arXiv subjects

Zlatko Papić

Publications and source records attributed to Zlatko Papić.

At least 37 records · Page 2Linked to original sources

arXiv2025

Entanglement scaling and charge fluctuations in a Fermi liquid of composite fermions

The composite fermion Fermi liquid (CFL) state at $ν=1/2$ filling of a Landau level is a paradigmatic non-Fermi liquid borne out purely by Coulomb interactions. But in what ways is this exotic state of matter different from a Fermi liquid? The CFL entanglement entropy was indeed found to exhibit a significant enhancement compared to free electrons [Shao et al., Phys. Rev. Lett. 114, 206402 (2015)], which was subsequently ruled out as a finite-size effect by the study of a lattice CFL analog [Mishmash and Motrunich, Phys. Rev. B 94, 081110 (2016)]. Moreover, the enhancement was not observed in a quasi-one-dimensional limit of the Coulomb ground state at $ν=1/2$ [Geraedts et al., Science 352, 197 (2016)]. Here, we revisit the problem of entanglement scaling in the CFL state realized in a two-dimensional electron gas. Using Monte Carlo evaluation of the second Rényi entropy $S_2$ for the CFL variational wave function, we show that the entanglement enhancement is present not only at $ν=1/2$ but also at $ν=1/4$, as well as in bosonic CFL states at $ν=1$ and $ν=1/3$ fillings. In all cases, we find the scaling of $S_2$ with subsystem size to be enhanced compared to the non-interacting case, and insensitive to the choice of geometry and projection to the lowest Landau level. We also demonstrate that, for CFL states, the variance of the particle number in a subsystem obeys area-law scaling with a universal subleading corner contribution, in stark contrast with free fermions. Our results establish the enhanced entanglement scaling and suppressed charge fluctuations as fingerprints of non-Fermi-liquid correlations in CFL states.

cond-mat.str-el↗

arXiv2025

Hilbert space fragmentation at the origin of disorder-free localization in the lattice Schwinger model

Lattice gauge theories, discretized cousins of continuum gauge theories arising in the Standard Model, have become important platforms for exploring non-equilibrium quantum phenomena. Recent works have reported the possibility of disorder-free localization in the lattice Schwinger model. Using degenerate perturbation theory and numerical simulations based on exact diagonalization and matrix product states, we perform a detailed characterization of thermalization breakdown in the Schwinger model, including its spectral properties, structure of eigenstates, and out-of-equilibrium quench dynamics. We scrutinize the strong-coupling limit of the model, in which an intriguing double-logarithmic-in-time growth of entanglement was previously proposed from the initial vacuum state. We identify the origin of this ultraslow growth of entanglement as due to approximate Hilbert space fragmentation and the emergence of a dynamical constraint on particle hopping, which gives rise to sharp jumps in the entanglement entropy dynamics within individual background charge sectors. Based on the statistics of jump times, we argue that entanglement growth, averaged over charge sectors, is more naturally explained as either single-logarithmic or a weak power law in time. Our results suggest the existence of a single ergodicity-breaking regime due to Hilbert space fragmentation, whose properties are reminiscent of conventional many-body localization within numerically accessible system sizes.

quant-ph↗

arXiv2024

Robust Finite-Temperature Many-Body Scarring on a Quantum Computer

Mechanisms for suppressing thermalization in disorder-free many-body systems, such as Hilbert space fragmentation and quantum many-body scars, have recently attracted much interest in foundations of quantum statistical physics and potential quantum information processing applications. However, their sensitivity to realistic effects such as finite temperature remains largely unexplored. Here, we have utilized IBM's Kolkata quantum processor to demonstrate an unexpected robustness of quantum many-body scars at finite temperatures when the system is prepared in a thermal Gibbs ensemble. We identify such robustness in the PXP model, which describes quantum many-body scars in experimental systems of Rydberg atom arrays and ultracold atoms in tilted Bose--Hubbard optical lattices. By contrast, other theoretical models which host exact quantum many-body scars are found to lack such robustness, and their scarring properties quickly decay with temperature. Our study sheds light on the important differences between scarred models in terms of their algebraic structures, which impacts their resilience to finite temperature.

cond-mat.quant-gas↗

arXiv2024

Disorder-tunable entanglement at infinite temperature

Emerging quantum technologies hold the promise of unraveling difficult problems ranging from condensed matter to high energy physics, while at the same time motivating the search for unprecedented phenomena in their setting. Here we utilize a custom-built superconducting qubit ladder to realize non-thermalizing states with rich entanglement structures in the middle of the energy spectrum. Despite effectively forming an "infinite" temperature ensemble, these states robustly encode quantum information far from equilibrium, as we demonstrate by measuring the fidelity and entanglement entropy in the quench dynamics of the ladder. Our approach harnesses the recently proposed type of non-ergodic behavior known as "rainbow scar", which allows us to obtain analytically exact eigenfunctions whose ergodicity-breaking properties can be conveniently controlled by randomizing the couplings of the model, without affecting their energy. The on-demand tunability of quantum correlations via disorder allows for in situ control over ergodicity breaking and it provides a knob for designing exotic many-body states that defy thermalization.

