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Browsing by Author "Harter, Andrew K."
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Item Fragile aspects of topological transition in lossy and parity-time symmetric quantum walks(Springer Nature, 2018-08-13) Harter, Andrew K.; Saxena, Avadh; Joglekar, Yogesh N.; Physics, School of ScienceQuantum walks often provide telling insights about the structure of the system on which they are performed. In PT-symmetric and lossy dimer lattices, the topological properties of the band structure manifest themselves in the quantization of the mean displacement of such a walker. We investigate the fragile aspects of a topological transition in these two dimer models. We find that the transition is sensitive to the initial state of the walker on the Bloch sphere, and the resultant mean displacement has a robust topological component and a quasiclassical component. In PT symmetric dimer lattices, we also show that the transition is smeared by nonlinear effects that become important in the PT-symmetry broken region. By carrying out consistency checks via analytical calculations, tight-binding results, and beam-propagation-method simulations, we show that our predictions are easily testable in today’s experimental systems.Item Observation of parity-time symmetry breaking transitions in a dissipative Floquet system of ultracold atoms(Springer Nature, 2019-02-20) Li, Jiaming; Harter, Andrew K.; Liu, Ji; de Melo, Leonardo; Joglekar, Yogesh N.; Luo, Le; Physics, School of ScienceOpen 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.Item Observation of slowly decaying eigenmodes without exceptional points in Floquet dissipative synthetic circuits(Springer Nature, 2018-12-03) León-Montiel, Roberto de J.; Quiroz-Juárez, Mario A.; Domínguez-Juárez, Jorge L.; Quintero-Torres, Rafael; Aragón, José L.; Harter, Andrew K.; Joglekar, Yogesh N.; Physics, School of ScienceParity-time symmetric systems allow one to study new types of Hamiltonians which could have potential impact on our understanding of nonlinear physics. The authors investigate the energy stored in an electronic Floquet system and demonstrate that such a setup can be used to study the dynamics of dissipative parity-time symmetric systems.Item Passive parity-time-symmetry-breaking transitions without exceptional points in dissipative photonic systems(OSA, 2018-08-01) Joglekar, Yogesh N.; Harter, Andrew K.; Physics, School of ScienceOver the past decade, parity-time (PT)-symmetric Hamiltonians have been experimentally realized in classical, optical settings with balanced gain and loss, or in quantum systems with localized loss. In both realizations, the PT-symmetry-breaking transition occurs at the exceptional point of the non-Hermitian Hamiltonian, where its eigenvalues and the corresponding eigenvectors both coincide. Here, we show that in lossy systems, the PT transition is a phenomenon that broadly occurs without an attendant exceptional point, and is driven by the potential asymmetry between the neutral and the lossy regions. With experimentally realizable quantum models in mind, we investigate dimer and trimer waveguide configurations with one lossy waveguide. We validate the tight-binding model results by using the beam-propagation-method analysis. Our results pave a robust way toward studying the interplay between passive PT transitions and quantum effects in dissipative photonic configurations.Item Photonic quantum simulations of coupled PT -symmetric Hamiltonians(APS, 2022-01-26) Maraviglia, Nicola; Yard, Patrick; Wakefield, Ross; Carolan, Jacques; Sparrow, Chris; Chakhmakhchyan, Levon; Harrold, Chris; Hashimoto, Toshikazu; Matsuda, Nobuyuki; Harter, Andrew K.; Joglekar, Yogesh N.; Laing, Anthony; Physics, School of ScienceParity-time-symmetric (PT -symmetric) Hamiltonians are generally non-Hermitian and give rise to exotic behavior in quantum systems at exceptional points, where eigenvectors coalesce. The recent realization of PT -symmetric Hamiltonians in quantum systems has ignited efforts to simulate and investigate many-particle quantum systems across exceptional points. Here, we use a programmable integrated photonic chip to simulate a model composed of twin pairs of PT -symmetric Hamiltonians, with each the time reverse of its twin. We simulate quantum dynamics across exceptional points including two- and three-particle interference, and a particle-trembling behavior that arises due to interference between subsystems undergoing time-reversed evolutions. These results show how programmable quantum simulators can be used to investigate foundational questions in quantum mechanics.Item PT symmetry breaking in the presence of random, periodic, long-range hopping(SPIE, 2016-09) Harter, Andrew K.; Assogba Onanga, Franck; Joglekar, Yogesh N.; Department of Physics, School of ScienceOver the past five years, open systems with balanced gain and loss have been investigated for extraordinary properties that are not shared by their closed counterparts. Non-Hermitian, Parity-Time (PT ) symmetric Hamiltonians faithfully model such systems. Such a Hamiltonian typically consists of a reflection-symmetric, Hermitian, nearest-neighbor hopping profile and a PT-symmetric, non-Hermitian, gain and loss potential, and has a robust PT -symmetric phase. Here we investigate the robustness of this phase in the presence of long-range hopping disorder that is not PT-symmetric, but is periodic. We find that the PT-symmetric phase remains robust in the presence of such disorder, and characterize the configurations where that happens. Our results are found using a tight-binding model, and we validate our predictions through the beam-propagation method.Item PT-breaking threshold in spatially asymmetric Aubry-André and Harper models: Hidden symmetry and topological states(APS, 2016-06) Harter, Andrew K.; Lee, Tony E.; Joglekar, Yogesh N.; Department of Physics, School of ScienceAubry-André-Harper lattice models, characterized by a reflection-asymmetric sinusoidally varying nearest-neighbor tunneling profile, are well known for their topological properties. We consider the fate of such models in the presence of balanced gain and loss potentials ±iγ located at reflection-symmetric sites. We predict that these models have a finite PT-breaking threshold only for specific locations of the gain-loss potential and uncover a hidden symmetry that is instrumental to the finite threshold strength. We also show that the topological edge states remain robust in the PT-symmetry-broken phase. Our predictions substantially broaden the possible experimental realizations of a PT-symmetric system.Item Sublattice signatures of transitions in a PT -symmetric dimer lattice(Springer, 2016) Harter, Andrew K.; Joglekar, Yogesh N.; Department of Physics, School of ScienceLattice models with non-hermitian, parity and time-reversal (PTPT) symmetric Hamiltonians, realized most readily in coupled optical systems, have been intensely studied in the past few years. A PTPT-symmetric dimer lattice consists of dimers with intra-dimer coupling νν, inter-dimer coupling ν′ν′, and balanced gain and loss potentials ±iγ±iγ within each dimer. This model undergoes two independent transitions, namely a PTPT-breaking transition and a topological transition. We numerically and analytically investigate the signatures of these transitions in the time-evolution of states that are initially localized on the gain-site or the loss-site.Item Time-invariant PT product and phase locking in PT -symmetric lattice models(APS, 2018-01) Joglekar, Yogesh N.; Assogba Onanga, Franck; Harter, Andrew K.; Physics, School of ScienceOver the past decade, non-Hermitian, PT -symmetric Hamiltonians have been investigated as candidates for both a fundamental, unitary, quantum theory and open systems with a nonunitary time evolution. In this paper, we investigate the implications of the former approach in the context of the latter. Motivated by the invariance of the PT (inner) product under time evolution, we discuss the dynamics of wave-function phases in a wide range of PT -symmetric lattice models. In particular, we numerically show that, starting with a random initial state, a universal, gain-site location dependent locking between wave-function phases at adjacent sites occurs in the PT -symmetry-broken region. Our results pave the way towards understanding the physically observable implications of time invariants in the nonunitary dynamics produced by PT -symmetric Hamiltonians.Item Veiled symmetry of disordered Parity-Time lattices: protected PT-threshold and the fate of localization(Nature Publishing Group, 2018-01-08) Harter, Andrew K.; Assogba Onanga, Franck; Joglekar, Yogesh N.; Physics, School of ScienceOpen, non-equilibrium systems with balanced gain and loss, known as parity-time ([Formula: see text])-symmetric systems, exhibit properties that are absent in closed, isolated systems. A key property is the [Formula: see text]-symmetry breaking transition, which occurs when the gain-loss strength, a measure of the openness of the system, exceeds the intrinsic energy-scale of the system. We analyze the fate of this transition in disordered lattices with non-Hermitian gain and loss potentials ±iγ at reflection-symmetric sites. Contrary to the popular belief, we show that the [Formula: see text]-symmetric phase is protected in the presence of a periodic disorder which leads to a positive [Formula: see text]-symmetry breaking threshold. We uncover a veiled symmetry of such disordered systems that is instrumental for the said protection, and show that this symmetry leads to new localization behavior across the [Formula: see text]-symmetry breaking transition. We elucidate the interplay between such localization and the [Formula: see text]-symmetry breaking phenomena in disordered [Formula: see text]-symmetric lattices, with Hermitian disorder or gain-loss disorder, and support our conclusions with a beampropagation- method analysis. Our theoretical predictions provide avenues for experimental realizations of -symmetric systems with engineered disorder.