arXiv subjects

Christopher W. Wächtler

Publications and source records attributed to Christopher W. Wächtler.

13 recordsLinked to original sources

arXiv2026

Quantum Synchronization of Perturbed Oscillating Coherences

Quantum mutual synchronization has recently been explored through the persistent oscillation of local observables that arises from undamped eigenmodes of dissipative dynamics. However, these oscillating modes require strictly fine-tuning the system to satisfy algebraic constraints. Here, we investigate the robustness of synchronization against generic perturbations that break these constraints. We identify conditions under which the steady state of the perturbed system exhibits correlations that indicate mutual synchronization, even as the oscillations decay. That synchronization persists as imprints in the time-independent, asymptotic steady state directly bridges the dynamical notion of synchronization with the steady-state notion, which have so far been treated as distinct phenomena. Moreover, we discover in a spin-1 model that the resulting steady-state synchronization is manifested in unexpected geometries of locked phases that are multiples of $π/3$. Our work establishes a link between the two primary paradigms of quantum synchronization while demonstrating its inherent robustness against generic perturbations.

quant-ph↗

arXiv2026

Synchronized Aharonov-Bohm Motifs via Engineered Dissipation

The interplay between external gauge fields and lattice geometry can induce extreme localization dynamics through complete destructive interference. We show that combining this flux-induced localization with engineered dissipation leads to robust spin synchronization in rotationally symmetric spin geometries, referred to as Aharonov-Bohm motifs, with cyclic symmetries of any order. The synchronized dynamics is independent of initial conditions and features entanglement among spins within each motif. We further demonstrate that multiple motifs can fully synchronize when coupled, which is achieved by applying additional collective dissipation acting on all intra-motif spins. These results reveal a direct connection between flux-induced localization, dissipative engineering, and collective quantum synchronization.

quant-ph↗

arXiv2025

Observation of synchronization between two quantum van der Pol oscillators in trapped ions

Synchronization is a hallmark of collective behavior that emerges when nonlinear systems interact, spanning scales from mechanical oscillators to planetary orbits. As a universal phenomenon it underpins the study of complex systems and has far-reaching technological implications. While classical synchronization has a long and rich history, it has not been observed experimentally between multiple quantum limit-cycle oscillators despite a decade of theoretical investigations. We realize synchronization between two quantum van der Pol oscillators by engineering dissipation in a mixed-isotope trapped-ion quantum simulator. The synchronized state is encoded in a fixed relative phase between the oscillators that is inaccessible to local measurements and only revealed through joint readout of both oscillators, in stark contrast to the classical case where synchronization can be observed via individual phase measurements. We further show that the relative phase can be precisely controlled, and that the chain of two oscillators can synchronize to an external field, suggesting applications in sensing. Our results provide a promising pathway for studying more complex synchronized quantum dynamics beyond two oscillators, where a theoretical treatment becomes increasingly challenging, and it remains to be understood whether genuinely quantum features persist in such cases.

quant-ph↗

arXiv2024

Topological quantum synchronization of fractionalized spins

The gapped symmetric phase of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model exhibits fractionalized spins at the ends of an open chain. We show that breaking SU(2) symmetry and applying a global spin-lowering dissipator achieves synchronization of these fractionalized spins. Additional local dissipators ensure convergence to the ground state manifold. In order to understand which aspects of this synchronization are robust within the entire Haldane-gap phase, we reduce the biquadratic term which eliminates the need for an external field but destabilizes synchronization. Within the ground state subspace, stability is regained using only the global lowering dissipator. These results demonstrate that fractionalized degrees of freedom can be synchronized in extended systems with a significant degree of robustness arising from topological protection. \rev{A direct consequence is that permutation symmetries are not required for the dynamics to be synchronized, representing a clear advantage of topological synchronization compared to synchronization induced by permutation symmetries.

quant-ph↗

arXiv2023

Entanglement generation between distant spins via quasilocal reservoir engineering

The generation and preservation of entanglement is a central goal in quantum technology. Traditionally, dissipation in quantum systems is thought to be detrimental to entanglement, however dissipation can also be utilised as a means of generating entanglement between quantum spins that are not directly interacting. In particular entanglement can be generated between two qubits, or multi qubit systems via a collective coupling to a reservoir. In this work, we explore multiple spin domains pairwise coupled to different reservoirs and show that entanglement can be generated between spins which are not coupled to each other, or even coupled to the same reservoir.

quant-ph↗

arXiv2023

Topological edge states in a Rydberg composite

We examine topological phases and symmetry-protected electronic edge states in the context of a Rydberg composite: a Rydberg atom interfaced with a structured arrangement of ground-state atoms. The electronic Hamiltonian of such a composite possesses a direct mapping to a tight-binding Hamiltonian, which enables the realization and study of a variety of systems with non-trivial topology by tuning the arrangement of ground-state atoms and the excitation of the Rydberg atom. The Rydberg electron moves in a combined potential including the long-ranged Coulomb interaction with the Rydberg core and short-ranged interactions with each neutral atom; the effective interactions between sites are determined by this combination. We first confirm the existence of topologically-protected edge states in a Rydberg composite by mapping it to the paradigmatic Su-Schrieffer-Heeger dimer model. Following that, we study more complicated systems with trimer unit cells which can be easily simulated with a Rydberg composite.

