Li, JiamingHarter, Andrew K.Liu, Jide Melo, LeonardoJoglekar, Yogesh N.Luo, Le2019-07-292019-07-292019-02-20Li, J., Harter, A. K., Liu, J., de Melo, L., Joglekar, Y. N., & Luo, L. (2019). Observation of parity-time symmetry breaking transitions in a dissipative Floquet system of ultracold atoms. Nature communications, 10(1), 855. doi:10.1038/s41467-019-08596-1https://hdl.handle.net/1805/20005Open physical systems with balanced loss and gain, described by non-Hermitian parity-time [Formula: see text] reflection symmetric Hamiltonians, exhibit a transition which could engender modes that exponentially decay or grow with time, and thus spontaneously breaks the [Formula: see text]-symmetry. Such [Formula: see text]-symmetry-breaking transitions have attracted many interests because of their extraordinary behaviors and functionalities absent in closed systems. Here we report on the observation of [Formula: see text]-symmetry-breaking transitions by engineering time-periodic dissipation and coupling, which are realized through state-dependent atom loss in an optical dipole trap of ultracold 6Li atoms. Comparing with a single transition appearing for static dissipation, the time-periodic counterpart undergoes [Formula: see text]-symmetry breaking and restoring transitions at vanishingly small dissipation strength in both single and multiphoton transition domains, revealing rich phase structures associated to a Floquet open system. The results enable ultracold atoms to be a versatile tool for studying [Formula: see text]-symmetric quantum systems.en-USAttribution-NonCommercial-NoDerivs 3.0 United StatesOpen physical systemsHamiltoniansUltracold atomsFloquet open systemsArticle