arXiv subjects

Stefan Yoshi Buhmann

Publications and source records attributed to Stefan Yoshi Buhmann.

At least 37 records · Page 2Linked to original sources

arXiv2021

Purcell modification of Auger and interatomic Coulombic decay

An excited two-atom system can decay via different competing relaxation processes. If the excess energy is sufficiently high the system may not only relax via spontaneous emission but can also undergo interatomic Coulombic decay (ICD) or even Auger decay. We provide analytical expressions for the rates by including them into the same quantum optical framework on the basis of macroscopic quantum electrodynamics. By comparing the rates in free space we derive the atomic properties determining which decay channel dominates the relaxation. We show that by modifying the excitation propagation of the respective process via macroscopic bodies, in the spirit of the Purcell effect, one can control the ratio between the two dominating decay rates. We can relate the magnitude of the effect to characteristic length scales of each process, analyse the impact of a simple close-by surface onto a general two-atom system in detail and discuss the effect of a cavity onto the decay rates. We finally apply our theory to the example of a doubly excited HeNe-dimer.

quant-ph↗

arXiv2021

Casimir-Polder interaction of neutrons with metal or dielectric surfaces

We predict a repulsive Casimir-Polder-type dispersion interaction between a single neutron and a metal or dielectric surface. We consider a scenario where a single neutron is subject to an external magnetic field. Due to its intrinsic magnetic moment, the neutron then forms a magnetisable two-level system which can exchange virtual photons with a nearby surface. The resulting dispersion interaction between a purely magnetic object (neutron) and a purely electric one (surface) is found to be repulsive, in contrast to the typical attractive interaction between electric objects. Its magnitude is considerably smaller than the standard atom--surface Casimir-Polder force due to the magnetic nature of the interaction and the smallness of the electron-to-neutron mass ratio. Nevertheless, we show that it can be comparable to the gravitational potential of the same surface and should be taken into consideration in future neutron interference experiments.

quant-ph↗

arXiv2021

Probing the Purcell effect without radiative decay: Lessons in the frequency and time domains

The microscopic processes underlying electro-optic sampling of quantum-vacuum fluctuations are discussed, leading to the interpretation of these experiments in terms of an exchange of virtual photons. With this in mind it is shown how one can directly study the Purcell effect, i.e. the changes induced by cavities upon the quantum vacuum, in the frequency and time domains. This forges a link between electro-optic sampling of the quantum vacuum and geometry-induced vacuum effects.

quant-ph↗

arXiv2021

Spectroscopic Effects of Velocity-Dependent Casimir-Polder Interactions Induced by Parallel Plates

Casimir-Polder interactions cause energy and momentum exchange between microscopic and macroscopic bodies, a process mediated by quantum fluctuations in the coupled matter-electromagnetic field system. The dynamics of such effects are yet to be experimentally investigated due to the dominance of static effects at currently attainable atomic velocities. However, Y. Guo and Z. Jacob [\textit{Opt. Express}, 22:26193-26202, 2014] have proposed a non-static two-plate set-up where quantum fluctuation mediated effects have a strong velocity-dependent resonance, leading to a giant friction force on the plates. Here a more easily realisable set-up, a moving atom between two stationary plates, is analysed within a QED framework to establish the spectroscopic Casimir-Polder effects on the atom, and their velocity dependence. While no large velocity-dependent enhancement is found, expressions for the plate-induced spectroscopic effects on the atom were found, and further shown to be equivalent to the Doppler-shifted static result within certain velocity constraints. A numerical analysis investigates the behaviour of this system for the well studied case of the $6D_{3/2}\rightarrow 7P_{1/2}$ transition in $^{133}$Cs interacting with sapphire plates.

quant-ph↗

arXiv2021

Superradiance from non-ideal initial states -- a quantum trajectory approach

Collective emission behavior is usually described by the decay dynamics of the completely symmetric Dicke states. To study a more realistic scenario, we investigate alternative initial states inducing a more complex time evolution. Superposition states of the fully inverted Dicke state and the Dicke ground state with unequal mutual weights are studied as examples as well as superradiance stemming from atoms in clusters separated by more than one wavelength. The Monte Carlo wave function method serves as framework to study the dynamics of quantum states, which is determined by quantum jumps on the one hand and continuous evolution dynamics on the other hand. We compare this method with the classical picture of a system of rate equations written for the diagonal components of the density matrix.

