arXiv subjects

A. R. Fritsch

Publications and source records attributed to A. R. Fritsch.

15 recordsLinked to original sources

arXiv2026

Q-GAIN: A Python Package for Machine Learning and Physically Informed Analysis Applications

Here we describe the quantum gas analysis and inference (Q-GAIN) Python package, which enables rapid deployment of machine learning (ML) and physics-informed analysis techniques for cold-atom experiments. Out of the box, Q-GAIN implements classification, object detection, and physics-informed metrics for feature detection in images of atomic Bose-Einstein condensates (BECs). Q-GAIN encourages a natural, module-based workflow: starting with data loading and preprocessing, followed by ML-based feature identification, and ending with conventional analysis techniques. We demonstrate this modularity by configuring Q-GAIN for three ML tasks. First, we demonstrate the basic workflow of the Q-GAIN framework by implementing the standard task of classifying handwritten digits from the MNIST dataset. Then, we re-implement our earlier soliton detection (SolDet) package in the Q-GAIN framework, enabling the detection and analysis of solitonic excitations in time-of-flight data. Finally, we develop an object-detection tool that identifies quantized vortices in images of ring-shaped BECs.

cond-mat.quant-gas↗

arXiv2026

Quantum Optimization Algorithms for Strongly Correlated Many-Body Systems

This perspective article analyzes the potential and critical challenges of employing quantum optimization algorithms to investigate phase transitions in quantum many-body systems during the Noisy Intermediate-Scale Quantum era. The simulation of strongly correlated systems is frequently intractable on classical computers due to the exponential growth of the Hilbert space and the fermionic sign problem. In this context, we review and compare the performance of traditional Variational Quantum Algorithms, such as the Variational Quantum Eigensolver and the Quantum Approximate Optimization Algorithm, against emerging heuristic approaches, specifically Feedback-based Quantum Algorithms, such as FALQON. We explore the applicability of these methods in the study of open phenomena in condensed matter physics, including Deconfined Quantum Criticality, strange metals, Many-Body Localization, topological phase transitions, and quantum spin liquids. We discuss how fundamental operational bottlenecks, notably expressibility- and noise-induced barren plateaus, severely compromise gradient-based optimization. We conclude that deterministic feedback-guided methods provide geometrically more robust trajectories for navigating the energy landscape of these systems, arguing that further advancement in the field will rely on deep hybridization and physics-informed circuit co-design towards fault tolerance.

quant-ph↗

arXiv2025

Observation of Relaxation Stages in a Nonequilibrium Closed Quantum System: Decaying Turbulence in a Trapped Superfluid

The dynamics of non-equilibrium closed quantum systems and their route to thermalization are of fundamental interest to several fields, from cosmology to particle physics. However, a comprehensive description of non-equilibrium phenomena still presents a significant challenge. In this work, we report the observation of distinct stages during the relaxation of the decaying turbulence in trapped Bose-Einstein condensates. Our findings show a direct particle cascade from low to high momenta, a consequence of the energy injection in the system, exhibiting a characteristic universal scaling. This stage is followed by an inverse particle cascade responsible for repopulating the previously depleted condensate. Both cascades can be explained through self-similar solutions provided by wave turbulence theory. These findings provide important insights into the relaxation stages of out-of-equilibrium quantum many-body systems.

cond-mat.quant-gas↗

arXiv2024

Universal scaling in far-from-equilibrium quantum systems: an equivalent differential approach

Recent progress in out-of-equilibrium closed quantum systems has significantly advanced the understanding of mechanisms behind their evolution towards thermalization. Notably, the concept of non-thermal fixed points (NTFPs) - responsible for the emergence of spatio-temporal universal scaling in far-from-equilibrium systems - has played a crucial role in both theoretical and experimental investigations. In this work, we introduce a differential equation that has the universal scaling associated with NTFPs as a solution. The advantage of working with a differential equation, rather than only with its solution, is that we can extract several insightful properties not necessarily present in the solution alone. How the differential equation is derived allows physical interpretation of the universal exponents in terms of the time dependence of the amplitude of the distributions and their momentum scaling. Employing two limiting cases of the equation, we determined the universal exponents related to the scaling using the distributions near just two momentum values. We established a solid agreement with previous investigations by validating this approach with three distinct physical systems. This consistency highlights the universal nature of scaling due to NTFPs and emphasizes the predictive capabilities of the proposed differential equation. Moreover, under specific conditions, the equation predicts a power-law related to the ratio of the two universal exponents, leading to implications concerning particle and energy transport. This suggests that the observed power-laws in far-from-equilibrium turbulent fluids could be related to the universal scaling due to NTFPs, potentially offering new insights into the study of turbulence.

