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Physical Review A

Dipolar collisions of ultracold polar molecules in a microwave field

А. V. Avdeenkov

Published Aug 14, 2012• 15 citations
Publisher

Abstract

Collisions at ultracold temperatures between ${}^{1}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Sigma}$-state diatomic polar molecules in a circularly polarized microwave field are theoretically analyzed. We demonstrate that elastic and inelastic collisions of polar molecules at ultracold temperatures are affected by an external microwave field and that they are enhanced at resonance frequencies. We show that the collision dynamics of polar molecules in a microwave field is largely determined by the long-range dipole-dipole interaction. Through this interaction, two polar molecules can resonantly exchange internal energy by undergoing a transition between neighboring rotational levels. This process may stabilize the system and enable the successive evaporative cooling of molecules. This paper discusses fermionic polar molecules at ultralow temperatures (1 $\ensuremath{\mu}$K).

DOI
10.1103/physreva.86.022707

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Collisions at ultracold temperatures between 1\ensuremathΣ{}^{1}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Sigma}1\ensuremathΣ-state diatomic polar molecules in a circularly polarized microwave field are theoretically analyzed. We demonstrate that elastic and inelastic collisions of polar molecules at ultracold temperatures are affected by an external microwave field and that they are enhanced at resonance frequencies. We show that the collision dynamics of polar molecules in a microwave field is largely determined by the long-range dipole-dipole interaction. Through this interaction, two polar molecules can resonantly exchange internal energy by undergoing a transition between neighboring rotational levels. This process may stabilize the system and enable the successive evaporative cooling of molecules. This paper discusses fermionic polar molecules at ultralow temperatures (1 \ensuremathμ\ensuremath{\mu}\ensuremathμK).
The collisions at ultracold temperatures between 1Σ state diatomic polar molecules in a microwave field with a circular polarization are theoretically analyzed. We demonstrate the possibility of trapping polar molecules in the standing-wave electromagnetic field and a successful evaporative cooling not only for samples of molecules in their absolute ground states but in some rotationally excited states in the appropriate regions of frequencies of a field. Here we only discuss fermionic polar molecules at ultralow temperatures(1μK), considering RbK as a prototype.
We theoretically study collisions between ultracold polar $^{1}\mathrm{\ensuremath{\Sigma}}$ molecules that are polarized by microwave or static electric fields. We systematically study the dependence on field strength, microwave polarization, and detuning from rotational transitions. We calculate the loss in two-body collisions that is observable experimentally and compare with the results expected for purely first-order dipolar interactions. For ground-state molecules polarized by a static electric field, the dynamics are accurately described by first-order dipolar interactions. For microwave dressing, instead, resonant dipolar collisions dominate the collision process, in which molecules reorient along the intermolecular axis and interact with the full strength of the transition dipole. For red detuning, reorientation can only be suppressed at extreme Rabi frequencies. For blue detuned microwaves, resonant dipolar interactions dominate even for the highest Rabi frequencies, leading to microwave shielding for circular polarization and structured losses due to resonances for linear polarization. The results are presented numerically for fermionic 23Na40K^{23}\mathrm{Na}^{40}\mathrm{K}23Na40K and bosonic 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K molecules.
The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. The microwave trap suggested by DeMille [1] seems to be rather advantageous in comparison with other traps. Here we have demonstrated that the microwave trap can provide a successful evaporative cooling for polar molecules in a rather broad range of frequencies of the AC-field. We suggested that not only ground state polar molecules but also molecules in some other states can be safely trapped. But the state in which molecules can be safely loaded and trapped depends on the frequency of the AC-field. 1 I.
The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. The microwave trap suggested by DeMille et al (2004 Eur. Phys. J. D 31 375) seems to be rather advantageous in comparison with other traps. Here we have demonstrated that the microwave trap can provide successful evaporative cooling for polar molecules in a rather broad range of frequencies of the ac field. We suggest that not only ground-state polar molecules but also molecules in some other states can be safely trapped. But the state in which molecules can be safely loaded and trapped depends on the frequency of the ac field.
We analyze the temporal evolution of the population of ultracold polar molecules in a microwave (mw) field with a circular polarization. The molecules are in their ground state and treated as rigid rotors with a permanent dipole moment which interact with each other via the dipole–dipole (DD) interaction V dd . The mw field mixes states with different quantum and photon numbers and the collisional dynamics in the mw field is mostly controlled by the ratios of the mw field frequency versus the rotational constant, and mw field Rabi frequency versus the rotational constant. There exists a special scattering process which is elastic by nature and due to a rotational energy exchange between the ground and the first excited rotational states. To analyze dynamics of polar molecules system in the mw field the equation of motion for the bare and dressed states is solved under different mw field parameters and molecular gas characteristics. Depending on the ratio of the Rabi frequency of a mw field and the magnitude of the DD interaction, beatings and oscillations occur in the bare and dressed states time-development. At a certain relation between the magnitudes of the mw detuning δ and the DD interaction , peak structures appear in the population of the excited bare state. Each peak is associated with an avoided crossing between the dressed states adiabatic curves at the same position of mw detuning.
Abstract Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter 1–9 , implement quantum information schemes 10–12 and test the fundamental symmetries of nature 13 . Realizing their full potential requires cooling interacting molecular gases deeply into the quantum-degenerate regime. However, the intrinsically unstable collisions between molecules at short range have so far prevented direct cooling through elastic collisions to quantum degeneracy in three dimensions. Here we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium–potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such cold and dense samples of polar molecules open the path to the exploration of many-body phenomena with strong dipolar interactions.