We report the experimental observation of tunable, non-reciprocal quantum transport of a Bose-Einstein condensate in a momentum lattice. By implementing a dissipative Aharonov-Bohm (AB) ring in momentum space and sending atoms through it, we demonstrate a directional atom flow by measuring the momentum distribution of the condensate at different times. While the dissipative AB ring is characterized by the synthetic magnetic flux through the ring and the laser-induced loss on it, both the propagation direction and transport rate of the atom flow sensitively depend on these highly tunable parameters. We demonstrate that the non-reciprocity originates from the interplay of the synthetic magnetic flux and the laser-induced loss, which simultaneously breaks the inversion and the time-reversal symmetries. Our results open up the avenue for investigating non-reciprocal dynamics in cold atoms, and highlight the dissipative AB ring as a flexible building element for applications in quantum simulation and quantum information. arXiv:2001.01859v1 [cond-mat.quant-gas]

The Su-Schrieffer-Heeger (SSH) model perhaps is the easiest and the most basic model for topological excitations. Many variations and extensions of the SSH model have been proposed and explored to better understand both fundamental and novel aspects of topological physics. The SSH4 model has been proposed theoretically as an extended SSH model with higher dimension (the internal dimension changes from two to four). It has been proposed that the winding number in this system can be determined through a higherdimensional extension of the mean chiral displacement measurement, however this has not yet been verified in experiment. Here we report the realization of this model with ultracold atoms in a momentum lattice.We verify the winding number through measurement of the mean chiral displacement in a system with higher internal dimension, we map out the topological phase transition in this system, and we confirm the topological edge state by observation of the quench dynamics when atoms are initially prepared at the system boundary. * yanbohang@zju.edu.cn 1 arXiv:1906.12019v1 [cond-mat.quant-gas]

We report the experimental implementation of discrete-time topological quantum walks of a Bose-Einstein condensate in momentum space. Introducing stroboscopic driving sequences to the generation of a momentum lattice, we show that the dynamics of atoms along the momentum lattice is dictated by a periodically driven Su-Schieffer-Heeger model, which is equivalent to a discretetime topological quantum walk. We directly measure the underlying topological invariants through time-averaged mean chiral displacements in different time frames, which are consistent with our experimental observation of topological phase transitions. The high tunability of the system further enables us to observe robust helical Floquet channels in the one-dimensional momentum lattice, which derive from the winding of Floquet quasienergy bands. Our experiment opens up the avenue of investigating discrete-time topological quantum walks using cold atoms, where the many-body environment and tunable interactions offer exciting new possibilities.Exploring topological phases is a main theme in modern physics. Characterized by topological invariants which reflect the global geometric properties of the system wave function, topological phases host a range of fascinating features, which are robust to local perturbations and are potentially useful for applications in quantum information and quantum computation [1,2]. Besides conventional topological materials in solid-state systems, topological phenomena also emerge away from equilibrium. For example, topological phases and emergent topological phenomena exist in non-Hermitian open systems [3][4][5][6][7][8][9][10][11][12][13][14], in periodically driven Floquet systems and quench processes [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], which have stimulated intense interest recently due to the rapid progress in synthetic quantum-simulation platforms such as cold atoms [34][35][36][37], photonics [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52], phononics [53], and superconducting qubits [54].A particularly interesting subject is topologies in periodically driven Floquet systems, which are shown to have a rich structure and host novel topological phases with no counterparts in static systems [20][21][22][23]. A paradigmatic example of topological Floquet dynamics is discrete-time quantum walks, which, besides potential applications in quantum information [55][56][57], have been widely used in photonics for the exploration of Floquet topological phases [38][39][40][41][42][43][44][45][46][47]. In cold atoms, whereas Floquet topological phases [34] and quantum walks [58] have been respectively implemented, quantum walks with topological properties are yet to be experimentally realized.Here we report the experimental implementation of discrete-time topological quantum walks in momentum space for a Bose-Einstein condensate (BEC). Combining the generation of momentum lattice [59-63] with stroboscopic driving sequences, dynamics of the conden-sate atoms is governed b...

We experimentally study quantum Zeno effects in a parity-time (PT) symmetric cold atom gas periodically coupled to a reservoir. Based on the state-of-the-art control of inter-site couplings of atoms in a momentum lattice, we implement a synthetic two-level system with passive PT symmetry over two lattice sites, where an effective dissipation is introduced through repeated couplings to the rest of the lattice. Quantum Zeno (anti-Zeno) effects manifest in our experiment as the overall dissipation of the two-level system becoming suppressed (enhanced) with increasing coupling intensity or frequency. We demonstrate that quantum Zeno regimes exist in the broken PT symmetry phase, and are bounded by exceptional points separating the PT symmetric and PT broken phases, as well as by a discrete set of critical coupling frequencies. Our experiment establishes the connection between PT-symmetry-breaking transitions and quantum Zeno effects, and is extendable to higher dimensions or to interacting regimes, thanks to the flexible control with atoms in a momentum lattice.

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Employing the external degrees of freedom of atoms as synthetic dimensions renders easy and new accesses to quantum engineering and quantum simulation. As a recent development, ultracold atoms suffering from two-photon Bragg transitions can be diffracted into a series of discrete momentum states to form a momentum lattice. Here we provide a detailed analysis on such a system, and, as a concrete example, report the observation of robust helical Floquet channels, by introducing periodic driving sequences. The robustness of these channels against perturbations is confirmed, as a test for their topological origin captured by Floquet winding numbers. The periodic switching demonstrated here serves as a testbed for more complicated Floquet engieering schemes, and offers exciting opportunities to study novel topological physics in a many-body setting with tunable interactions.