arXiv subjects

Viktoria Noel

Publications and source records attributed to Viktoria Noel.

9 recordsLinked to original sources

arXiv2026

Quantum Fisher information of driven-dissipative systems from Keldysh path integrals

Driven-dissipative many-body quantum systems can be exploited in quantum enhanced sensing protocols. Here, non-classical correlations may allow one to reach a measurement precision that surpasses the standard quantum limit. The relevant figure of merit for assessing such quantum advantage is the quantum Fisher information (QFI). In a many-body setting its computation is challenging as it requires knowledge of the full system-environment state. Here we make use of a field-theoretical approach, which is particularly well suited for systems with many degrees of freedom. In particular, we show that the QFI can be linked to a Keldysh path-integral. This route leads to a semiclassical expression of the QFI, which is obtained through a controlled expansion in the so- called quantum fields. We benchmark the framework on three systems of increasing complexity: the driven-dissipative harmonic oscillator, the boundary time crystal - which becomes semiclassical at large system sizes - and a two-dimensional array of collectively emitting atoms. We assess in which parameter regimes the semiclassical description holds and gives access to system sizes beyond the reach of exact methods.

quant-ph↗

arXiv2026

Nonthermal fixed point for massless scalar field theories

We compute universal scaling exponents and scaling functions for massless scalar field theories far from equilibrium. Using a large-$N$ expansion to next-to-leading order, we investigate the nonperturbative infrared behaviour associated with the transport of a) energy or b) effective particle number towards low frequencies $ω$ and momenta $\bar p$. For massless dispersion $ω= \bar{p}$, we determine the scaling form of the distribution function whose universal power-law tail $f_S(\bar p) \sim 1/\bar{p}^κ$ is found to be a) $κ= d+1$ and b) $κ= d$ in $d=3$ spatial dimensions. Our results establish that relativistic and nonrelativistic theories do not belong to the same universality class in this case, which opens up new applications to far-from-equilibrium phenomena in high-energy and (quantum) many-body physics with linear dispersions.

hep-ph↗

arXiv2026

Quantum to classical relaxation dynamics of the dissipative Rydberg gas

We investigate the relaxation dynamics of a Rydberg gas in regimes where coherent processes and dissipation compete. In the strongly dissipative limit, the dynamics is known to be governed by an effective classical rate equation and to exhibit kinetically constrained, glassy relaxation towards a trivial stationary state. This behaviour originates from the Rydberg blockade, which prevents simultaneous excitations within a characteristic blockade radius. However, the fate of kinetic constraints in the weakly dissipative limit remains unexplored in large systems above one dimension. To access large system sizes and two-dimensional geometries, we employ the truncated Wigner approximation, a phase-space method that captures correlated many-body dynamics beyond classical rate equations. To probe the emergence of kinetic constraints on timescales where coherent and dissipative processes are comparable, we analyse the relaxation dynamics starting from two initial states: a fully polarised state and a Néel state, which belongs to a manifold of so-called quantum scars. In both cases, we observe a pronounced slowdown in the relaxation of the magnetisation towards the stationary state and identify transient signatures of quantum kinetically constrained dynamics in one and two dimensions.

cond-mat.quant-gas↗

arXiv2026

Truncated Wigner approximation for spins in continuous phase space

We review the truncated Wigner approximation (TWA) for spins as a computationally inexpensive numerical approximation method to describe interacting and / or dissipative many-body spin systems. Using the Wigner-Moyal mapping from Hilbert space to a suitable phase space, the many-body density matrix is represented by a c-number distribution, the Wigner function. The gauge freedom in continuous phase space can be exploited to find positive Wigner functions for a large class of spin states, including entangled ones. Employing different sets of correspondence rules, we derive equations of motion for the Wigner function, which, applying controlled approximations, can be mapped to stochastic differential equations. This allows a computationally inexpensive simulation of expectation values. Using a phase-space analog of the quantum regression theorem also multi-time correlations and spectra can be obtained. To illustrate the potential of the method, we benchmark the TWA for spins with some exactly solvable problems of interacting, dissipative spin systems, and then discuss its application to collective processes, such as the superradiant emission of light. Extending the TWA to imaginary time furthermore provides a tool to approximately calculate thermal and ground states of spin Hamiltonians. Finally, we show that the TWA stochastic equations can equivalently be derived within a path-integral approach, provided that the operator products in the dissipator are rigorously mapped onto the curved phase space.

quant-ph↗

arXiv2026

Path integral approach to the truncated Wigner approximation of driven-dissipative spins

Phase-space approaches such as the truncated Wigner approximation (TWA) provide an efficient semiclassical framework for performing approximate simulations of the dynamics of open quantum many-body systems outside the reach of exact numerical methods but beyond the mean-field level. For bosonic systems, TWA is known to be equivalent to a Keldysh path-integral formulation truncated at second order in the so-called quantum fluctuations. This semiclassical approach provides an alternative transparent route towards approximate stochastic equations of motion, which can be efficiently solved. Here we establish the corresponding path-integral formulation for interacting open spin-$1/2$ systems using the continuous $\mathrm{SU}(2)$ phase space. We show, in particular, that a consistent treatment of dissipation requires correctly mapping operator products onto the curved spin phase space, leading to stochastic equations that coincide with those obtained from the continuous TWA formulation and thus reproduce the exact dynamics of a single dissipative spin. Our results provide a unified field-theoretic foundation for the TWA to dissipative spin dynamics and offer a systematic starting point for extensions beyond the semiclassical approximation.

