87Rb molecules in 3D. The favorable ratio of elastic to inelastic collisions enables direct thermalization, the rate of which depends on the angle between the collisional axis and the dipole orientation controlled by an external electric field. This is a direct manifestation of the anisotropic dipolar interaction. We further achieve dipolar-interaction-mediated evaporative cooling in 3D. This work demonstrates control of a long-lived bulk quantum gas system with tunable long-range interactions, paving the way for the study of collective quantum many-body physics.
We investigate collisions of polar molecules in quasi-2D traps in the presence of an external electric field perpendicular to the collision plane. We use the quantum-defect model characterized by two dimensionless parameters: yyy and sss. The first of them is related to the probability of the reaction at short distances, whereas the latter one defines the wave function phase at short distances. For yyy close to unity we obtain universal collision rates determined by the quantum reflection process from the long-range part of the interaction potential that depends only on the van der Waals coefficient, dipole-dipole interaction and the trap frequency. For small short-range reaction probabilities collision rates are highly nonuniversal and trap induced shape resonances are visible. For high dipole moments we observe the damping of reactive collisions, which can stabilize the ultracold gas of polar molecules. The calculations were performed with help of multichannel wave funcion propagation by imposing short-range boundary condition derived from the quantum-defect model.
Abstract The control of molecules is key to the investigation of quantum phases, in which rich degrees of freedom can be used to encode information and strong interactions can be precisely tuned1. Inelastic losses in molecular collisions2–5, however, have greatly hampered the engineering of low-entropy molecular systems6. So far, the only quantum degenerate gas of molecules has been created via association of two highly degenerate atomic gases7, 8. Here we use an external electric field along with optical lattice confinement to create a two-dimensional Fermi gas of spin-polarized potassium–rubidium (KRb) polar molecules, in which elastic, tunable dipolar interactions dominate over all inelastic processes. Direct thermalization among the molecules in the trap leads to efficient dipolar evaporative cooling, yielding a rapid increase in phase-space density. At the onset of quantum degeneracy, we observe the effects of Fermi statistics on the thermodynamics of the molecular gas. These results demonstrate a general strategy for achieving quantum degeneracy in dipolar molecular gases in which strong, long-range and anisotropic dipolar interactions can drive the emergence of exotic many-body phases, such as interlayer pairing and p-wave superfluidity.
We study polar molecule scattering in quasi-one-dimensional geometries. Elastic and reactive collision rates are computed as a function of collision energy and electric dipole moment for different confinement strengths. The numerical results are interpreted in terms of first order scattering and of adiabatic models. Universal dipolar scattering is also discussed. Our results are relevant to experiments where control of the collision dynamics through one-dimensional confinement and an applied electric field is envisioned.