arXiv subjects

Sebastian Erne

Publications and source records attributed to Sebastian Erne.

At least 19 recordsLinked to original sources

arXiv2026

Dispersive Shock Waves in a 1D Quantum Liquid

We implement a moving boundary condition in a 1D quantum liquid to study nonlinear wave breaking and its regularization by dispersion. Programmable optical potentials allow us to compress a weakly interacting ultra-cold Bose gas trapped on an atomchip at tunable speeds of up to three times the speed of sound and we subsequently measure the quasi-in-situ density distribution to extract the shock wave edge dynamics. We resolve both leading and trailing edge velocities and observe a shock wave width that increases linearly in time, which are distinguishing features of dispersive shock waves, consistent with asymptotic predictions using Whitham's method. Quantitative agreement is found with finite temperature non-polynomial Schr\"odinger equation simulations, taking into account the imaging process and the finite height of the piston potential. Our results constitute a controlled, quantitative test of dispersive shock dynamics in a 1D quantum fluid and demonstrate that the coarse-grained dispersive-shock phenomenology remains robust even as the microscopic dynamics depart from the strictly integrable 1D regime.

cond-mat.quant-gas↗

arXiv2026

Experimentally probing the Quantum Physics in the Inverted Harmonic Oscillator

When a quantum system passes through an unstable fixed point the local dynamics reduces to the inverted harmonic oscillator (IHO). It exponentially amplifies along one quadrature while squeezing the other, producing macroscopically extended quantum states from microscopic zero-point fluctuations. We realize this dynamics with a Bose-Einstein condensate on an AtomChip. Radio-frequency dressing flips the transverse harmonic confinement into an IHO. Through phase-space tomography we follow the full Wigner function of the evolving quantum state, observe sub-vacuum squeezing of 10.6(1.3) dB, and test coherent reversibility by time-reversing the IHO evolution. Matter-wave interference between the two daughter clouds confirms quantum coherence over timescales far beyond the initial expansion. Our experiment establishes ultra-cold atoms as a clean, controlled, many-body platform for unstable quantum dynamics opening a route to force sensing with time-reversal-based coherence certification and to analog studies of the amplification of quantum fluctuations in inflationary field dynamics.

quant-ph↗

arXiv2025

Damping of phonons in one-dimensional quantum fluids

Collective excitations in one-dimensional (1D) quantum fluids are expected to propagate almost without dissipation. Here we directly excite phonon modes in a weakly interacting 1D Bose gas and study their time evolution. In the linear response regime, damping is surprisingly fast and quantitatively follows the non-analytic scaling predicted by Andreev's hydrodynamic description. For stronger excitations, we observe a crossover to a highly nonlinear regime characterized by wave breaking, captured by the finite-temperature nonlinear Schr\"odinger evolution. Our results resolve a long-standing question on the fate of phonons in 1D Bose gases, and open new pathways to study non-linear relaxation in quantum many-body systems.

cond-mat.quant-gas↗

arXiv2025

Momentum-resolved two-dimensional spectroscopy as a probe of nonlinear quantum field dynamics

Emergent collective excitations constitute a hallmark of interacting quantum many-body systems, yet in solid-state platforms their study has been largely limited by the constraints of linear-response probes and by finite momentum resolution. We propose to overcome these limitations by combining the spatial resolution of ultracold atomic systems with the nonlinear probing capabilities of two-dimensional spectroscopy (2DS). As a concrete illustration, we analyze momentum-resolved 2DS of the quantum sine-Gordon model describing the low energy dynamics of two weakly coupled one-dimensional Bose-Einstein condensates. This approach reveals distinctive many-body signatures, most notably asymmetric cross-peaks reflecting the interplay between isolated ($B_2$ breather) and continuum ($B_1$ pair) modes. The protocol further enables direct characterization of anharmonicity and disorder, establishing momentum-resolved 2DS as both a powerful diagnostic for quantum simulators and a versatile probe of correlated quantum matter.

