Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Q65: Non-Hermitian Physics |
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Sponsoring Units: DAMOP Chair: Manuel Valiente, Universidad de La Laguna Room: Room 414 |
Wednesday, March 8, 2023 3:00PM - 3:12PM |
Q65.00001: Quantum geometry of non-Hermitian systems Jan Behrends, Roni Ilan, Moshe Goldstein The geometry of quantum states determines their response to external perturbations, their noise, and wave packets dynamic. For example, the Berry curvature gives anomalous velocity of wave packets and the Hall conductivity of extended systems, thereby linking these observables to topological invariants. In non-Hermitian systems, wave packet dynamics are enriched by additional anomalous terms arising from generalizations of the Berry curvature to non-orthogonal eigenstates. In this work, we contextualize these anomalous contributions by showing that they directly arise from the geometry of the underlying quantum states. We discuss possible generalizations of the quantum geometric tensor, which we relate to the localization of qunatum states and wave packets. We discuss experimental signatures in terms of response functions and transport signatures. |
Wednesday, March 8, 2023 3:12PM - 3:24PM |
Q65.00002: Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier Chandrashekhar Gaikwad, Daria Kowsari, Weijian Chen, Kater Murch Parity-Time (PT) symmetry, first devised as a theoretical concept and later realized in optics has quickly grown to become a major field with significant applications. The simplest realization of such systems is a PT dimer with two modes exhibiting gain and loss respectively. The important phenomena that make these systems interesting are PT symmetry broken and unbroken regions separated by an exceptional point degeneracy. PT-symmetric Hamiltonians may be realized by engineering controlled interactions with an environment of a quantum system but there are multiple other ways to design such an arrangement. In this work, we design a narrowband JPA working in a three-wave mixing mode to observe PT symmetry-broken and unbroken regions in the transient response. Here the two quadratures are two modes of the PT dimer. The non-Hermitian nature of the equations of motion of the cavity modes gives insight into the functionality of a Josephson parametric amplifier and sets the stage for harnessing the physics of exceptional points in large arrays of such parametric devices. |
Wednesday, March 8, 2023 3:24PM - 3:36PM |
Q65.00003: Non-Hermitian systems subject to classical noise Pablo Martinez-Azcona, Aritra Kundu, Aurelia Chenu, Adolfo del Campo, Avadh B Saxena No quantum system can ever be truly isolated, for this reason the study of Open Quantum Systems is of both fundamental and practical interest. Non Hermitian systems can account for dissipation and decoherence, while showing new phenomena like exceptional points and PT symmetry. The experimental realization of these systems is subject to all sorts of noise. Motivated by this fact, in this study we consider the effect of classical noise on the dynamics of Non-Hermitian Hamiltonians. Using tools from stochastic calculus, we find the evolution for the noise-averaged density matrix to evolve with an exotic Liouvillian. Furthermore we also characterize the evolution of the fluctuations around the noise-averaged density matrix. This allows us to replicate noisy Non-Hermitian evolution seen experimentally and find several exceptional points, not shown by the standard Lindbladian. Non-Hermitian dynamics and exceptional points show many exciting potential applications, therefore our results shed light on the experimental realization of such systems, as well as on the role of stochasticity in Non-Hermitian extensions of Quantum Mechanics. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q65.00004: Zoology of non-Hermitian spectra and their graph topology Tommy Tai, Ching Hua Lee We uncover the very rich graph topology of generic bounded non-Hermitian spectra, distinct from the topology of conventional band invariants and complex spectral winding. The graph configurations of complex spectra are characterized by the algebraic structures of their corresponding energy dispersions, drawing new intimate links between combinatorial graph theory, algebraic geometry, and non-Hermitian band topology. Spectral graphs conformally related belong to the same equivalence class, and are characterized by emergent symmetries not necessarily present in the physical Hamiltonian. The simplest class encompasses well-known examples such as the Hatano-Nelson and non-Hermitian SSH models, while more sophisticated classes represent novel multi-component models with interesting spectral graphs resembling stars, flowers, and insects. With recent rapid advancements in metamaterials, ultracold atomic lattices, and quantum circuits, it is now feasible to not only experimentally realize such esoteric spectra but also investigate the non-Hermitian flat bands and anomalous responses straddling transitions between different spectral graph topologies.? |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q65.00005: Scaling laws for non-Hermitian skin effect with long-range couplings Jhih-Shih You, Yi-Cheng