arXiv subjects

Sebastian Wüster

Publications and source records attributed to Sebastian Wüster.

At least 19 recordsLinked to original sources

arXiv2024

Transferring vibrational states of trapped atoms via a Rydberg electron

We show theoretically that it is possible to coherently transfer vibrational excitation between trapped neutral atoms over a micrometer apart. To this end we consider three atoms, where two are in the electronic ground state and one is excited to a Rydberg state whose electronic orbital overlaps with the positional wave functions of the two ground-state atoms. The resulting scattering of the Rydberg electron with the ground-state atoms provides the interaction required to transfer vibrational excitation from one trapped atom to the other. By numerically investigating the dependence of the transfer dynamics on the distance between traps and their relative frequencies we find that there is a "sweet spot" where the transfer of a vibrational excitation is nearly perfect and fast compared to the Rydberg lifetime. We investigate the robustness of this scenario with respect to changes of the parameters. In addition, we derive a intuitive effective Hamiltonian which explains the observed dynamics.

physics.atom-ph↗

arXiv2024

Quantum network tomography of Rydberg arrays by machine learning

Configurable arrays of optically trapped Rydberg atoms are a versatile platform for quantum computation and quantum simulation, also allowing controllable decoherence. We demonstrate theoretically, that they also enable proof-of-principle demonstrations for a technique to build models for open quantum dynamics by machine learning with artificial neural networks, recently proposed in [Mukherjee et al. [arXiv:2409.18822] (2024)]. Using the outcome of quantum transport through a network of sites that correspond to excited Rydberg atoms, the multi-stage neural network algorithm successfully identifies the number of atoms (or nodes in the network), and subsequently their location. It further extracts an effective interaction Hamiltonian and decoherence operators induced by the environment. To probe the Rydberg array, one initiates dynamics repeatedly from the same initial state and then measures the transport probability to an output atom. Large datasets are generated by varying the position of the latter. Measurements are required in only one single basis, making the approach complementary to e.g. quantum process tomography. The cold atom platform discussed in this article can be used to explore the performance of the proposed protocol when training the neural network with simulation data, but then applying it to construct models based on experimental data.

quant-ph↗

arXiv2024

Automated quantum system modeling with machine learning

Despite the complexity of quantum systems in the real world, models with just a few effective many-body states often suffice to describe their quantum dynamics, provided decoherence is accounted for. We show that a machine learning algorithm is able to construct such models, given a straightforward set of quantum dynamics measurements. The effective Hilbert space can be a black box, with variations of the coupling to just one accessible output state being sufficient to generate the required training data. We demonstrate through simulations of a Markovian open quantum system that a neural network can automatically detect the number $N $ of effective states and the most relevant Hamiltonian terms and state-dephasing processes and rates. For systems with $N\leq5$ we find typical mean relative errors of predictions in the $10 \%$ range. With more advanced networks and larger training sets, it is conceivable that a future single software can provide the automated first stop solution to model building for an unknown device or system, complementing and validating the conventional approach based on physical insight into the system.

quant-ph↗

arXiv2024

Macroscopic self-trapping in the dynamical tunneling of a Bose-Einstein condensate

A Bose-Einstein condensate in a modulated, one-dimensional, anharmonic potential can exhibit dynamical tunneling between islands of regular motion in phase space. With increasingly repulsive atomic interactions, dynamical tunneling is predicted to cease due to self-trapping [S. Wüster et al. Phys. Rev. Lett. 109 080401 (2012)]. This suppression of tunneling oscillations is related to the same phenomenon that occurs in the two-mode dynamics of a repulsively interacting Bose-Einstein condensate in a double-well potential. Here we present a two-mode model for dynamical tunnelling based on nonlinear Floquet states and examine the range of validity of the approximation. We characterise nonlinear dynamical tunneling for different trap strengths, modulation amplitudes, and effective Planck constants. Using the linear Floquet states we derive an expression for the critical nonlinearity beyond which tunneling ceases. Finally we demonstrate the dynamical instability of selected nonlinear Floquet states and show how to initialise some Floquet states in experiments. Our detailed survey will enable experiments to target accessible parameter regimes for the study of nonlinear dynamical tunneling.