cond-mat.quant-gas↗

arXiv2025

Nondestructive optomechanical detection scheme for Bose-Einstein condensates

We present a two-tone heterodyne optical readout scheme to extract unequal-time density correlations along an arbitrary stationary interaction path from a pancake-shaped Bose-Einstein condensate, using a modulated laser probe. Analysing the measurement noise both from imprecision and backaction, we identify the standard quantum limit for the signal-extraction scheme, and examine how a class of two-mode squeezed initial states can be used to push beyond this limit. As an application, we show how the readout scheme can be used for an experimentally feasible realisation of acceleration-dependence of quantum-vacuum fluctuations in the system, including the analogue spacetime circular motion Unruh effect. The scheme is adaptable beyond Bose-Einstein condensates, providing nondestructive access to unequal-time correlations in quantum fluids.

cond-mat.quant-gas↗

arXiv2025

Universal non-thermal fixed point for quasi-1D Bose gases

Spatio-temporal scaling dynamics connected to non-thermal fixed points has been suggested as a universal framework to describe the relaxation of isolated far-from-equilibrium systems. Experimental studies in weakly-interacting cold atom systems have found scaling dynamics connected to specific attractors. In our experiments, we study a quantum gas of strongly interacting $^6$Li$_2$ Feshbach molecules, brought far out of equilibrium by imprinting a white-noise phase profile. The observed relaxation follows the same universal dynamics as for the previously observed formation of the order parameter in a shock-cooled gas of weakly interacting $^{87}$Rb atoms. Our results point to a single universal fixed point with a large basin of attraction governing the relaxation of quasi-1D bosonic systems, independent of their specific initial conditions and microscopic details.

cond-mat.quant-gas↗

arXiv2024

Tachyonic and parametric instabilities in an extended bosonic Josephson junction

We study the dynamics and decay of quantum phase coherence for Bose-Einstein condensates in tunnel-coupled quantum wires. The two elongated Bose-Einstein condensates exhibit a wide variety of dynamic phenomena where quantum fluctuations can lead to a rapid loss of phase coherence. We investigate the phenomenon of self-trapping in the relative population imbalance of the two condensates, particularly $\pi$-trapped oscillations that occur when also the relative phase is trapped. Though this state appears stable in mean-field descriptions, the $\pi$-trapped state becomes dynamically unstable due to quantum fluctuations. Nonequilibrium instabilities result in the generation of pairs excited from the condensate to higher momentum modes. We identify tachyonic instabilities, which are associated with imaginary parts of the dispersion relation, and parametric resonance instabilities that are triggered by oscillations of the relative phase and populations. At early times, we compute the instability chart of the characteristic modes through a linearized analysis and identify the underlying physical process. At later times, the primary instabilities trigger secondary instabilities due to the build-up of non-linearities. We perform numerical simulations in the Truncated Wigner approximation in order to observe the dynamics also in this non-linear regime. Furthermore, we discuss realistic parameters for experimental realizations of the $\pi$-trapped state in ultracold atom setups.

cond-mat.quant-gas↗

arXiv2024

Measurement of total phase fluctuation in cold-atomic quantum simulators

Studying the dynamics of quantum many-body systems is often constrained by the limitations in probing relevant observables, especially in continuous systems. A powerful method to gain information about such systems is the reconstruction of local currents from the continuity equation. We show that this approach can be used to extract the total phase fluctuation of adjacent Bose gases. We validate our technique numerically and demonstrate its effectiveness by analyzing data from selected experiments simulating 1D quantum field theories through the phase difference of two parallel 1D Bose gases. This analysis reveals the previously hidden sector of the sum mode of the phase, which is important for studying long-time thermalization and out-of-equilibrium dynamics of the system. Our method is general and can be applied to other cold atom systems with spatial phase gradients, thereby expanding the scope and capabilities of cold-atomic quantum simulators.

cond-mat.quant-gas↗

arXiv2024

Systematic analysis of relative phase extraction in one-dimensional Bose gases interferometry