Wang, Hsiang-Hua Jen Non-Hermitian systems are sensitive to the boundary conditions. Recent years have seen various studies on non-Hermitian skin effect (NHSE), where extensive bulk eigenstates pile at open boundary. Yet, little is known about the role of long-range coupling in a non-Hermitian system. Here we study the non-Hermitian skin effect in a one-dimensional lattice model with long-range couplings. We show that the non-locality gives rise to the scale-free localization, where the localization length of eigenstate is proportional to the system size. By tuning the coupling range, the competition between the nearest-neighbor coupling and non-locality results in the real-to-complex spectral transition and a crossover in the size dependence of localization length. Furthermore, we study the scaling of nonequilibrium steady-state entanglement entropy, where complex spectrum and NHSE contribute to subextensive and area laws, respectively. Our results shed light on the interplay between long-range coupling and non-Hermiticity. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q65.00006: A 1d realization of lattice chiral fermions with a non-Hermitian Hamiltonian Gen YUE It is generally believed that a 1+1D chiral fermion state can not exist by itself on lattice. The obstruction to such a lattice realization is originated from the quantum anomalies of chiral fermion in quantum field theory. It has been recognized that such an anomalous state have a topologically nontrivial bulk. For example, a chiral fermion can be realized as the boundary state of a 2+1D Quantum Hall state. In this talk, we demonstrate that such a 1+1D chiral fermion state can be realized, without its corresponding bulk, by a one-spatial-dimensional non-Hermitian lattice Hamiltonian. The model possesses correct chiral anomaly and gravitational anomaly. Furthermore, the low energy effective theory of the model is a field theory of unitary chiral fermion. |
Wednesday, March 8, 2023 4:12PM - 4:24PM |
Q65.00007: Employing Non-Hermitian matrices in understanding open quantum systems Maria Zelenayova Various quantum systems can be viewed as open quantum systems due to the presence of noise, dissipations and other sources of decoherence in experimental setups. Theoretically, these systems are studied by either solving the full master equation or using effective formulations based on non-Hermitian physics. While the former approach is widely known, it is challenging in obtaining analytical and numerical solutions and connecting their outcome with experimental measurements. The latter approach evaluates effective descriptions employing non-Hermitian physics, which has been the focus of numerous studies in recent years. Interestingly, for a given open quantum system multiple non-Hermitian matrices can be determined. In this talk, I will present similarities and distinctions between various non-Hermitian treatments and focus on several experimentally feasible examples. I will show how unique features of non-Hermitian systems, namely exceptional points, emerge in these systems. |
Wednesday, March 8, 2023 4:24PM - 4:36PM |
Q65.00008: Unidirectional segregation of Bright-Bright soliton through a $mathcal{PT}$-symmetric potential Majed Alotaibi We study the dynamics of two-component vector solitons, namely, bright-bright (BB) solitons interacting with parity-time-(PT ) symmetric potentials. We employ direct numerical simulations to demonstrate the unidirectional segregation of the BB soliton. Using a modified perturbed dynamical variational Lagrangian approximation, we develop an analytical model to verify the results obtained from numerical calculations. Simplified variational equations of motion suggest that the splitting of BB solitons can be explained by considering the effective force between the two components. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q65.00009: Continuum of Bound States in a Non-Hermitian Model Changyan Zhu, QIANG WANG, Yidong Chong In a Hermitian system, bound states must have quantized energies, whereas extended states can form a continuum. We demonstrate how this principle fails for non-Hermitian systems, by analyzing non-Hermitian continuous Hamiltonians with an imaginary momentum and Landau-type vector potential. The eigenstates, which we call "continuum Landau modes" (CLMs), have gaussian spatial envelopes and form a continuum filling the complex energy plane. We present experimentallyrealizable 1D and 2D lattice models that can be used to study CLMs; the lattice eigenstates are localized and have other features that are the same as in the continuous model. One of these lattices can serve as a rainbow trap, whereby the response to an excitation is concentrated at a position proportional to the frequency. Another lattice can act a wave funnel, concentrating an input excitation onto a boundary over a wide frequency bandwidth. Unlike recent funneling schemes based on the non-Hermitian skin effect, this requires only a simple lattice design without nonreciprocal couplings. