arXiv subjects

Gerhard Zürn

Publications and source records attributed to Gerhard Zürn.

At least 19 recordsLinked to original sources

arXiv2026

Universal Defect Statistics in Reverse Quenches

We derive an extension of the generalized Kibble-Zurek mechanism (KZM), which provides a stochastic approach to defect formation, to reverse quench protocols. While the universal scaling behavior of defects in reverse quenches has been observed previously in certain 1D chains, we argument from a probability-theoretical perspective that this scaling persists in general, and is given by the double of the defect density variance in forward quenches. We validate these results analytically for the one-dimensional transverse field Ising model and numerically for its bond-disordered variant governed by an infinite randomness fixed point. Starting from the paramagnetic phase, this protocol relies solely on global control of the system's magnetic field and global magnetization measurements, enabling the extraction of critical exponents without microscopic access to individual defects. This approach offers a robust and experimentally feasible method for probing quantum critical behavior through entirely global operations.

quant-ph↗

arXiv2026

Quantum information scrambling in strongly disordered Rydberg spin systems

Despite the fact that power-law interactions occur in a plethora of physical systems, their many-body dynamics is far less understood than that of nearest-neighbor interacting systems. Here, we study information scrambling in strongly disordered spin systems with power-law interactions via out-of-time-order correlators (OTOCs). Numerically, we find pronounced differences in the dynamical spreading of OTOCs between nearest-neighbor and power-law interacting systems. This deviation persists even for short-range interactions, opposing the common view that these interactions produce dynamics equivalent to the nearest-neighbor case. In a detailed experimental proposal, tailored but not limited to Rydberg tweezer setups, we present a protocol to extract OTOCs in XXZ Heisenberg spin systems with tunable anisotropy and programmable disorder based on currently available techniques.

quant-ph↗

arXiv2025

Observation of hysteresis in an isolated quantum system of disordered Heisenberg spins

We find energy-dependent hysteresis in an isolated Heisenberg quantum spin system, similar to thermomagnetic hysteresis in canonical spin glasses in contact with a thermal reservoir. Analogous to zero-field cooling and field cooling in conventional magnetic materials, an annealing protocol is devised to control the energy in an isolated system. Depending on the strength of disorder, the susceptibilities at zero field bifurcate at a specific energy, which signals the presence of different magnetic regimes. This behavior is apparent both in a numerical simulation by exact diagonalization of the Heisenberg Hamiltonian with twelve particles, as well as in an experiment with thousands of Rydberg atoms representing dipolar interacting quantum spins. The annealing protocols open a new path to explore the energy-dependent phase structure of spin systems at low energies. Our observation of a nonthermal metastable regime might indicate the existence of a phase transition to a novel state of isolated quantum spin systems.

quant-ph↗

arXiv2024

Observation of anisotropy-independent magnetization dynamics in spatially disordered Heisenberg spin systems

An important step towards a comprehensive understanding of far-from-equilibrium dynamics of quantum many-body systems is the identification of unifying features that are independent of microscopic details of the system. We experimentally observe such robust features in the magnetization relaxation dynamics of disordered Heisenberg XX-, XXZ- and Ising Hamiltonians. We realize these Heisenberg spin models with tunable anisotropy parameter and power-law interactions in an ensemble of Rydberg atoms by encoding the spin in suitable Rydberg state combinations. We consistently observe stretched-exponential relaxation of magnetization for all considered spin models, collapsing onto a single curve after appropriate rescaling of time. This robust short-time relaxation behavior is explained by a perturbative treatment that exploits the strong disorder in pairwise couplings, which leads to a description in terms of approximately independent pairs of spins. In numerical simulations of small systems, we show that these pairs of spins constitute approximate local integrals of motion, which remain at least partially conserved on a timescale exceeding the duration of the relaxation dynamics of the magnetization.