quant-ph↗

arXiv2024

Enhanced many-body quantum scars from the non-Hermitian Fock skin effect

In contrast with extended Bloch waves, a single particle can become spatially localized due to the so-called skin effect originating from non-Hermitian pumping. Here we show that in kinetically-constrained many-body systems, the skin effect can instead manifest as dynamical amplification within the Fock space, beyond the intuitively expected and previously studied particle localization and clustering. We exemplify this non-Hermitian Fock skin effect in an asymmetric version of the PXP model and show that it gives rise to ergodicity-breaking eigenstates, the non-Hermitian analogs of quantum many-body scars. A distinguishing feature of these non-Hermitian scars is their enhanced robustness against external disorders. We propose an experimental realization of the non-Hermitian scar enhancement in a tilted Bose-Hubbard optical lattice with laser-induced loss. Additionally, we implement digital simulations of such scar enhancement on the IBM quantum processor. Our results show that the Fock skin effect provides a powerful tool for creating robust non-ergodic states in generic open quantum systems.

cond-mat.quant-gas↗

arXiv2024

Fingerprints of Composite Fermion Lambda Levels in Scanning Tunneling Microscopy

Composite fermion (CF) is a topological quasiparticle that emerges from a non-perturbative attachment of vortices to electrons in strongly correlated two-dimensional materials. Similar to non-interacting fermions that form Landau levels in a magnetic field, CFs can fill analogous ``Lambda'' levels, giving rise to the fractional quantum Hall (FQH) effect of electrons. Here, we show that Lambda levels can be directly visualized through the characteristic peak structure in the signal obtained via spectroscopy with the scanning tunneling microscopy (STM) on a FQH state. Complementary to transport, which probes low-energy properties of CFs, we show that \emph{high-energy} features in STM spectra can be interpreted in terms of Lambda levels. We numerically demonstrate that STM spectra can be accurately modeled using Jain's CF theory. Our results show that STM provides a powerful tool for revealing the anatomy of FQH states and identifying physics beyond the non-interacting CF paradigm.

cond-mat.mes-hall↗

arXiv2024

Stirring the false vacuum via interacting quantized bubbles on a 5564-qubit quantum annealer

False vacuum decay is a potential mechanism governing the evolution of the early Universe, with profound connections to non-equilibrium quantum physics, including quenched dynamics, the Kibble-Zurek mechanism, and dynamical metastability. The non-perturbative character of the false vacuum decay and the scarcity of its experimental probes make the effect notoriously difficult to study, with many basic open questions, such as how the bubbles of true vacuum form, move and interact with each other. Here we utilize a quantum annealer with 5564 superconducting flux qubits to directly observe quantized bubble formation in real time -- the hallmark of false vacuum decay dynamics. Moreover, we develop an effective model that describes the initial bubble creation and subsequent interaction effects. We demonstrate that the effective model remains accurate in the presence of dissipation, showing that our annealer can access coherent scaling laws in driven many-body dynamics of 5564 qubits for over $1μ$s, i.e., more than 1000 intrinsic qubit time units. This work sets the stage for exploring late-time dynamics of the false vacuum at computationally intractable system sizes, dimensionality, and topology in quantum annealer platforms.

quant-ph↗

arXiv2024

Microscopic Model for Fractional Quantum Hall Nematics

Geometric fluctuations of the density mode in a fractional quantum Hall (FQH) state can give rise to a nematic FQH phase, a topological state with a spontaneously broken rotational symmetry. While experiments on FQH states in the second Landau level have reported signatures of putative FQH nematics in anisotropic transport, a realistic model for this state has been lacking. We show that the standard model of particles in the lowest Landau level interacting via the Coulomb potential realizes the FQH nematic transition, which is reached by a progressive reduction of the strength of the shortest-range Haldane pseudopotential. Using exact diagonalization and variational wave functions, we demonstrate that the FQH nematic transition occurs when the system's neutral gap closes in the long-wavelength limit while the charge gap remains open. We confirm the symmetry-breaking nature of the transition by demonstrating the existence of a "circular moat" potential in the manifold of states with broken rotational symmetry, while its geometric character is revealed through the strong fluctuations of the nematic susceptibility and Hall viscosity.