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q65.00010: Experimentally-realizable PT phase transitions in reflectionless quantum systems Micheline B Soley, Carl M Bender, A. Douglas Stone Although parity-time (PT) reversal symmetry has been measured in classical wave equations, the fundamental physical symmetry has yet to be measured in lossless fundamental quantum mechanics, where PT-symmetry theory was originally developed. We show theoretically that standard cold-atom experiments with programmable traps could be used to observe both eigenstates of PT-symmetric systems and PT-symmetry breaking behavior in fundamental quantum scattering systems. We demonstrate that weakly bound states predicted for the upside-down PT-symmetric potentials V(x)=-x^4, -x^6, -x^8 can be measured to arbitrarily high accuracy as reflectionless states in the truncated purely real potential V(x)=-|x|^p for positive parameter p. Quantum scattering calculations indicate the measurements are robust to experimental error. In addition, spontaneous PT-symmetry-breaking can be measured as a function of p. In the unbroken phase, there exist an infinite number of reflectionless states at real energies; in the broken phase, there are no real-eigenenergy solutions; and in the mixed phase, exceptional points are measurable with their signature quartic dips in the reflection coefficient. The findings invite a hunt for PT-symmetry behaviors in near-threshold atomic scattering systems. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q65.00011: Robust qudit Hamiltonian engineering and dynamical decoupling Haoyang Gao, Hengyun Zhou, Nathaniel T Leitao, Oksana A Makarova, Iris Cong, Alexander M Douglas, Leigh S Martin, Mikhail D Lukin We develop a formalism for the Hamiltonian engineering and decoupling in qudit systems beyond spin-1/2. Overcoming the lack of easily visualized geometric structures in the qudit case, we uncover a graph structure that generalizes the qubit Bloch sphere, and identify a choice of pulses that significantly simplifies the incorporation of robustness conditions. Exploiting certain structures in the "decoupling frame graph" that have close analogies with the qubit case, we design a decoupling sequence for strongly disordered, interacting ensembles of spin-1 nitrogen-vacancy centers, in which we experimentally demonstrate an order of magnitude improvement in coherence time over existing sequences. Motivated by recent results on universal decoupling using spherical t-designs, we discuss the outlook toward higher spin sensing, which promises superior sensitivities over their spin-1/2 counterparts. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q65.00012: Non-Hermitian Floquet-Free Analytically Solvable Time Dependant Systems Hamed Ghaemidizicheh, Hamidreza Ramezani The non-Hermitian models, which are symmetric under parity (P) and time-reversal (T) operators, are the cornerstone for the fabrication of new ultra-sensitive optoelectronic devices. However, providing the gain in such systems usually demands nonlinear processes, limiting their application mainly in the quantum realm. In this paper, to achieve this obstacle, we introduce a class of time-dependent non-Hermitian Hamiltonians (not necessarily Floquet) that can describe a two-level system with temporal modulated on-site potential and couplings. We show that implementing an appropriate non-Unitary gauge transformation converts the original system to an effective one with a balanced gain/loss. This will allow us to analytically derive the evolution of states. Our proposed class of Hamiltonians can be employed in different platforms, such as electronic circuits, acoustics, and photonics, to design PT-symmetric structures without amplification and absorption mechanisms. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q65.00013: Coherent pair driving as a resource for many-body physics Emmanouil K Grigoriou Under adequate conditions, many body quantum systems are known to form macroscopic phases displaying exotic properties. Superfluid zero viscosity or superconducting infinite conductivity still hold the promise for key technological breakthroughs. Such expressions of collective behavior emerge from the complex interplay between quantum processes in the limit of an ever increasing number of degrees of freedom, quickly becoming numerically intractable and setting the stage for the appraising of quantum simulators. As a result, a lot of effort has been devoted to the understanding of the conceptual ingredients underlying emergent physics and an active area of research is concerned with the experimental generation of these phases in controlled environments. Today, modern experimental platforms allow for the implementation of processes not traditionally considered in a many body context. Processes that do not conserve the number of excitations such as coherent pair drivings are the focus of this work. We show how these new ingredients, lead to a novel kind of Bogoliubov inestability, inducing new phases in spatially non extended systems or enriching the phase diagram of already well established models in unexplored ways. Among others, we observe the enhancement of lattice supersolidity in the extended Bose-Hubbard model. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q65.00014: Critical Dynamics in Nonhermitian Many-body System Xiaoyuan Huang Recently, there are observations on critical phenom- |
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