quant-ph↗

arXiv2024

Emergent pair localization in a many-body quantum spin system

Understanding how closed quantum systems dynamically approach thermal equilibrium presents a major unresolved problem in statistical physics. Generically, non-integrable quantum systems are expected to thermalize as they comply with the Eigenstate Thermalization Hypothesis. However, in the presence of strong disorder, the dynamics can possibly slow down to a degree that systems fail to thermalize on experimentally accessible timescales, as in spin glasses or many-body localized systems. In general, particularly in long-range interacting quantum systems, the specific nature of the disorder necessary for the emergence of a prethermal, metastable state--distinctly separating the timescales of initial relaxation and subsequent slow thermalization--remains an open question. We study an ensemble of Heisenberg spins with a tunable distribution of random coupling strengths realized by a Rydberg quantum simulator. We observe a drastic change in the late-time magnetization when increasing disorder strength. The data is well described by models based on pairs of strongly interacting spins, which are treated as thermal for weak disorder and isolated for strong disorder. Our results indicate a crossover into a pair-localized prethermal regime in a closed quantum system of thousands of spins in the critical case where the exponent of the power law interaction matches the spatial dimension.

quant-ph↗

arXiv2024

Time-reversal in a dipolar quantum many-body spin system

Time reversal in a macroscopic system is contradicting daily experience. It is practically impossible to restore a shattered cup to its original state by just time reversing the microscopic dynamics that led to its breakage. Yet, with the precise control capabilities provided by modern quantum technology, the unitary evolution of a quantum system can be reversed in time. Here, we implement a time-reversal protocol in a dipolar interacting, isolated many-body spin system represented by Rydberg states in an atomic gas. By changing the states encoding the spin, we flip the sign of the interaction Hamiltonian, and demonstrate the reversal of the relaxation dynamics of the magnetization by letting a demagnetized many-body state evolve back-in-time into a magnetized state. We elucidate the role of atomic motion using the concept of a Loschmidt echo. Finally, by combining the approach with Floquet engineering, we demonstrate time reversal for a large family of spin models with different symmetries. Our method of state transfer is applicable across a wide range of quantum simulation platforms and has applications far beyond quantum many-body physics, reaching from quantum-enhanced sensing to quantum information scrambling.

quant-ph↗

arXiv2021

Nonlinear absorption in interacting Rydberg electromagnetically-induced-transparency spectra on two-photon resonance

We experimentally investigate the nonlinear transmission spectrum of coherent light fields propagating through a Rydberg EIT medium with strong atomic interactions. In contrast to previous investigations, which have largely focused on resonant control fields, we explore here the full two-dimensional spectral response of the Rydberg gas. Our measurements confirm previously observed spectral features for a vanishing control-field detuning, but also reveal significant differences on two-photon resonance. In particular, we find qualitative deficiencies of mean-field models and rate-equation simulations as well as a third-order nonlinear susceptibility that accounts for pair-wise interaction effects at low probe-field intensities in describing the nonlinear probe-field response under EIT conditions. Our results suggest that a more complete understanding of Rydberg-EIT and emerging photon interactions requires to go beyond existing simplified models as well as few-photon theories.

physics.atom-ph↗

arXiv2021

A stationary Rydberg polariton

We propose a novel scheme for coupling a Rydberg state to a stationary light polariton, based on a dual-V level scheme. We investigate the properties of the resulting stationary Rydberg polariton, and show that its form and its quadratic dispersion relation closely resemble that of the stationary light polariton of the underlying dual-V scheme. We consider the influence of a Rydberg impurity on the system and find strong interaction-induced absorption of the involved probe field. The proposed scheme for a stationary Rydberg polariton might find applications for realizing interacting polaritons with increased interaction time.

