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# Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas

## Abstract

The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations1,2, high-resolution spectroscopy of complex molecules3,4,5, cold chemistry6,7 and astrochemistry8. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion9,10 or a cryogenic trap environment11,12. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation6,7,13,14,15,16,17,18,19,20,21,22,23. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH+ molecular ions, which have been trapped and sympathetically cooled13 in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second—that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings—we have cooled a single molecular ion to a rotational temperature of  kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions24. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy25 of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest4,5.

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## Acknowledgements

M.D. appreciates generous support through the Danish National Research Foundation Center for Quantum Optics – QUANTOP; The Danish Agency for Science, Technology and Innovation; the Carlsberg Foundation; the Lundbeck Foundation; and the European Commission under the Seventh Framework Programme FP7 GA 607491 COMIQ. O.O.V., M.S., L.K. and A.W. acknowledge funding from STSM travel grants from COST-Action IOTA. Finally, the MPIK mechanical workshops have been of crucial importance for the construction of the cryogenic trap.

## Author information

Authors

### Contributions

L.K., O.O.V., A.K.H., S.B.K. and A.G. contributed equally to the work by carrying out the experiments, taking part in the data analysis and writing the manuscript. M.S., A.W., J.U. and J.R.C.L.-U. contributed significantly by designing and constructing the cryogenically cooled trap. M.D. contributed to the trap design, had the idea for the experiment, devised the experimental protocol, led the data analysis and wrote most of the manuscript.

### Corresponding author

Correspondence to M. Drewsen.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

This file contains Supplementary Text. (PDF 409 kb)

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Hansen, A., Versolato, O., Kłosowski, Ł. et al. Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas. Nature 508, 76–79 (2014). https://doi.org/10.1038/nature12996

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• DOI: https://doi.org/10.1038/nature12996