Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E27: Topological Physics in AMO Systems IFocus

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Sponsoring Units: DAMOP DCMP Chair: Bryce Gadway, UIUC Room: LACC 404B 
Tuesday, March 6, 2018 8:00AM  8:36AM 
E27.00001: Topological physics with atoms and with photons Invited Speaker: Iacopo Carusotto In this talk I will give a parallel review of the latest developments in the theoretical and experimental study of topological and magnetic effects in ultracold gases of neutral atoms and in optical systems. In spite of the apparent differences, these two families of systems share in fact the same basic concepts and are benefitting from a continuous crossfertilization effect. 
Tuesday, March 6, 2018 8:36AM  8:48AM 
E27.00002: Confinement effects in topological photonics Gleb Siroki, Paloma Arroyo Huidobro, Vincenzo Giannini Photonic topological insulators have polarized edge states which support unidirectional propagation of light. In practice, one would like to make devices exploiting these states as small as possible. In this work we theoretically investigate the effects of confinement on topological edge states using a particular photonic crystal design as an example [1]. We show that the edge bands become discretized but still support the unidirectional propagation which can be switched with the polarization. An analytical formula is derived which predicts the number of edge states and their field profiles agreeing with numerical simulations. These results will be relevant for experiments on topological photonic crystals containing less than a few hundred unit cells. 
Tuesday, March 6, 2018 8:48AM  9:00AM 
E27.00003: NonHermitian Topological Photonics Bo Zhen, Hengyun Zhou, Chao Peng, Yoseob Yoon, Chia Wei Hsu, Huitao Shen, Liang Fu, Keith Nelson, John Joannopoulos, Marin Soljacic The ideas of topology have found tremendous success in Hermitian physical systems, but even richer properties exist in the more general nonHermitian framework. Here, we theoretically propose and experimentally demonstrate a new topologicallyprotected bulk Fermi arc which—unlike the wellknown surface Fermi arcs arising from Weyl points in Hermitian systems—develops from nonHermitian radiative losses in photonic crystal slabs. Moreover, we discover halfinteger topological charges in the polarization of farfield radiation around the Fermi arc. We show that both phenomena are direct consequences of the nonHermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, nonHermitian physics, and singular optics, and paves the way for future exploration of nonHermitian topological systems. 
Tuesday, March 6, 2018 9:00AM  9:12AM 
E27.00004: Observation of Vortex Reflection Phase Induced by Synthetic Weyl Points Qiang Wang, Meng Xiao, Hui Liu, Shining Zhu, CheTing Chan We demonstrate that generalized Weyl points can exist in a parameter space and we report the first observation of such nodal points in onedimensional photonic crystals in the optical range. The reflection phase inside the band gap of a truncated photonic crystal exhibits vortexes in the parameter space where the Weyl points are defined and they share the same topological charges as the Weyl points. 
Tuesday, March 6, 2018 9:12AM  9:24AM 
E27.00005: Knotted Hopfion Texture of Polarization States in Tightly Focused Light Danica Sugic, Mark Dennis Topology is a key tool in engineering new states of matter, particularly in smooth configurations such as skyrmions and hopfions in nuclear physics, liquid crystals and magnetism [1, 2]. We find that the topology of optical polarization around a dark focus can be described in terms of spacefilling topological hopfion textures, corresponding to the celebrated Hopf fibration or its generalizations. The textures consist of lines in the focal volume mapped to points on the Poincaré sphere of polarization; these lines are interlinked loops, which can be knotted [3]. These knotted hopfions arise naturally from linear interference of optical helicity eigenstates in the lowintensity focus, due to optical spinorbit effects. We describe a potential experiment, which, if realized, could be used to imprint new topological stable states into soft or electronic matter, which might have application for information storage and processing, and opens the possibility of exploring new regimes of lightmatter interaction. 
Tuesday, March 6, 2018 9:24AM  9:36AM 
E27.00006: Experimental demonstration of nonreciprocal lasing in topological cavities of arbitrary geometries Babak Bahari, Abdouaye Ndao, Felipe Valini, Abdelkrim El Amili, Yeshaiahu Fainman, Boubacar Kante Topological insulator is a material in which helical conducting states exist on the surface of the bulk insulator. These states can transport electrons or photons at the boundary without any back scattering, even in presence of obstacles enabling to make topological cavities with arbitrary geometries that light can propagate in one direction. Here we present nonreciprocal photonic topological cavities for laser applications that operate at telecommunication wavelengths. The unidirectional stimulated emission from edge states is coupled to a selected waveguide output port with an isolation ratio of 11 dB. Topological cavities are made of two different photonic crystals with distinct topological phase invariants, which are bonded on a magnetic material of yttrium iron garnet to break the timereversal symmetry. Our experimental demonstration, paves the way to develop complex nonreciprocal topological devices of arbitrary geometries for integrated and robust generation and transport of light in classical and quantum regimes. 
