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arXiv:0904.4144 · Physical Review A

Optoelectrical cooling of polar molecules

Martin Zeppenfeld, M. Motsch, Pepijn W. H. Pinkse, Gerhard Rempe

Published Oct 5, 2009• 52 citations
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Abstract

We present an optoelectrical cooling scheme for polar molecules based on a Sisyphus-type cooling cycle in suitably tailored electric trapping fields. Dissipation is provided by spontaneous vibrational decay in a closed level scheme found in symmetric-top rotors comprising six low-field-seeking rovibrational states. A generic trap design is presented. Suitable molecules are identified with vibrational decay rates on the order of 100 Hz. A simulation of the cooling process shows that the molecular temperature can be reduced from 1 K to 1 mK in approximately 10 s. The molecules remain electrically trapped during this time, indicating that the ultracold regime can be reached in an experimentally feasible scheme.

DOI
10.1103/physreva.80.041401

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We present an optoelectrical cooling scheme for polar molecules based on a Sisyphus-type cooling cycle in suitably tailored electric trapping fields. Dissipation is provided by spontaneous vibrational decay in a closed level scheme found in symmetric-top rotors comprising six low-field-seeking rovibrational states. A generic trap design is presented. Suitable molecules are identified with vibrational decay rates on the order of 100 Hz. A simulation of the cooling process shows that the molecular temperature can be reduced from 1 K to 1 mK in approximately 10 s. The molecules remain electrically trapped during this time, indicating that the ultracold regime can be reached in an experimentally feasible scheme.
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In this thesis, direct cooling of gaseous formaldehyde (H2CO) to the ultracold regime (T < 1 mK) is experimentally demonstrated. Employing optoelectrical Sisyphus cooling, a dissipative cooling method applicable to electrically trapped dipolar molecules, an ensemble of about 300,000 molecules with a temperature of around 400 µK is generated. More than 80% of these molecules populate a single rotational state. These results are used to implement a technique for high-resolution spectroscopy on electrically trapped molecules.
We present a study of Sisyphus cooling of molecules: the scattering of a single-photon remove a substantial amount of the molecular kinetic energy and an optical pumping step allow to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperature. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using Kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase space density by a factor of 10^7 . Even in the case of molecules with both low Franck-Condon coefficients and a non-closed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperature in few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam
We present a study of Sisyphus cooling of molecules: The scattering of a single photon removes a substantial amount of the molecular kinetic energy and an optical pumping step allows one to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperatures. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase-space density by a factor of 107{}^{7}7. Even in the case of molecules with both low Franck-Condon coefficients and a nonclosed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperatures in a few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam.
Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid relaxation rates. Although atoms are routinely laser cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational-vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here, we cool trapped AlH+ molecules to their ground rotational-vibrational quantum state using an electronically-exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH+. We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3.8(+0.9/-0.3) K, with the ground state population increasing from ~3% to 95.4(+1.3/-2.0) %. This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications, and precision tests of fundamental symmetries.