quant-ph↗

arXiv2023

Topological synchronization of quantum van der Pol oscillators

To observe synchronization in a large network of classical or quantum systems demands both excellent control of the interactions between the nodes and very accurate preparation of the initial conditions due to the involved nonlinearities and dissipation. This limits the applicability of this phenomenon for future devices. Here, we demonstrate a route towards significantly enhancing the robustness of synchronized behavior in open nonlinear systems that utilizes the power of topology. In a lattice of quantum van der Pol oscillators with topologically motivated couplings, boundary synchronization emerges in the classical mean field as well as the quantum model. In addition to its robustness against disorder and initial state perturbations, the observed dynamics is independent of the underlying topological insulator model provided the existence of zero-energy modes. Our work extends the notion of topology to the general nonlinear dynamics and open quantum system realm with applications to networks where specific nodes need special protection like power grids or quantum networks.

quant-ph↗

arXiv2023

Synchronized states in dissipatively coupled harmonic oscillator networks

The question under which conditions oscillators with slightly different frequencies synchronize appears in various settings. We show that synchronization can be achieved even for harmonic oscillators that are bilinearly coupled via a purely dissipative interaction. By appropriately tuned gain/loss stable dynamics may be achieved where for the cases studied in this work all oscillators are synchronized. These findings are interpreted using the complex eigenvalues and eigenvectors of the non-Hermitian matrix describing the dynamics of the system.

nlin.CD↗

arXiv2022

Electron shuttle as an autonomous single-electron source

The majority of experimental realizations of single-electron sources rely on the periodic manipulation of the tunnel junctions through their gate voltages, and thus require a high level of control over the system. To circumvent the necessity of external driving, we utilize the self-oscillatory behavior of the electron shuttle. By means of waiting time distributions, which had not been applied to this autonomous system before, we extensively assess the performance of the shuttle as a single-electron source. We unveil a smooth transition between three regimes, whereas previous studies at the same mean field level of description only predict a sharp bifurcation based on the time-averaged electron current. Over the parameter range of already existing experimental realizations the electron shuttle can perform as a single-electron source, albeit with moderate precision. We propose an alternative design of the position-dependent tunneling rates, which significantly decreases the relative error of charge transmission, and thus paves the way for the realization of autonomous single-electron sources.

cond-mat.mes-hall↗

arXiv2021

Reservoir-assisted energy migration through multiple spin-domains

The transfer of energy through a network of nodes is fundamental to both how nature and current technology operates. Traditionally we think of the nodes in a network being coupled to channels that connect them and then energy is passed from node to channel to node until it reaches its targeted site. Here we introduce an alternate approach to this where our channels are replaced by collective environments (or actually reservoirs) which interact with pairs of nodes. We show how energy initially located at a specific node can arrive at a target node - even though that environment may be at zero temperate. Further we show that such a migration occurs on much faster time scales than the damping rate associated with a single spin coupled to the reservoir. Our approach shows the power of being able to tailor both the system & environment and the symmetries associated with them to provide new directions for future quantum technologies.

quant-ph↗

arXiv2021

Autonomous implementation of thermodynamic cycles at the nanoscale

There are two paradigms to study nanoscale engines in stochastic and quantum thermodynamics. Autonomous models, which do not rely on any external time-dependence, and models that make use of time-dependent control fields, often combined with dividing the control protocol into idealized strokes of a thermodynamic cycle. While the latter paradigm offers theoretical simplifications, its utility in practice has been questioned due to the involved approximations. Here, we bridge the two paradigms by constructing an autonomous model, which implements a thermodynamic cycle in a certain parameter regime. This effect is made possible by self-oscillations, realized in our model by the well studied electron shuttling mechanism. Based on experimentally realistic values, we find that a thermodynamic cycle analysis for a single-electron working fluid is {\it not} justified, but a few-electron working fluid could suffice to justify it. Furthermore, additional open challenges remain to autonomously implement the more studied Carnot and Otto cycles.

quant-ph↗

arXiv2021

Nanoelectromechanical rotary current rectifier

Nanoelectromechanical systems (NEMS) are devices integrating electrical and mechanical functionality on the nanoscale. Because of individual electron tunneling, such systems can show rich self-induced, highly non-linear dynamics. We show theoretically that rotor shuttles, fundamental NEMS without intrinsic frequencies, are able to rectify an oscillatory bias voltage over a wide range of external parameters in a highly controlled manner, even if subject to the stochastic nature of electron tunneling and thermal noise. Supplemented by a simple analytic model, we identify different operational modes of charge rectification. Intriguingly, the direction of the current depends sensitively on the external parameters.

cond-mat.mes-hall↗

arXiv2021

Coherently delocalized states in dipole interacting Rydberg ensembles: the role of internal degeneracies

We investigate the effect of degenerate atomic states on the exciton delocalization of dipole-dipole interacting Rydberg assemblies. Using a frozen gas and regular one-, two-, and three-dimensional lattice arrangements as examples, we see that degeneracies can enhance the delocalization compared to the situation when there is no degeneracy. Using the Zeeman splitting provided by a magnetic field, we controllably lift the degeneracy to study in detail the transition between degenerate and non-degenerate regimes.

physics.atom-ph↗