Spatially resolved relative phase measurement of two adjacent 1D Bose gases is enabled by matter-wave interference upon free expansion. However, longitudinal dynamics is typically ignored in the analysis of experimental data. We provide an analytical formula showing a correction to the readout of the relative phase due to longitudinal expansion and mixing with the common phase. We numerically assess the error propagation to the estimation of the gases' physical quantities such as correlation functions and temperature. Our work characterizes the reliability and robustness of interferometric measurements, directing us to the improvement of existing phase extraction methods necessary to observe new physical phenomena in cold-atomic quantum simulators.

cond-mat.quant-gas↗

arXiv2024

Hamiltonian Learning in Quantum Field Theories

We discuss Hamiltonian learning in quantum field theories as a protocol for systematically extracting the operator content and coupling constants of effective field theory Hamiltonians from experimental data. Learning the Hamiltonian for varying spatial measurement resolutions gives access to field theories at different energy scales, and allows to learn a flow of Hamiltonians reminiscent of the renormalization group. Our method, which we demonstrate in both theoretical studies and available data from a quantum gas experiment, promises new ways of addressing the emergence of quantum field theories in quantum simulation experiments.

cond-mat.quant-gas↗

arXiv2023

The Whitham approach to Generalized Hydrodynamics

The formation of dispersive shock waves in the one-dimensional Bose gas represents a limitation of Generalized Hydrodynamics (GHD) due to the coarse-grained nature of the theory. Nevertheless, GHD accurately captures the long wavelength behavior indicating an implicit knowledge of the underlying microscopic physics. Such representation are already known through the Whitham modulation theory, where dispersion-less equations describe the evolution of the slowly varying shock wave parameters. Here we study the correspondence between Whithams approach to the Gross-Pitaevskii equation and GHD in the semi-classical limit. Our findings enable the recovery of the shock wave solution directly from GHD simulations, which we demonstrate for both zero and finite temperature. Additionally, we study how free expansion protocols affect the shock wave density and their implications for experimental detection. The combined picture of Whitham and GHD lends itself to additional physical interpretation regarding the formation of shock waves. Further, this picture exhibits clear analogies to the theory of Quantum GHD, and we discuss possible routes to establish an explicit connection between them.

cond-mat.stat-mech↗

arXiv2023

Optimal control of quasi-1D Bose gases in optical box potentials

In this paper, we investigate the manipulation of quasi-1D Bose gases that are trapped in a highly elongated potential by optimal control methods. The effective meanfield dynamics of the gas can be described by a one-dimensional non-polynomial Schr\"odinger equation. We extend the indirect optimal control method for the Gross-Pitaevskii equation by Winckel and Borzi (2008) to obtain necessary optimality conditions for state and energy cost functionals. This approach is then applied to optimally compress a quasi-1D Bose gase in an (optical) box potential, i.e., to find a so-called short-cut to adiabaticity, by solving the optimality conditions numerically. The behavior of the proposed method is finally analyzed and compared to simple direct optimization strategies using reduced basis functions. Simulations results demonstrate the feasibility of the proposed approach.

eess.SY↗

arXiv2023

The discrete Green's function method for wave packet expansion via the free Schr\"odinger equation

We consider the expansion of wave packets governed by the free Schr\"odinger equation. This seemingly simple task plays an important role in simulations of various quantum experiments and in particular in the field of matter-wave interferometry. The initial tight confinement results in a very fast expansion of the wave function at later times which significantly complicates an efficient and precise numerical evaluation. In many practical cases the expansion time is too short for the validity of the stationary phase approximation and too long for an efficient application of Fourier collocation-based methods. We develop an alternative method based on a discretization of the free-particle propagator. This simple approach yields highly accurate results which readily follows from the exceptionally fast convergence of the trapezoidal rule approximation of integrals involving smooth and rapidly decaying functions. We discuss and analyze our approach in detail and demonstrate how to estimate the numerical error in the one-dimensional setting. Furthermore, we show that by exploiting the separability of the Green's function, the numerical effort of the multi-dimensional approximation is considerably reduced. Our method is very fast, highly accurate, and easy to implement on modern hardware.