cond-mat.quant-gas↗

arXiv2024

Stationary solitons in F=1 spin-orbit coupled Bose-Einstein condensates

We consider solitary wave excitations above the ground state of $F=1$ spin-orbit coupled Bose-Einstein condensates (SOBECs). The low energy properties of SOBECs in any of the three branches of the single particle dispersion relation can be described by suitable scalar nonlinear Schrödinger (NLS) equations which we obtain using multiple-scale expansions. This enables us to examine a variety of different configurations, such as dark solitary waves associated with higher energy branches, as well as dark and bright structures in the lowest branch. The lowest branch can also exhibit a ``superstripe'' phase that supports solitary waves. In all cases, we provide explicit expressions for the NLS coefficients, and confirm their validity with full numerical simulations of the SOBEC system including a harmonic confining potential.

cond-mat.quant-gas↗

arXiv2022

Universal dynamics of a turbulent superfluid Bose gas

We study the emergence of universal scaling in the time-evolving momentum distribution of a harmonically trapped three-dimensional Bose-Einstein condensate, parametrically driven to a turbulent state. We demonstrate that the out-of-equilibrium dynamics post excitation is described by a single function due to nearby non-thermal fixed points. The observed behavior connects the dynamics of a quantum turbulent state to several far-from-equilibrium phenomena. We present a controllable protocol to explore universality in such systems, obtaining scaling exponents that can serve as reference for future theoretical investigations. Our experimental results thus offer a promising route to investigate the complex dynamics of the quantum turbulent regime under a novel perspective.

cond-mat.quant-gas↗

arXiv2022

Dynamical Instability of 3d Stationary and Traveling Planar Dark Solitons

Here we revisit the topic of stationary and propagating solitonic excitations in self-repulsive three-dimensional Bose-Einstein condensates by quantitatively comparing theoretical analysis and associated numerical computations with our experimental results. Using fully 3d numerical simulations, we explore the existence, stability, and evolution dynamics of planar dark solitons, as well as their instability-induced decay products including solitonic vortices and vortex rings. In the trapped case and with no adjustable parameters, our numerical findings are in correspondence with experimentally observed coherent structures. Without a longitudinal trap, we identify numerically exact traveling solutions and quantify how their transverse destabilization threshold changes as a function of the solitary wave speed.

cond-mat.quant-gas↗

arXiv2022

Observation of dynamical topology in 1D

Nontrivial topology in lattices is characterized by invariants--such as the Zak phase for one dimensional (1D) lattices--derived from wave functions covering the Brillouin zone. We realized the 1D bipartite Rice-Mele (RM) lattice using ultracold $^{87}$Rb and focus on lattice configurations possessing various combinations of chiral, time-reversal and particle-hole symmetries. We quenched between configurations and used a form of quantum state tomography, enabled by diabatically tuning lattice parameters, to directly follow the time evolution of the Zak phase as well as a chiral winding number. The Zak phase evolves continuously; however, when chiral symmetry transiently appears in the out-of-equilibrium system, the chiral winding number is well defined and can take on different integer values. When quenching between two configurations obeying all three symmetries the Zak phase is time independent; we confirm the contrasting prediction of [M. McGinley and N. R.Cooper, PRL 121 090401 (2018)] that chiral symmetry is periodically restored, at which times the winding number changes by $\pm 2$, yielding values that are not present in the native RM Hamiltonian.

cond-mat.quant-gas↗

arXiv2022

Floquet engineering topological Dirac bands

We experimentally realized a time-periodically modulated 1D lattice for ultracold atoms featuring a pair of linear bands, each associated with a Floquet winding number: a topological invariant. These bands are spin-momentum locked and almost perfectly linear everywhere in the Brillouin zone (BZ), making this system a near-ideal realization of the 1D Dirac Hamiltonian. We characterized the Floquet winding number using a form of quantum state tomography, covering the BZ and following the micromotion through one Floquet period. Lastly, we altered the modulation timing to lift the topological protection, opening a gap at the Dirac point that grew in proportion to the deviation from the topological configuration.