quant-ph↗

arXiv2021

Glassy quantum dynamics of disordered Ising spins

We study the out-of-equilibrium dynamics in the quantum Ising model with power-law interactions and positional disorder. For arbitrary dimension $d$ and interaction range $α\geq d$ we analytically find a stretched exponential decay of the global magnetization and ensemble-averaged single-spin purity with a stretch-power $β= d/α$ in the thermodynamic limit. Numerically, we confirm that glassy behavior persists for finite system sizes and sufficiently strong disorder. We identify dephasing between disordered coherent pairs as the main mechanism leading to a relaxation of global magnetization, whereas genuine many-body interactions lead to a loss of single-spin purity which signifies the build-up of entanglement. The emergence of glassy dynamics in the quantum Ising model extends prior findings in classical and open quantum systems, where the stretched exponential law is explained by a scale-invariant distribution of time scales, to both integrable and non-integrable quantum systems.

quant-ph↗

arXiv2021

Semiclassical simulations predict glassy dynamics for disordered Heisenberg models

We numerically study out-of-equilibrium dynamics in a family of Heisenberg models with $1/r^6$ power-law interactions and positional disorder. Using the semi-classical discrete truncated Wigner approximation (dTWA) method, we investigate the time evolution of the magnetization and ensemble-averaged single-spin purity for a strongly disordered system after initializing the system in an out-of-equilibrium state. We find that both quantities display robust glassy behavior for almost any value of the anisotropy parameter of the Heisenberg Hamiltonian. Furthermore, a systematic analysis allows us to quantitatively show that, for all the scenarios considered, the stretch power lies close to the one analytically obtained in the Ising limit. This indicates that glassy relaxation behavior occurs widely in disordered quantum spin systems, independent of the particular symmetries and integrability of the Hamiltonian.

cond-mat.dis-nn↗

arXiv2021

Floquet Hamiltonian Engineering of an Isolated Many-Body Spin System

Controlling interactions is the key element for quantum engineering of many-body systems. Using time-periodic driving, a naturally given many-body Hamiltonian of a closed quantum system can be transformed into an effective target Hamiltonian exhibiting vastly different dynamics. We demonstrate such Floquet engineering with a system of spins represented by Rydberg states in an ultracold atomic gas. Applying a sequence of spin manipulations, we change the symmetry properties of the effective Heisenberg XYZ Hamiltonian. As a consequence, the relaxation behavior of the total spin is drastically modified. The observed dynamics can be qualitatively captured by a semi-classical simulation. Synthesising a wide range of Hamiltonians opens vast opportunities for implementing quantum simulation of non-equilibrium dynamics in a single experimental setting.

cond-mat.quant-gas↗

arXiv2019

Experimental Characterization of Two-Particle Entanglement through Position and Momentum Correlations

Quantum simulation is a rapidly advancing tool to gain insight into complex quantum states and their dynamics. Trapped ion systems have pioneered deterministic state preparation and comprehensive state characterization, operating on localized and thus distinguishable particles. With ultracold atom experiments, one can prepare large samples of delocalized particles, but the same level of characterization has not yet been achieved. Here, we present a method to measure the positions and momenta of individual particles to obtain correlations and coherences. We demonstrate this with deterministically prepared samples of two interacting ultracold fermions in a coupled double well. As a first application, we use our technique to certify and quantify different types of entanglement.

cond-mat.quant-gas↗

arXiv2019

Blockade-induced resonant enhancement of the optical nonlinearity in a Rydberg medium

We predict a resonant enhancement of the nonlinear optical response of an interacting Rydberg gas under conditions of electromagnetically induced transparency. The enhancement originates from a two-photon process which resonantly couples electronic states of a pair of atoms dressed by a strong control field. We calculate the optical response for the three-level system by explicitly including the dynamics of the intermediate state. We find an analytical expression for the third order susceptibility for a weak classical probe field. The nonlinear absorption displays the strongest resonant behavior on two-photon resonance where the detuning of the probe field equals the Rabi frequency of the control field. The nonlinear dispersion of the medium exhibits various spatial shapes depending on the interaction strength. Based on the developed model, we propose a realistic experimental scenario to observe the resonance by performing transmission measurements.