physics.comp-ph↗

arXiv2022

Optimizing optical potentials with physics-inspired learning algorithms

We present our new experimental and theoretical framework which combines a broadband superluminescent diode (SLED/SLD) with fast learning algorithms to provide speed and accuracy improvements for the optimization of 1D optical dipole potentials, here generated with a Digital Micromirror Device (DMD). To characterize the setup and potential speckle patterns arising from coherence, we compare the superluminescent diode to a single-mode laser by investigating interference properties. We employ Machine Learning (ML) tools to train a physics-inspired model acting as a digital twin of the optical system predicting the behavior of the optical apparatus including all its imperfections. Implementing an iterative algorithm based on Iterative Learning Control (ILC) we optimize optical potentials an order of magnitude faster than heuristic optimization methods. We compare iterative model-based offline optimization and experimental feedback-based online optimization. Our methods provide a new route to fast optimization of optical potentials which is relevant for the dynamical manipulation of ultracold gases.

cond-mat.quant-gas↗

arXiv2022

Experimental Observation of Curved Light-Cones in a Quantum Field Simulator

We investigate signal propagation in a quantum field simulator of the Klein-Gordon model realized by two strongly coupled parallel one-dimensional quasi-condensates. By measuring local phononic fields after a quench, we observe the propagation of correlations along sharp light-cone fronts. If the local atomic density is inhomogeneous, these propagation fronts are curved. For sharp edges, the propagation fronts are reflected at the system's boundaries. By extracting the space-dependent variation of the front velocity from the data, we find agreement with theoretical predictions based on curved geodesics of an inhomogeneous metric. This work extends the range of quantum simulations of non-equilibrium field dynamics in general spacetime metrics.

cond-mat.quant-gas↗

arXiv2022

Primary thermalisation mechanism of Early Universe observed from Faraday-wave scattering on liquid-liquid interfaces

For the past two hundred years, parametric instabilities have been studied in various physical systems, such as fluids, mechanical devices and even inflationary cosmology. It was not until a few decades ago that this subharmonic unstable response arose as a central mechanism for the thermalisation of the Early Universe, in a theory known as preheating. Here we study a parametrically driven two-fluid interface to simulate the key aspects of inflationary preheating dynamics through the onset of nonlinear Faraday waves. We present a detailed analysis of the effective field theory description for interfacial waves through the factorization properties of higher-order correlations. Despite the intricacies of a damped and highly interacting hydrodynamical system, we show that the scattering of large amplitude Faraday waves is connected to a broadening of primary resonance bands and the subsequent appearance of secondary instabilities as predicted in preheating dynamics.

gr-qc↗

arXiv2022

Bridging Effective Field Theories and Generalized Hydrodynamics

Generalized Hydrodynamics (GHD) has recently been devised as a method to solve the dynamics of integrable quantum many-body systems beyond the mean-field approximation. In its original form, a major limitation is the inability to predict equal-time correlations. Here we present a new method to treat thermal fluctuations of a 1D bosonic degenerate gas within the GHD framework. We show how the standard results using the thermodynmaic Bethe ansatz can be obtained through sampling of collective bosonic excitations, revealing the connection or duality between GHD and effective field theories such as the standard hydrodynamic equations. As an example, we study the damping of a coherently excited density wave and show how equal-time phase correlation functions can be extracted from the GHD evolution. Our results present a conceptually new way of treating fluctuations beyond the linearized regime of GHD.

cond-mat.quant-gas↗

arXiv2022

Floquet engineering a bosonic Josephson junction

We study Floquet engineering of the tunnel coupling between a pair of one-dimensional bosonic quasi-condensates in a tilted double-well potential. By modulating the energy difference between the two wells, we re-establish tunnel coupling and precisely control its amplitude and phase. This allows us to initiate coherence between two initially uncorrelated Bose gases and prepare different initial states in the emerging sine-Gordon Hamiltonian. We fully characterize the Floquet system and study the dependence of both equilibrium properties and relaxation on the modulation.

cond-mat.quant-gas↗