quant-ph↗

arXiv2021

Effective screening of medium-assisted Van der Waals interactions between embedded particles

The effect of an implicit medium on dispersive interactions of particle pairs is discussed and simple expressions for the correction relative to vacuum are derived. We show that a single point Gauss quadrature leads to the intuitive result that the vacuum van der Waals $C_6$ coefficient is screened by the permittivity squared of the environment evaluated near to the resonance frequencies of the interacting particles. This approximation should be particularly relevant if the medium is transparent at these frequencies. In the manuscript, we provide simple models and sets of parameters for commonly used solvents, atoms and small molecules.

cond-mat.mes-hall↗

arXiv2020

Quantum sensing protocol for motionally chiral Rydberg atoms

A quantum sensing protocol is proposed for demonstrating the motion-induced chirality of circularly polarised Rydberg atoms. To this end, a cloud of Rydberg atoms is dressed by a bichromatic light field. This allows to exploit the long-lived ground states for implementing a Ramsey interferometer in conjunction with a spin echo pulse sequence for refocussing achiral interactions. Optimal parameters for the dressing lasers are identified. Combining a circularly polarised dipole transition in the Rydberg atom with atomic centre-of-mass motion, the system becomes chiral. The resulting discriminatory chiral energy shifts induced by a chiral mirror are estimated using a macroscopic quantum electrodynamics approach.

quant-ph↗

arXiv2020

Comparison of theory and experiments on Van der Waals forces in media -- a survey

We present a critical overview comparing theoretical predictions and measurements of Van der Waals dispersion forces in media on the basis of the respective Hamaker constants. To quantify the agreement, we complement the reported experimental errors with those for the theoretical predictions, which are due to uncertainties in the underlying spectroscopic data. Our main finding is that the theoretical errors are often larger than their experimental counterparts. Within these uncertainties, the comparison confirms the standard Lifshitz theory based on the Abraham electromagnetic stress tensor against the recently suggested alternative account on the basis of the Maxwell stress tensor.

physics.chem-ph↗

arXiv2020

Macroscopic Quantum Electrodynamics and Density Functional Theory Approaches to Dispersion Interactions between Fullerenes

The processing and material properties of commercial organic semiconductors, for e.g. fullerenes is largely controlled by their precise arrangements, specially intermolecular symmetries, distances and orientations, more specifically, molecular polarisabilities. These supramolecular parameters heavily influence their electronic structure, thereby determining molecular photophysics and therefore dictating their usability as n-type semiconductors. In this article we evaluate van der Waals potentials of a fullerene dimer model system using two approaches: a) Density Functional Theory and, b) Macroscopic Quantum Electrodynamics, which is particularly suited for describing long-range van der Waals interactions. Essentially, we determine and explain the model symmetry, distance and rotational dependencies on binding energies and spectral changes. The resultant spectral tuning is compared using both methods showing correspondence within the constraints placed by the different model assumptions. We envision that the application of macroscopic methods and structure/property relationships laid forward in this article will find use in fundamental supramolecular electronics.

cond-mat.mes-hall↗

arXiv2020

Non-trivial retardation effects in dispersion forces: From anomalous distance dependence to novel traps

In the study of dispersion forces, nonretarded, retarded and thermal asymptotes with their distinct scaling laws are regarded as cornerstone results governing interactions at different separations. Here, we show that when particles interact in a medium, the influence of retardation is qualitatively different, making it necessary to consider the non-monotonous potential in full. We discuss different regimes for several cases and find an anomalous behaviour of the retarded asymptote. It can change sign, and lead to a trapping potential.

cond-mat.mes-hall↗

arXiv2020

Lateral interatomic dispersion forces

Van der Waals forces between atoms and molecules are universally assumed to act along the line separating them. Inspired by recent works on effects which can propel atoms parallel to a macroscopic surface via the Casimir--Polder force, we predict a lateral van der Waals force between two atoms, one of which is in an excited state with non-zero angular momentum and the other is isotropic and in its ground state. The resulting force acts in the same way as a planetary gear, in contrast to the rack-and-pinion motion predicted in works on the lateral Casimir--Polder force in the analogous case, for which the force predicted here is the microscopic origin. We illustrate the effect by predicting the trajectories of an excited caesium in the vicinity of ground-state rubidium, finding behaviour qualitatively different to that if lateral forces are ignored.

