Rotational Cooling of Trapped Polyatomic Molecules

Rosa Glöckner, Alexander Prehn, Barbara G. U. Englert, Gerhard Rempe, and Martin Zeppenfeld

Phys. Rev. Lett. 115, 233001 – Published 3 December 2015

Abstract

Controlling the internal degrees of freedom is a key challenge for applications of cold and ultracold molecules. Here, we demonstrate rotational-state cooling of trapped methyl fluoride molecules ( ${CH}_{3} F$ ) by optically pumping the population of 16 $M$ sublevels in the rotational states $J = 3$ , 4, 5 and 6 into a single level. By combining rotational-state cooling with motional cooling, we increase the relative number of molecules in the state $J = 4$ , $K = 3$ , $M = 4$ from a few percent to over 70%, thereby generating a translationally cold ( $\approx 30 mK$ ) and nearly pure state ensemble of about $10^{6}$ molecules. Our scheme is extendable to larger sets of initial states, other final states, and a variety of molecule species, thus paving the way for internal-state control of ever-larger molecules.

Received 18 February 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.233001

Authors & Affiliations

Rosa Glöckner, Alexander Prehn, Barbara G. U. Englert, Gerhard Rempe, and Martin Zeppenfeld^*

Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, D-85748 Garching, Germany

^*Martin.Zeppenfeld@mpq.mpg.de

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Vol. 115, Iss. 23 — 4 December 2015

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Images

Figure 1
Simplified scheme for RSC. Optical pumping via a vibrational excited state with infrared (IR) and microwave (MW) radiation is used to accumulate population in a lower rotational state.
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Figure 2
Experimental setup and level scheme. (a) Experimental setup as described in the main text. (b) Level scheme of ${CH}_{3} F$ for the four lowest rotational states of the $| K | = 3$ manifold. Transition frequencies are given for zero electric field. As indicated by the zoom, the rotational states split in an electric field according to the linear Stark effect. Only the low-field-seeking states (positive $M$ values) are trapped with our dc electric trap. The splitting in the homogeneous-field region of our trap ( $815 V / cm$ ) is 188, 113, 75, and 54 MHz for $J = 3$ , $J = 4$ , $J = 5$ , and $J = 6$ , respectively. Additional figures of the optical pumping schemes described in the main text are provided in [28].
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Figure 3
Results for RSC. We plot the rotational-state-discriminated unloading signal detected with the quadrupole mass spectrometer. (a) A time-dependent measurement of RSC. The single black cross was measured while leaving the microwaves off (see main text). (b) Contribution of single $M$ sublevels to RSC. Only a single MW frequency is applied and scanned across all relevant transitions coupling $J = 5$ , 6 (dashed vertical lines show the calculated transition frequencies). To increase the resolution of the individual transitions, a higher electric-field strength was used. This results in a larger separation of the $M$ substates and, thus, a larger splitting of the transition frequencies. Vertical bars denote the $1 σ$ statistical error.
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Figure 4
Relative population of all trapped molecules in the $| 0; 4; 3; 4 ⟩$ state for the following experimental sequences: Reference: no state manipulation $(t_{s} = 3 s)$ ; RSC: rotational cooling as presented in Fig. 3 $(t_{s} = 6 s)$ ; $RSC + Sisyphus$ : motional cooling combined with RSC $(t_{s} = 23 s)$ ; SSP: the previous sequence with subsequent optical pumping to $| 0; 4, 3, 4 ⟩$ $(t_{s} = 25 s)$ ; $SSP + cleaning$ : in addition to SSP, removal of all states except for the $| 0; 4, 3, 4 ⟩$ state from the trap $(t_{s} = 27 s)$ . The internal-state purity increases to 70%, while the mean kinetic energy is reduced by more than an order of magnitude. The vertical solid lines represent the $1 σ$ statistical error (which is smaller than the edge of the bar if not visible). The values for uncooled ensembles overestimate the population relative to the other sequences due to an imperfect state detection [27].
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