Time reflection and refraction are temporal analogies of the spatial boundary effects derived from Fermat’s principle. They occur when classical waves strike a time boundary where an abrupt change in the properties of the medium is introduced. The main features of time-reflected and time-refracted waves are the shift in frequency and conservation of momentum, which offer a new degree of freedom for steering extreme waves and controlling the phases of matter. The concept was originally proposed for manipulating optical waves more than five decades ago. However, due to the extreme challenges in the ultrafast engineering of optical materials, the experimental realization of the time boundary effects remains elusive. Here we introduce a time boundary into a momentum lattice of ultracold atoms and simultaneously demonstrate time reflection and refraction experimentally. Through launching a Gaussian-superposed state into the Su–Schrieffer–Heeger atomic chain, we observe the time-reflected and time-refracted waves when the input state strikes a time boundary. Furthermore, we detect a transition from time reflection/refraction to localization with increasing strength of disorder and show that the time boundary effects are robust against considerable disorder. Our work opens a new avenue for the future exploration of time boundaries and spatiotemporal lattices, as well as their interplay with non-Hermiticity and many-body interactions.
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We acknowledge funding support from the National Key Research and Development Program of China under grant no. 2022YFA1404203; the National Natural Science Foundation of China under grant nos. U21A20437, 12074337, and 12174339; Zhejiang Provincial Natural Science Foundation of China under grant nos. LR21A040002 and LR23A040003; Zhejiang Provincial Plan for Science and Technology grant no. 2020C01019; the Fundamental Research Funds for the Central Universities under grant no. 2021FZZX001-02; the Excellent Youth Science Foundation Project (Overseas); and China Postdoctoral Science Foundation under grant no. 2023M733122.
The authors declare no competing interests.
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Dong, Z., Li, H., Wan, T. et al. Quantum time reflection and refraction of ultracold atoms. Nat. Photon. (2023). https://doi.org/10.1038/s41566-023-01290-1