cond-mat.quant-gas↗

arXiv2024

In situ observation of chemistry in Rydberg molecules within a coherent solvent

We often infer the state of systems in nature indirectly, for example, in high-energy physics by the interaction of particles with an ambient medium. We adapt this principle to energies $9$ orders of magnitude smaller, to classify the final state of exotic molecules after internal conversion of their electronic state, through their interaction with an ambient quantum fluid, a Bose-Einstein condensate (BEC). The BEC is the ground-state of a million bosonic atoms near zero temperature, and a single embedded ultra-long range Rydberg molecule can coherently excite waves in this fluid, which carry telltale signatures of its dynamics. Bond lengths exceeding a micrometer allow us to observe the molecular fingerprint on the BEC in-situ, via optical microscopy. Interpreting images in comparison with simulations strongly suggests that the molecular electronic state rapidly converts from the initially excited S and D orbitals to a much more complex molecular state (called "trilobite''), marked by a maximally localized electron. This internal conversion liberates energy, such that one expects final-state particles to move rapidly through the medium, which is however ruled out by comparing experiment and simulations. The molecule thus must strongly decelerate in the medium, for which we propose a plausible mechanism. Our experiment demonstrates a medium that facilitates and records an electronic state change of embedded exotic molecules in ultra-cold chemistry, with sufficient sensitivity to constrain velocities of final-state particles.

cond-mat.quant-gas↗

arXiv2024

Quantum transport enabled by non-adiabatic transitions

Quantum transport of charge or energy in networks with discrete sites is central to diverse quantum technologies, from molecular electronics to light harvesting and quantum opto-mechanical metamaterials. A one dimensional network can be viewed as waveguide. We show that if such waveguide is hybridised with a control unit that contains a few sites, then transmission through the waveguide depends sensitively on the motion of the sites in the control unit. Together, the hybrid waveguide and its control-unit form a Fano-Anderson chain whose Born-Oppenheimer surfaces inherit characteristics from both components: A bandstructure from the waveguide and potential energy steps as a function of site coordinates from the control-unit. Using time-dependent quantum wave packets, we reveal conditions under which the hybrid structure becomes transmissive only if the control unit contains mobile sites that induce non-adiabatic transitions between the surfaces. Hence, our approach provides functional synthetic Born-Oppenheimer surfaces for hybrid quantum technologies combining mechanic and excitonic elements, and has possible applications such as switching and temperature sensing.

quant-ph↗

arXiv2023

The disordered Dicke model

We introduce and study the disordered Dicke model in which the spin-boson couplings are drawn from a random distribution with some finite width. Regarding the quantum phase transition we show that when the standard deviation $σ$ of the coupling strength gradually increases, the critical value of the mean coupling strength $μ$ gradually decreases and after a certain $σ$ there is no quantum phase transition at all; the system always lies in the super-radiant phase. We derive an approximate expression for the quantum phase transition in the presence of disorder in terms of $μ$ and $σ$, which we numerically verify. Studying the thermal phase transition in the disordered Dicke model, we obtain an analytical expression for the critical temperature in terms of the mean and standard deviation of the coupling strength. We observe that even when the mean of the coupling strength is zero, there is a finite temperature transition if the standard deviation of the coupling is sufficiently high. Disordered couplings in the Dicke model will exist in quantum dot superlattices, and we also sketch how they can be engineered and controlled with ultracold atoms or molecules in a cavity.

cond-mat.stat-mech↗

arXiv2023

Anisotropic Inflation in Dipolar Bose-Einstein Condensates

Early during the era of cosmic inflation, rotational invariance may have been broken, only later emerging as a feature of low-energy physics. This motivates ongoing searches for residual signatures of anisotropic space-time, for example in the power spectrum of the cosmic microwave background. We propose that dipolar Bose-Einstein condensates (BECs) furnish a laboratory quantum simulation platform for the anisotropy evolution of fluctuation spectra during inflation, exploiting the fact that the speed of dipolar condensate sound waves depends on direction. We construct the anisotropic analogue space-time metric governing sound, by linking the time-varying strength of dipolar and contact interactions in the BEC to the scale factors in different coordinate directions. Based on these, we calculate the dynamics of phonon power spectra during an inflation that renders the initially anisotropic universe isotropic. We find that the expansion speed provides an experimental handle to control and study the degree of final residual anisotropy. Gravity analogues using dipolar condensates can thus provide tuneable experiments for a field of cosmology that was until now confined to a single experiment, our universe.