cond-mat.str-el↗

arXiv2024

Persistent non-Gaussian correlations in out-of-equilibrium Rydberg atom arrays

Gaussian correlations emerge in a large class of many-body quantum systems quenched out of equilibrium, as demonstrated in recent experiments on coupled one-dimensional superfluids [Schweigler et al., Nature Physics 17, 559 (2021)]. Here, we present a mechanism by which an initial state of a Rydberg atom array can retain persistent non-Gaussian correlations following a global quench. This mechanism is based on an effective kinetic blockade rooted in the ground state symmetry of the system, which prevents thermalizing dynamics under the quench Hamiltonian. We propose how to observe this effect with Rydberg atom experiments and we demonstrate its resilience against several types of experimental errors. These long-lived non-Gaussian states may have practical applications as quantum memories or stable resources for quantum-information protocols due to the protected non-Gaussianity away from equilibrium.

quant-ph↗

arXiv2024

Ergodicity Breaking Under Confinement in Cold-Atom Quantum Simulators

The quantum simulation of gauge theories on synthetic quantum matter devices has gained a lot of traction in the last decade, making possible the observation of a range of exotic quantum many-body phenomena. In this work, we consider the spin-$1/2$ quantum link formulation of $1+1$D quantum electrodynamics with a topological $θ$-angle, which can be used to tune a confinement-deconfinement transition. Exactly mapping this system onto a PXP model with mass and staggered magnetization terms, we show an intriguing interplay between confinement and the ergodicity-breaking paradigms of quantum many-body scarring and Hilbert-space fragmentation. We map out the rich dynamical phase diagram of this model, finding an ergodic phase at small values of the mass $μ$ and confining potential $χ$, an emergent integrable phase for large $μ$, and a fragmented phase for large values of both parameters. We also show that the latter hosts resonances that lead to a vast array of effective models. We propose experimental probes of our findings, which can be directly accessed in current cold-atom setups.

cond-mat.quant-gas↗

arXiv2023

Deep thermalization in constrained quantum systems

The concept of "deep thermalization" has recently been introduced to characterize moments of an ensemble of pure states, resulting from projective measurements on a subsystem, which lie beyond the purview of conventional Eigenstate Thermalization Hypothesis (ETH). In this work, we study deep thermalization in systems with kinetic constraints, such as the quantum East and the PXP models, which have been known to weakly break ETH by the slow dynamics and high sensitivity to the initial conditions. We demonstrate a sharp contrast in deep thermalization between the first and higher moments in these models by studying quench dynamics from initial product states in the computational basis: while the first moment shows good agreement with ETH, higher moments deviate from the uniform Haar ensemble at infinite temperature. We show that such behavior is caused by an interplay of time-reversal symmetry and an operator that anticommutes with the Hamiltonian. We formulate sufficient conditions for violating deep thermalization, even for systems that are otherwise "thermal" in the ETH sense. By appropriately breaking these properties, we illustrate how the PXP model fully deep-thermalizes for all initial product states in the thermodynamic limit. Our results highlight the sensitivity of deep thermalization as a probe of physics beyond ETH in kinetically-constrained systems.

quant-ph↗

arXiv2023

Embedding semiclassical periodic orbits into chaotic many-body Hamiltonians

Protecting coherent quantum dynamics from chaotic environment is key to realizations of fragile many-body phenomena and their applications in quantum technology. We present a general construction that embeds a desired periodic orbit into a family of non-integrable many-body Hamiltonians, whose dynamics is otherwise chaotic. Our construction is based on time dependent variational principle that projects quantum dynamics onto a manifold of low-entangled states, and it complements earlier approaches for embedding non-thermal eigenstates, known as quantum many-body scars, into thermalizing spectra. By designing terms that suppress "leakage" of the dynamics outside the variational manifold, we engineer families of Floquet models that host exact scarred dynamics, as we illustrate using a driven Affleck-Kennedy-Lieb-Tasaki model and a recent experimental realization of scars in a dimerized superconducting qubit chain.

quant-ph↗

arXiv2023

Deformed Fredkin model for the $ν{=}5/2$ Moore-Read state on thin cylinders

We propose a frustration-free model for the Moore-Read quantum Hall state on sufficiently thin cylinders with circumferences $\lesssim 7$ magnetic lengths. While the Moore-Read Hamiltonian involves complicated long-range interactions between triplets of electrons in a Landau level, our effective model is a simpler one-dimensional chain of qubits with deformed Fredkin gates. We show that the ground state of the Fredkin model has high overlap with the Moore-Read wave function and accurately reproduces the latter's entanglement properties. Moreover, we demonstrate that the model captures the dynamical response of the Moore-Read state to a geometric quench, induced by suddenly changing the anisotropy of the system. We elucidate the underlying mechanism of the quench dynamics and show that it coincides with the linearized bimetric field theory. The minimal model introduced here can be directly implemented as a first step towards quantum simulation of the Moore-Read state, as we demonstrate by deriving an efficient circuit approximation to the ground state and implementing it on IBM quantum processor.