quant-ph↗

arXiv2025

Kelvin waves in nonequilibrium universal dynamics of relativistic scalar field theories

We investigate the different degrees of freedom underlying far-from-equilibrium scaling behaviour in a relativistic, single-component $\mathrm{O}(1)$ scalar field theory in two and three spatial dimensions. In such a strongly correlated many-body system, identifying the respective roles of nonlinear wave excitations and defect dynamics is a prerequisite for understanding the universal character of time evolution far from equilibrium and thus the different possible universality classes of nonthermal fixed points. Using unequal-time two-point correlation functions, we extract information about the dominant infrared excitations and study their connections to the turbulent dynamics of topological defects created in the system. In three dimensions, the primary excitations are identified as kelvon quasiparticles, which are quantised Kelvin waves propagating along vortex lines, while in two dimensions, vortices are point defects, and the infrared dynamics is dominated by bound-state like excitations similar to Kelvin waves. In both cases, the kelvon excitations are found to be characterised by distinct time-evolving dispersion relations, subject to the coarsening dynamics close to the respective nonthermal fixed point and, thus, to the decay of superfluid turbulence in the system. Our results underline the role of topological defects and their influence on the universal dynamics of strongly correlated systems near nonthermal fixed points, complementing the analysis of large-$N$ models in $\mathrm{O}(N)$ systems.

cond-mat.quant-gas↗

arXiv2025

Extracting the symmetries of nonequilibrium quantum many-body systems

Symmetries play a pivotal role in our understanding of the properties of quantum many-body systems. While there are theorems and a well-established toolbox for systems in thermal equilibrium, much less is known about the role of symmetries and their connection to dynamics out of equilibrium. This arises due to the direct link between a system's thermal state and its Hamiltonian, which is generally not the case for nonequilibrium dynamics. Here we present a pathway to identify the effective symmetries and to extract them from data in nonequilibrium quantum many-body systems. Our approach is based on exact relations between correlation functions involving different numbers of spatial points, which can be viewed as nonequilibrium versions of (equal-time) Ward identities encoding the symmetries of the system. We derive symmetry witnesses, which are particularly suitable for the analysis of measured or simulated data at different snapshots in time. To demonstrate the potential of the approach, we apply our method to numerical and experimental data for a spinor Bose gas. We investigate the important question of a dynamical restoration of an explicitly broken symmetry of the Hamiltonian by the initial state. Remarkably, it is found that effective symmetry restoration can occur long before the system equilibrates. We also use the approach to define and identify spontaneous symmetry breaking far from equilibrium, which is of great relevance for applications to nonequilibrium phase transitions. Our work opens new avenues for the classification and analysis of quantum as well as classical many-body dynamics in a large variety of systems, ranging from ultracold quantum gases to cosmology.

cond-mat.quant-gas↗

arXiv2024

Time-Resolved Rubidium-Assisted Electron Capture by Barium (II) Cation

Non-local energy transfer between bound electronic states close to the ionisation threshold is employed for efficient state preparation in dilute atom systems from technological foundations to quantum computing. The generalisation to electronic transitions into and out of the continuum is lacking quantum simulations necessary to motivate such potential experiments. Here, we present the first development of a electron-dynamical model simulating fully three-dimensional atomic systems for this purpose. We investigate the viability of this model for the prototypical case of recombination of ultracold barium(II) by environment-assisted electron capture thanks to a rubidium atom in its vicinity. Both atomic sites are modelled as effective one-electron systems using the Multi Configuration Time Dependent Hartree (MCTDH) algorithm and can transfer energy by dipole-dipole interaction. We find that the simulations are robust enough to realise assisted capture over a dilute interatomic distance which we are able to quantify by comparing to simulations without interatomic energy exchange. For our current parameters not yet optimised for reaction likelihood, an environment-ionising assisted capture has a probability of $1.9\times10^{-5}~\%$ over the first $15~\mathrm{fs}$ of the simulation. The environment-exciting assisted-capture path to $[\text{Ba}^{+*}\text{Rb}^{*}]$ appears as a stable long-lived intermediate state with a probability of $8.2\times10^{-4}~\%$ for at least $20~\mathrm{fs}$ after the capture has been completed. This model shows potential to predict optimised parameters as well as to accommodate the conditions present in experimental systems as closely as possible. We put the presented setup forward as a suitable first step to experimentally realise environment-assisted electron capture with current existing technologies.

physics.atom-ph↗

arXiv2024

Detecting defect dynamics in relativistic field theories far from equilibrium using topological data analysis

We study nonequilibrium dynamics of relativistic $N$-component scalar field theories in Minkowski space-time in a classical-statistical regime, where typical occupation numbers of modes are much larger than unity. In this strongly correlated system far from equilibrium, the role of different phenomena such as nonlinear wave propagation and defect dynamics remains to be clarified. We employ persistent homology to infer topological features of the nonequilibrium many-body system for different numbers of field components $N$ via a hierarchy of cubical complexes. Specifically, we show that the persistent homology of local energy density fluctuations can give rise to signatures of self-similar scaling associated with topological defects, distinct from the scaling behaviour of nonlinear wave modes. This contributes to the systematic understanding of the role of topological defects for far-from-equilibrium time evolutions of nonlinear many-body systems.

hep-ph↗