gr-qc↗

arXiv2023

Maximally entangled Rydberg-atom pairs via Landau-Zener sweeps

We analyze the formation of maximally entangled Rydberg atom pairs subjected to Landau-Zener sweeps of the atom-light detuning. Though the populations reach a steady value at longer times, the phases evolve continuously, leading to periodic oscillations in the entanglement entropy. The local unitary equivalence between the obtained maximally entangled states and the Bell states is verified by computing the polynomial invariants. Finally, we study the effect of spontaneous emission from the Rydberg state of rubidium atoms on the correlation dynamics and show that the oscillatory dynamics persists for high-lying Rydberg states. Our study may offer novel ways to generate maximally entangled states, quantum gates and exotic quantum matter in arrays of Rydberg atoms through Landau Zener sweeps.

quant-ph↗

arXiv2022

Hyper-entangling mesoscopic bound states

We predict hyper-entanglement generation during binary scattering of mesoscopic bound states, solitary waves in Bose-Einstein condensates containing thousands of identical Bosons. The underlying many-body Hamiltonian must not be integrable, and the pre-collision quantum state of the solitons fragmented. Under these conditions, we show with pure state quantum field simulations that the post-collision state will be hyper-entangled in spatial degrees of freedom and atom number within solitons, for realistic parameters. The effect links aspects of non-linear systems and quantum-coherence and the entangled post-collision state challenges present entanglement criteria for identical particles. Our results are based on simulations of colliding quantum solitons in a quintic interaction model beyond the mean-field, using the truncated Wigner approximation.

cond-mat.quant-gas↗

arXiv2022

Trapping and binding by dephasing

The binding and trapping of particles usually rely on conservative forces, described by unitary quantum dynamics. We show that both can also arise solely from spatially dependent dephasing, the simplest type of decoherence. This can be based on continuous weak position measurements in only selected regions of space, for which we propose a practical realisation. For a single particle, we demonstrate a quantum particle-in-the-box based on dephasing. For two particles, we demonstrate their binding despite repulsive interactions, if their molecular states are dephased at large separations only. Both mechanisms are experimentally accessible, as we show for an example with Rydberg atoms in a cold gas background.

quant-ph↗

arXiv2022

Density correlations from analogue Hawking radiation in the presence of atom losses

The sonic analogue of Hawking radiation can now be experimentally recreated in Bose-Einstein Condensates that contain an acoustic black hole. In these experiments the signal strength and analogue Hawking temperature increase for denser condensates, which however also suffer increased atom losses from inelastic collisions. To determine how these affect analogue Hawking radiation, we numerically simulate creation of the latter in a Bose-Einstein Condensate in the presence of atomic losses. In particular we explore modifications of density-density correlations through which the radiation has been analyzed so far. We find that losses increase the contrast of the correlation signal, which we attribute to heating that in turn leads to a component of stimulated radiation in addition to the spontaneous one. Another indirect consequence is the modification of the white hole instability pattern.

cond-mat.quant-gas↗

arXiv2022

Collisions of solitary waves in condensates beyond mean-field theory

Bright solitary waves in a Bose-Einstein condensate contain thousands of identical atoms held together despite their only weakly attractive contact interactions. They nonetheless behave like a compound object, staying whole in collisions, with their collision properties strongly affected by inter-soliton quantum coherence. We show that separate solitary waves decohere due to phase diffusion, dependent on their effective ambient temperature, after which their initial mean-field relative phases are no longer well defined or relevant for collisions. In this situation, collisions occur predominantly repulsively and can no longer be described within mean field theory. When considering the time-scales involved in recent solitary wave experiments where non-equilibrium phenomena play an important role, these features could explain the predominantly repulsive collision dynamics observed in most condensate soliton train experiments.