cond-mat.quant-gas↗

arXiv2020

Creating solitons with controllable and near zero velocity in Bose-Einstein condensates

Established techniques for deterministically creating dark solitons in repulsively interacting atomic Bose-Einstein condensates (BECs) can only access a narrow range of soliton velocities. Because velocity affects the stability of individual solitons and the properties of soliton-soliton interactions, this technical limitation has hindered experimental progress. Here we create dark solitons in highly anisotropic cigar-shaped BECs with arbitrary position and velocity by simultaneously engineering the amplitude and phase of the condensate wavefunction, improving upon previous techniques which only explicitly manipulated the condensate phase. The single dark soliton solution present in true 1D systems corresponds to the kink soliton in anisotropic 3D systems and is joined by a host of additional dark solitons including vortex ring and solitonic vortex solutions. We readily create dark solitons with speeds from zero to half the sound speed. The observed soliton oscillation frequency suggests that we imprinted solitonic vortices, which for our cigar-shaped system are the only stable solitons expected for these velocities. Our numerical simulations of 1D BECs show this technique to be equally effective for creating kink solitons when they are stable. We demonstrate the utility of this technique by deterministically colliding dark solitons with domain walls in two-component spinor BECs.

cond-mat.quant-gas↗

arXiv2019

Thermodynamic measurement of the sound velocity of a Bose gas across the transition to Bose-Einstein condensation

We present an alternative method for determining the sound velocity in atomic Bose-Einstein condensates, based on thermodynamic global variables. The total number of trapped atoms was as a function of temperature carefully studied across the phase transition, at constant volume. It allowed us to evaluate the sound velocity resulting in consistent values from the quantum to classical regime, in good agreement with previous results found in literature. We also provide some insight about the dominant sound mode (thermal or superfluid) across a wide temperature range.

cond-mat.quant-gas↗

arXiv2016

Twisted unwinding of multi-charged quantum vortex and generation of turbulence in atomic Bose-Einstein condensates

We report the observation of the twisted decay of quadruply charged vortices in an atomic Bose-Einstein condensate. Supporting numerical simulations show that the singly-charged vortices, which result from the decay of a multi-charged vortex, twist around intertwined in the shape of helical Kelvin waves. Finally, we propose to apply this effect to generate an almost isotropic state of turbulence which we characterize in terms of the velocity statistics.

cond-mat.quant-gas↗

arXiv2016

Isothermal compressibility determination across Bose-Einstein condensation

We apply the global thermodynamic variables approach to experimentally determine the isothermal compressibility parameter $κ_T$ of a trapped Bose gas across the phase transition. We demonstrate the behavior of $κ_T$ around the critical pressure, revealing the second order nature of the phase transition. Compressibility is the most important susceptibility to characterize the system. The use of global variables shows advantages with respect to the usual local density approximation method and can be applied to a broad range of situations.

cond-mat.quant-gas↗

arXiv2015

Investigation of the momentum distribution of an excited BEC by free expansion: coupling with collective modes

We investigate the evolution of the momentum distribution of a Bose-Einstein condensate subjected to an external small oscillatory perturbation as a function of the in-trap evolution of the condensate after the external perturbation is switched-off. Besides changing its momentum distribution, we observe that the cloud distributes the input energy among its normal collective modes, displaying center-of-mass dipolar mode and quadrupolar mode. While the dipolar mode can be easily disregarded, we show that the momentum distribution is closely tied to the quadrupolar oscillation mode. This convolution hinders the actual momentum distribution.

cond-mat.quant-gas↗

arXiv2015

Nonlinear dependence observed in quadrupolar collective excitation of a trapped BEC

We report the experimental observation of the collective excitations induced in a magnetically trapped 87Rb Bose-Einstein condensate. Low-lying mode excitations were studied by tracking the condensate's center-of-mass displacement, and its aspect ratio as a function of the hold time in the trap. We were able to partially control the modes onset by modulating the amplitude of the additional field gradient used to excite the BEC. The measured excitation frequencies were found to be in good agreement with the literature. We have also found that the modulation amplitude was able to change the phase of the center-of-mass oscillation. Finally, an interesting, non-linear dependence was observed on the condensate aspect ratio as a function of the perturbing amplitude which induces the quadrupolar mode.

cond-mat.quant-gas↗