cond-mat.str-el↗

arXiv2023

Integrability breaking and bound states in Google's decorated XXZ circuits

Recent quantum simulation by Google [Nature 612, 240 (2022)] has demonstrated the formation of bound states of interacting photons in a quantum-circuit version of the XXZ spin chain. While such bound states are protected by integrability in a one-dimensional chain, the experiment found the bound states to be unexpectedly robust when integrability was broken by decorating the circuit with additional qubits, at least for small numbers of qubits ($\leq 24$) within the experimental capability. Here we scrutinize this result by state-of-the-art classical simulations, which greatly exceed the experimental system sizes and provide a benchmark for future studies in larger circuits. We find that the bound states consisting of a small and finite number of photons are indeed robust in the non-integrable regime, even after scaling to the infinite time and infinite system size limit. Moreover, we show that such systems possess unusual spectral properties, with level statistics that deviates from the random matrix theory expectation. On the other hand, for low but finite density of photons, we find a much faster onset of thermalization and significantly weaker signatures of bound states, suggesting that anomalous dynamics may only be a property of dilute systems with zero density of photons in the thermodynamic limit. The robustness of the bound states is also influenced by the number of decoration qubits and, to a lesser degree, by the regularity of their spatial arrangement.

cond-mat.stat-mech↗

arXiv2023

Anatomy of Dynamical Quantum Phase Transitions

Global quenches of quantum many-body models can give rise to periodic dynamical quantum phase transitions (DQPTs) directly connected to the zeros of a Landau order parameter (OP). The associated dynamics has been argued to bear close resemblance to Rabi oscillations characteristic of two-level systems. Here, we address the question of whether this DQPT behavior is merely a manifestation of the limit of an effective two-level system or if it can arise as part of a more complex dynamics. We focus on quantum many-body scarring as a useful toy model allowing us to naturally study state transfer in an otherwise chaotic system. We find that a DQPT signals a change in the dominant contribution to the wave function in the degenerate initial-state manifold, with a direct relation to an OP zero only in the special case of occurring at the midpoint of an evenly degenerate manifold. Our work generalizes previous results and reveals that, in general, periodic DQPTs comprise complex many-body dynamics fundamentally beyond that of two-level systems.

quant-ph↗

arXiv2023

Evidence for a topological "exciton Fermi sea" in bilayer graphene

The quantum Hall physics of bilayer graphene is extremely rich due to the interplay between a layer degree of freedom and delicate fractional states. Recent experiments show that when an electric field perpendicular to the bilayer causes Landau levels of opposing layers to cross in energy, a even-denominator Hall plateau can coexist with a finite density of inter-layer excitons. We present theoretical and numerical evidence that this observation is due to a new phase of matter -- a Fermi sea of topological excitons.

cond-mat.str-el↗

arXiv2023

Observation of many-body scarring in a Bose--Hubbard quantum simulator

The ongoing quest for understanding nonequilibrium dynamics of complex quantum systems underpins the foundation of statistical physics as well as the development of quantum technology. Quantum many-body scarring has recently opened a window into novel mechanisms for delaying the onset of thermalization by preparing the system in special initial states, such as the $\mathbb{Z}_2$ state in a Rydberg atom system. Here we realize many-body scarring in a Bose-Hubbard quantum simulator from previously unknown initial conditions such as the unit-filling state. We develop a quantum-interference protocol for measuring the entanglement entropy and demonstrate that scarring traps the many-body system in a low-entropy subspace. Our work makes the resource of scarring accessible to a broad class of ultracold-atom experiments, and it allows one to explore the relation of scarring to constrained dynamics in lattice gauge theories, Hilbert space fragmentation, and disorder-free localization.

cond-mat.quant-gas↗

arXiv2023

Signatures of Supersymmetry in the $ν{=}5/2$ Fractional Quantum Hall Effect

The Moore-Read state, one of the leading candidates for describing the fractional quantum Hall effect at filling factor $ν{=}5/2$, is a paradigmatic $p$-wave superconductor with non-Abelian topological order. Among its many exotic properties, the state hosts two collective modes: a bosonic density wave and a neutral fermion mode that arises from an unpaired electron in the condensate. It has recently been proposed that the descriptions of the two modes can be unified by postulating supersymmetry (SUSY) that relates them in the long-wavelength limit. Here we extend the SUSY description to construct wave functions of the two modes on closed surfaces, such as the sphere and torus, and we test the resulting states in large-scale numerical simulations. We demonstrate the equivalence in the long-wavelength limit between SUSY wave functions and previous descriptions of collective modes based on the Girvin-MacDonald-Platzman ansatz, Jack polynomials, and bipartite composite fermions. Leveraging the first-quantized form of the SUSY wave functions, we study their energies using the Monte Carlo method and show that realistic $ν{=}5/2$ systems are close to the putative SUSY point, where the two collective modes become degenerate in energy.

cond-mat.str-el↗