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# Dipolar evaporation of reactive molecules to below the Fermi temperature

## 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 collisions2,3,4,5, 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.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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## Acknowledgements

We acknowledge funding from NIST, DARPA DRINQS, ARO MURI and NSF Phys-1734006. We thank J. L. Bohn, A. M. Kaufman, and C. Miller for careful reading of the manuscript and T. Brown for technical assistance.

## Author information

Authors

### Contributions

All authors contributed to carrying out the experiments, interpreting the results, and writing the manuscript.

### Corresponding authors

Correspondence to Giacomo Valtolina or Jun Ye.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Georgy Shlyapnikov and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Layer occupancy.

Histogram of the average number per layer (relative population) for the data shown in Fig. 1c.

### Extended Data Fig. 2 Trend of ωx/(2π) versus γ.

Grey points are the experimental measurements at EDC = 5 kV cm−1, the solid grey line is a linear fit to guide the eye, and the dashed line is the prediction (Sim) from the finite-element model. All error bars are 1 standard deviation of the mean.

### Extended Data Fig. 3 Evaporation sequence.

a, Ramp in EDC. b, Ramp in γ. c, Trap depth versus time from the finite-element model of electro-optical potential. d, Evolution of η, calculated by taking the ratio of the trap depth and temperature at each time point. e, Evolution of T/TF during the ramp. All error bars are 1 standard error of the mean.

### Extended Data Fig. 4 Fermi gas thermometry.

Trend of Tout/Trel as a function of the excluded region from the centre of the Gaussian fit for T/TF = 0.81(15) (orange diamonds) and T/TF = 2.0(1) (black circles). Solid lines are Gaussian fits to simulated density profiles for T/TF = 2.0 (black) and T/TF = 0.8 (orange). All error bars are 1 standard error of the mean.

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Valtolina, G., Matsuda, K., Tobias, W.G. et al. Dipolar evaporation of reactive molecules to below the Fermi temperature. Nature 588, 239–243 (2020). https://doi.org/10.1038/s41586-020-2980-7

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