physics.atom-ph↗

arXiv2018

Spin-resolved single-atom imaging of $^6$Li in free space

We present a versatile imaging scheme for fermionic $^6$Li atoms with single-particle sensitivity. Our method works for freely propagating particles and completely eliminates the need for confining potentials during the imaging process. We illuminate individual atoms in free space with resonant light and collect their fluorescence on an electron-multiplying CCD camera using a high-numerical-aperture imaging system. We detect approximately \num{20} photons per atom during an exposure of 20 $μ$s and identify individual atoms with a fidelity of $99.4\pm0.3$ % . By addressing different optical transitions during two exposures in rapid succession, we additionally resolve the hyperfine spin state of each particle. The position uncertainty of the imaging scheme is 4.0 $μ$m, given by the diffusive motion of the particles during the imaging pulse. The absence of confining potentials enables readout procedures, such as the measurement of single-particle momenta in time of flight, which we demonstrate here. Our imaging scheme is technically simple and easily adapted to other atomic species.

cond-mat.quant-gas↗

arXiv2017

High temperature pairing in a strongly interacting two-dimensional Fermi gas

We observe many-body pairing in a two-dimensional gas of ultracold fermionic atoms at temperatures far above the critical temperature for superfluidity. For this, we use spatially resolved radio-frequency spectroscopy to measure pairing energies spanning a wide range of temperatures and interaction strengths. In the strongly interacting regime where the scattering length between fermions is on the same order as the inter-particle spacing, the pairing energy in the normal phase significantly exceeds the intrinsic two-body binding energy of the system and shows a clear dependence on local density. This implies that pairing in this regime is driven by many-body correlations, rather than two-body physics. We find this effect to persist at temperatures close to the Fermi temperature which demonstrates that pairing correlations in strongly interacting two-dimensional fermionic systems are remarkably robust against thermal fluctuations.

cond-mat.quant-gas↗

arXiv2016

Interaction-Enhanced Imaging of Rydberg P states

The Interaction Enhanced Imaging technique allows to detect the spatial distribution of strongly interacting impurities embedded within a gas of background atoms used as a contrast medium. Here we present a detailed study of this technique, applied to detect Rydberg $P$ states. We experimentally realize fast and efficient three-photon excitation of $P$ states, optimized according to the results of a theoretical effective two-level model. Few Rydberg $P$-state atoms, prepared in a small cloud with dimensions comparable to the blockade radius, are detected with a good sensitivity by averaging over 50 shots. The main aspects of the technique are described with a hard-sphere model, finding good agreement with experimental data. This work paves the way to a non-destructive optical detection of single Rydberg atoms with high spatial and temporal resolution.

physics.atom-ph↗

arXiv2015

Antiferromagnetic Heisenberg Spin Chain of a Few Cold Atoms in a One-Dimensional Trap

We report on the deterministic preparation of antiferromagnetic Heisenberg spin chains consisting of up to four fermionic atoms in a one-dimensional trap. These chains are stabilized by strong repulsive interactions between the two spin components without the need for an external periodic potential. We independently characterize the spin configuration of the chains by measuring the spin orientation of the outermost particle in the trap and by projecting the spatial wave function of one spin component on single-particle trap levels. Our results are in good agreement with a spin-chain model for fermionized particles and with numerically exact diagonalizations of the full few-fermion system.

cond-mat.quant-gas↗

arXiv2015

Two Fermions in a double well: Exploring a fundamental building block of the Hubbard model

We have prepared two ultracold fermionic atoms in an isolated double-well potential and obtained full control over the quantum state of this system. In particular, we can independently control the interaction strength between the particles, their tunneling rate between the wells and the tilt of the potential. By introducing repulsive (attractive) interparticle interactions we have realized the two-particle analog of a Mott-insulating (charge-density-wave) state. We have also spectroscopically observed how second-order tunneling affects the energy of the system. This work realizes the first step of a bottom-up approach to deterministically create a single-site addressable realization of a ground-state Fermi-Hubbard system.

cond-mat.quant-gas↗