quant-ph↗

arXiv2020

The Langevin Noise Approach for Lossy Media and the Lossless Limit

The Langevin noise approach for quantization of macroscopic electromagnetics for three-dimensional, inhomogeneous environments is compared with normal mode quantization. Recent works on the applicability of the method are discussed, and several examples are provided showing that for closed systems the Langevin noise approach reduces to the usual cavity mode expansion method when loss is eliminated.

quant-ph↗

arXiv2020

Open quantum systems decay across time

The description of an open quantum system's decay almost always requires several approximations as to remain tractable. Here, we first revisit the meaning, domain and seeming contradictions of a few of the most widely used of such approximations: semi-group Markovianity, linear response theory, Wigner--Weisskopf and rotating-wave approximation. Secondly, we derive an effective time-dependent decay theory and corresponding generalized quantum regression relations for an open quantum system linearly coupled to an environment. This theory covers all timescales, and subsumes the Markovian and linear-response results as limiting cases. Finally, we apply our theory to the phenomenon of quantum friction.

quant-ph↗

arXiv2020

Inverse design of light-matter interactions

Inverse design represents a paradigm shift in the development of nanophotonic devices, where optimal geometries and materials are discovered by an algorithm rather than symmetry considerations or intuition. Here we present a very general formulation of inverse design that is applicable to atomic interactions in external environments, and derive from this some explicit formulae for optimisation of spontaneous decay rates, Casimir-Polder forces and resonant energy transfer. Using the Purcell factor of latter as an example, we use finite-difference time-domain techniques to demonstrate the ability of inverse design algorithms to go far beyond what can be achieved by intuition-based approaches, opening up a new route to their technological exploitation.

quant-ph↗

arXiv2020

Theory of quantum-vacuum detection

Recent progress in electro-optic sampling has allowed direct access to the fluctuations of the electromagnetic ground state. Here, we present a theoretical formalism that allows for an in-depth characterisation and interpretation of such quantum-vacuum detection experiments by relating their output statistics to the quantum statistics of the electromagnetic vacuum probed. In particular, we include the effects of absorption, dispersion and reflections from general environments. Our results agree with available experimental data while leading to significant corrections to previous theoretical predictions and generalises them to new parameter regimes. Our formalism opens the door for a detailed experimental analysis of the different characteristics of the polaritonic ground state, e.g. we show that transverse (free-field) as well as longitudinal (matter or near-field) fluctuations can be accessed individually by tuning the experimental parameters.

quant-ph↗

arXiv2019

Superradiance and symmetry in resonant energy transfer

Closely-spaced quantum emitters coherently sharing excitation can release their energy faster than suggested by a simple sum over their individual emission rates - a phenomenon known as superradiance. Here, we show that the assumption of closely-spaced emitters can be relaxed in the context of resonant energy transfer, instead finding that certain symmetrical arrangements of donors are just as effective. We derive exact expressions for the superradiant fidelity in such situations, finding some surprising results such as complete suppression of the rate for a single acceptor within a homogeneous spherically symmetric distribution of coherent donors.

quant-ph↗

arXiv2019

Medium-assisted van der Waals dispersion interactions involving chiral molecules

The van der Waals dispersion interaction between two chiral molecules in the presence of arbirary magnetoelectric media is derived using perturbation theory. To be general, the molecular polarisabilities are assumed to be of electric, paramagnetic and diamagnetic natures and the material environment is considered to possess a chiral electromagnetic response. The derived formulas of electric-chiral, paramagnetic-chiral, diamagnetic-chiral and chiral-chiral interaction potentials when added to the previously obtained contributions in literature, form a complete set of dispersion interaction formulas. We present them in a unified form making use of electric--magnetic duality. As an application, the case of two anisotropic molecules in free space is considered where we drive the retarded and non-retarded limits with respect to intermolecular distance.

quant-ph↗

arXiv2019

Spectroscopy on nanoparticles without light

One of the most important tools in modern science is the analysis of electromagnetic properties via spectroscopy. The various types of spectroscopy can be classified by the underlying type of interactions between energy and material. In this paper we demonstrate a new class of spectroscopy based on Casimir interactions between a solid investigated object and a reference surface embedded in an environmental liquid medium. Our main example is based on the measurement of Hamaker constants upon changing the concentration of an intervening two-component liquid, where we demonstrate a possible reconstruction algorithm to estimate the frequency-dependent dielectric function of the investigated particle.

physics.optics↗