# Diffraction of strongly interacting molecular Bose-Einstein condensate from standing wave light pulses

### Submission summary

 As Contributors: Chen Li · Qi Liang · Pradyumna Paranjape · Jörg Schmiedmayer Arxiv Link: https://arxiv.org/abs/2201.01620v2 (pdf) Date accepted: 2022-04-25 Date submitted: 2022-04-01 16:41 Submitted by: Li, Chen Submitted to: SciPost Physics Academic field: Physics Specialties: Atomic, Molecular and Optical Physics - Experiment Quantum Physics Approach: Experimental

### Abstract

We study the effects of strong inter-particle interaction on diffraction of a Bose-Einstein condensate of $^6Li_2$ molecules from a periodic potential created by pulses of a far detuned optical standing wave. For short pulses we observe the standard Kapitza-Dirac diffraction, with the contrast of the diffraction pattern strongly reduced for very large interactions due to interaction dependent loss processes. For longer pulses diffraction shows the characteristic for matter waves impinging on an array of tubes and coherent channeling transport. We observe a slowing down of the time evolution governing the population of the momentum modes caused by the strong atom interaction. A simple physical explanation of that slowing down is the phase shift caused by the self-interaction of the forming matter wave patterns inside the standing light wave. Simple 1D mean field simulations qualitatively capture the phenomenon, however to quantitatively reproduce the experimental results the molecular scattering length has to be multiplied by factor of 4.2. In addition, two contributions to interaction-dependent degradation of the coherent diffraction patterns were identified: (i) in-trap loss of molecules during the lattice pulse, which involves dissociation of Feshbach molecules into free atoms, as confirmed by radio-frequency spectroscopy and (ii) collisions between different momentum modes during separation. This was confirmed by interferometrically recombining the diffracted momenta into the zero-momentum peak, which consequently removed the scattering background.

Published as SciPost Phys. 12, 154 (2022)

We thank the referees for the constructive suggestions. Their comments are addressed with the changes listed below. And more details are described in our replies to the referees.

### List of changes

- According to Referee 1's comment 1 and 5, in Sec. 4.1 Para. 6, we add additional discussion on effective reduction of the lattice depth.
- According to Referee 1's comment 2, in Appendix. A1 Para 6, we state the detail of lattice depth calibration.
- According to Referee 1's comment 3, in caption of Fig.3 and 4, we clarify the calculation of theory curves and the use of free parameters.
- According to Referee 1's comments, in Sec. 4.1 Para 2, explanation given for different lattice depth chosen.
- According to Referee 1's comments, in Fig.4, a zoom on the relevant region is added to the figure, and the caption changed accordingly.
- According to Referee 2's comments, in Sec. 1 Para 2, we add discussion of strongly interacting molecular system.
- According to Referee 2's comment 1, in abstract, we clarify the discussion on mean field simulation.
- According to Referee 2's comment 2, in Sec. 4.2 Para. 5, we comment on secondary collisions.
- According to Referee 2's comment 5, in Fig.6 caption, the factor $\eta$ is explained.
- According to Referee 2's comment 6, in Sec. 4.2 Para 3, we add a discussion on different contributions of loss.
- According to Referee 2's comment 7, in Appendix. A3, we include quantitative information for the GPE simulation.
- Correction of typos.
- Figure improvement according to Referee 1’s comments.

### Submission & Refereeing History

#### Published as SciPost Phys. 12, 154 (2022)

Resubmission 2201.01620v2 on 1 April 2022
Submission 2201.01620v1 on 6 January 2022

## Reports on this Submission

### Report

I am happy with the answers provided to my questions and with the revised manuscript. This work pushes the widely used technique of diffracting matter waves from a standing light wave into the regime of a strongly interacting quantum gas. It raises some interesting question and thus I have no doubt that this work will inspire interesting and valuable follow-up work. I support publication of the current version of the manuscript in SciPost Physics.

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### Report

The authors addressed all comments and suggestions satisfactorily, including an added discussion of effectively reduced lattice depth, a new version of Fig. 4, updated figures, and corrected typos.

I can recommend the manuscript for publication as-is.

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