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# Universal prethermal dynamics of Bose gases quenched to unitarity

## Abstract

Understanding strongly correlated phases of matter, such as the quark–gluon plasma and neutron stars, and in particular the dynamics of such systems, for example, following a Hamiltonian quench (a sudden change in some Hamiltonian parameter, such as the strength of interparticle interactions) is a fundamental challenge in modern physics. Ultracold atomic gases are excellent quantum simulators for these problems, owing to their tunable interparticle interactions and experimentally resolvable intrinsic timescales. In particular, they provide access to the unitary regime, in which the interactions are as strong as allowed by quantum mechanics. This regime has been extensively studied in Fermi gases1,2. The less-explored unitary Bose gases3,4,5,6,7,8,9,10,11 offer possibilities12 such as universal physics controlled solely by the gas density13,14 and new forms of superfluidity15,16,17. Here, through momentum- and time-resolved studies, we explore degenerate and thermal homogeneous Bose gases quenched to unitarity. In degenerate samples, we observe universal post-quench dynamics in agreement with the emergence of a prethermal state18,19,20,21,22,23,24 with a universal non-zero condensed fraction22,24. In thermal gases, the dynamic and thermodynamic properties generally depend on the gas density and the temperature, but we find that they can still be expressed in terms of universal dimensionless functions. Surprisingly, we find that the total quench-induced correlation energy is independent of the gas temperature. These measurements provide quantitative benchmarks and challenges for the theory of unitary Bose gases.

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## Data availability

The data that support the findings of this study are available in the Apollo repository (https://doi.org/10.17863/CAM.30242). Any additional information is available from the corresponding authors on reasonable request.

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## Acknowledgements

We thank R. Fletcher, N. Navon and T. Hilker for discussions and comments on the manuscript. This work was supported by the Royal Society, EPSRC (grant numbers EP/N011759/1 and EP/P009565/1), ERC (QBox), AFOSR and ARO. R.L. acknowledges support from the EU Marie Curie programme (grant number MSCA-IF-2015 704832) and Churchill College, Cambridge. E.A.C. acknowledges hospitality and support from Trinity College, Cambridge.

### Reviewer information

Nature thanks M. Kolodrubetz and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

Authors

### Contributions

C.E., J.A.P.G. and R.L. collected the data. C.E. analysed the data and produced the figures. C.E., E.A.C., R.P.S. and Z.H. interpreted the data and wrote the manuscript.

### Corresponding authors

Correspondence to Christoph Eigen or Zoran Hadzibabic.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Extrapolation of $${\bar{{\boldsymbol{n}}}}_{{\boldsymbol{k}}}{{\boldsymbol{k}}}_{{\boldsymbol{n}}}^{{\bf{3}}}$$ in a degenerate gas to lower k/kn.

Solid symbols show directly measured values (also shown in Fig. 2b), here combining the data for all three BEC densities. Open symbols show experimentally extrapolated values, for all three densities, as described in Methods. The solid line is the same as in Fig. 2b.

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Reprints and Permissions

Eigen, C., Glidden, J.A.P., Lopes, R. et al. Universal prethermal dynamics of Bose gases quenched to unitarity. Nature 563, 221–224 (2018). https://doi.org/10.1038/s41586-018-0674-1

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• DOI: https://doi.org/10.1038/s41586-018-0674-1

### Keywords

• Bose Gas
• Thermal Gas
• Universal Dimensionless Function
• Ultracold Atomic Gases
• Lower Hyperne Ground State

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