cond-mat.quant-gas↗

arXiv2021

Fano Resonances in Quantum Transport with Vibrations

Quantum mechanical scattering involving continuum states coupled to a scatterer with a discrete spectrum gives rise to Fano resonances. Here we consider scatterers that possess internal vibrational degrees of freedom in addition to discrete states. Entanglement between the scattered excitation and vibrational modes complicates analytical and numerical calculations considerably. For the example of one-dimensional scattering we develop a multichannel quantum scattering approach which can determine reflection and transmission probabilities in the presence of vibrations. Application to a linear chain coupled to a control unit containing vibrating sites shows that vibrational degrees of freedom can have a profound effect on quantum transport. For suitable parameters, spectral regions which are opaque in the static case can be rendered transparent when vibrations are included. The formalism is general enough to be applicable to a variety of platforms for quantum transport including molecular aggregates, cold atom chains, quantum-dot arrays and molecular wires based on conjugated polymers.

quant-ph↗

arXiv2021

Non-adiabatic dynamics in Rydberg gases with random atom positions

Assemblies of highly excited Rydberg atoms in an ultracold gas can be set into motion by a combination of van-der-Waals and resonant dipole-dipole interactions. Thereby, the collective electronic Rydberg state might change due to non-adiabatic transitions, in particular if the configuration encounters a conical interaction. For the experimentally most accessible scenario, in which the Rydberg atoms are initially randomly excited in a three-dimensional bulk gas under blockade conditions, we numerically show that non-adiabatic transitions can be common when starting from the most energetic repulsive BO-surface. We outline how this state can be selectively excited using a microwave resonance, and demonstrate a regime where almost all collisional ionization of Rydberg atoms can be traced back to a prior non-adiabatic transition. Since Rydberg ionisation is relatively straightforward to detect, the excitation and measurement scheme considered here renders non-adiabatic effects in Rydberg motion easier to demonstrate experimentally than in scenarios considered previously.

physics.atom-ph↗

arXiv2021

Population trapping in a pair of periodically driven Rydberg atoms

We study the population trapping extensively in a periodically driven Rydberg pair. The periodic modulation of the atom-light detuning effectively suppresses the Rabi couplings and, together with Rydberg-Rydberg interactions, leads to the state-dependent population trapping. We identify a simple yet a general scheme to determine population trapping regions using driving induced resonances, the Floquet spectrum, and the inverse participation ratio. Contrary to the single atom case, we show that the population trapping in the two-atom setup may not necessarily be associated with level crossings in the Floquet spectrum. Further, we discuss under what criteria population trapping can be related to dynamical stabilization, taking specific and experimentally relevant initial states, which include both product and the maximally entangled Bell states. The behavior of the entangled states is further characterized by the bipartite entanglement entropy.

quant-ph↗

arXiv2020

Two-dimensional spectroscopy of Rydberg gases

Two-dimensional (2D) spectroscopy uses multiple electromagnetic pulses to infer the properties of a complex system. A paradigmatic class of target systems are molecular aggregates, for which one can obtain information on the eigenstates, various types of static and dynamic disorder and on relaxation processes. However, two-dimensional spectra can be difficult to interpret without precise knowledge of how the signal components relate to microscopic Hamiltonian parameters and system-bath interactions. Here we show that two-dimensional spectroscopy can be mapped in the microwave domain to highly controllable Rydberg quantum simulators. By porting 2D spectroscopy to Rydberg atoms, we firstly open the possibility of its experimental quantum simulation, in a case where parameters and interactions are very well known. Secondly, the technique may provide additional handles for experimental access to coherences between system states and the ability to discriminate different types of decoherence mechanisms in Rydberg gases. We investigate the requirements for a specific implementation utilizing multiple phase coherent microwave pulses and a phase cycling technique to isolate signal components.

cond-mat.quant-gas↗

arXiv2020

Transmission in a Fano-Anderson chain with a topological defect

The Fano-Anderson chain consists of a linear lattice with a discrete side-unit, and exhibits Fano-resonant scattering due to coupling between the discrete states of the side-unit with the tight-binding continuum. We study Fano-resonance-assisted transport for the case of a topologically non-trivial side unit. We find that the topology of the side unit influences the transmission characteristics which thus can be an effective detection tool of the topological phases of the side unit. Furthermore, we explore the role of dual links between the linear tight-binding chain and the side unit. The secondary connection between the main chain and the side unit can modify the position or width of the Fano resonance dip in the transmission probability, and thus yield additional control.

cond-mat.mtrl-sci↗