Tuesday, March 6, 2018 9:36AM  9:48AM 
E27.00007: Topological phases from dipolar interactions on kagome optical lattices HoiYin Hui, Mengsu Chen, Sumanta Tewari, Vito Scarola Topological phases identified by a quantum Hall effect are typically generated from an applied gauge field. But the quantum anomalous Hall effect can arise from a spontaneously broken time reversal symmetry even without an applied gauge field. While a number of proposals have been put forward to realize ordinary topological phases in quantum gas experiments, most of them require extra lasers (in addition to those generating the lattice) for creating effective gauge fields. Applied fields unfortunately lead to additional heating and losses, and, in realistic experiments, often makes it difficult to reach the low temperatures required to realize topological phases. Here, we propose a different system in which this problem is circumvented: dipolar fermions on a kagome optical lattice. We find that a quantum anomalous Hall phase naturally emerges in the \emph{absence} of artificial gauge fields, thus avoiding the additional lasers and the concomitant heating and losses. The complementary approaches of exact diagonalization and meanfield theory are used to establish the remarkable robustness of this phase and estimate parameters needed to observe it with dipolar fermions in a kagome optical lattice. 
Tuesday, March 6, 2018 9:48AM  10:00AM 
E27.00008: Probing topological superfluidity in a system of repulsive alkalineearth atoms in optical lattices Leonid Isaev, Adam Kaufman, Gerardo Ortiz, Ana Maria Rey Topological superfluids are of technological relevance since they are believed to host Majorana bound states, a powerful resource for quantum computation and memory. In this talk, I will describe an experimentally feasible realization of topological superfluidity with fermionic atoms in an optical lattice. We consider a situation where atoms in two internal states experience different lattice potentials: one species is localized and the other itinerant, and show how quantum fluctuations of the localized fermions give rise to an attraction and strong spinorbit coupling in the itinerant band. At low temperature, these effects stabilize a topological superfluid of mobile atoms even if their bare interactions are repulsive. This emergent state can be engineered with Sr87 atoms in a superlattice with a dimerized unit cell. To probe its unique properties we describe protocols that use high spectral resolution and controllability of the Sr clock transition, such as momentumresolved spectroscopy and emergent magnetoelectric phenomena when the system exhibits a supercurrent response to a synthetic (laserinduced) magnetic field. 
Tuesday, March 6, 2018 10:00AM  10:12AM 
E27.00009: Breakdown of topological Thouless pumping in strongly interacting fermions Masaya Nakagawa, Tsuneya Yoshida, Robert Peters, Norio Kawakami In 2016, experimentalists succeeded in demonstrating the realization of topological Thouless pumping using ultracold atoms. The topological Thouless pumping is characterized by the Chern number which gives the quantized pumped charge. Since the Chern number cannot be changed unless the energy gap collapses, the topological pumping of free fermions is considered to be stable at least against weak interactions. In this talk, however, we show that the topological Thouless pumping is forced to break down in a strongly correlated regime. This result is demonstrated using a spinful RiceMele model of repulsively interacting fermions. The key element for the breakdown of pumping is a crossover of a symmetryprotected topological phase from fermions to spin systems due to the increase of the Hubbard repulsion. Our result can be readily tested in current experimental setups. 
Tuesday, March 6, 2018 10:12AM  10:24AM 
E27.00010: Majorana Edge Mode in a Numberconserving Fermi Gas with Tunable pwave Interaction Xiangyu Yin, TinLun Ho, Xiaoling Cui <!StartFragment>The remarkable exchange properties of Majorana fermions and their potential application in quantum information processing have led to considerable effort to realize topological matters that host these excitations in recent years. In cold atom research, it has been pointed out that coupled chain systems that allow fermion pair hopping between chains can have Majorana ground states even though these system are number conserving. In this work, we show that Majorana ground state can occur in a Fermi gas on a 1D lattice with tunable pwave interaction, where a pair of fermions can be converted into a closechannel boson and vice versa, thereby allowing fermion pair fluctuation in a single chain. This scheme requires neither spinorbit coupling nor multichain set up and can be implemented easily. We offer evidence of Majorana ground states through DMRG calculations on boson compressibility, entanglement entropy, and edgeedge Majorana fermion correlation. The signature of the Majorana phase can be identified from its momentum distribution. <!EndFragment> 
Tuesday, March 6, 2018 10:24AM  10:36AM 
E27.00011: Dipolar fermions as an experimental platform of correlated topological classification Tsuneya Yoshida, Ippei Danshita, Robert Peters, Norio Kawakami One of the most challenging problems in correlated topological systems is a realization of the reduction of topological classification. However, so far, very few experimental platforms have been proposed. 
Tuesday, March 6, 2018 10:36AM  10:48AM 
E27.00012: Monopoles Constructed by Cold Atoms Cheng Li, TinLun Ho Monopoles are important topological structures in many important areas in physics. In the last few years, cold atom experiments are able to simulated both Dirac monopoles and Yang Monopoles, which appear in the manifold of nondegenerate ground states and degenerate ground states, are characterized by nonvanishing first and second Chern numbers respectively. Motivated by the recent experiment at NIST (arXiv:1610.06228) on Yang monopole constructed by ^{87}Rb, we have studied how interactions between atoms affect these Chern numbers. We show that without quantum fluctuation, interaction will stretch the monopole into an extended manifold of singularity in the parameter space. With quantum fluctuation, the singular manifold is transformed into volume of fragmented condensate. The interaction effect will cause a gradual change of Chern numbers as a monopole leaves the closed surface where Chern numbers are calculated. The interaction effect is contained in the data of the NIST experiment. 
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