arXiv subjects

Stefan Yoshi Buhmann

Publications and source records attributed to Stefan Yoshi Buhmann.

At least 73 records · Page 4Linked to original sources

arXiv2016

Spectroscopic signatures of quantum friction

We present a formula for the spectroscopically accessible level shifts and decay rates of an atom moving at an arbitrary angle relative to a surface. Our Markov formulation leads to an intuitive analytic description whereby the shifts and rates are obtained from the coefficients of the Heisenberg equation of motion for the atomic flip operators but with complex Doppler-shifted (velocity-dependent) transition frequencies. Our results conclusively demonstrate that for the limiting case of parallel motion the shifts and rates are quadratic or higher in the atomic velocity. We show that a stronger, linear velocity dependence is exhibited by the rates and shifts for perpendicular motion, thus opening the prospect of experimentally probing the Markovian approach to the phenomenon of quantum friction.

quant-ph↗

arXiv2016

Dynamical Casimir-Polder force between an excited atom and a conducting wall

We consider the dynamical atom-surface Casimir-Polder force in the non-equilibrium configuration of an atom near a perfectly conducting wall, initially prepared in an excited state with the field in its vacuum state. We evaluate the time-dependent Casimir-Polder force on the atom, and find that it shows an oscillatory behavior from attractive to repulsive both in time and in space. We also investigate the asymptotic behavior in time of the dynamical force and of related local field quantities, showing that the static value of the force, as obtained by a time-independent approach, is recovered for times much larger than the timescale of the atomic self-dressing, but smaller than the atomic decay time. We then discuss the evolution of global quantities such as atomic and field energies, and their asymptotic behavior. We also compare our results for the dynamical force on the excited atom with analogous results recently obtained for an initially bare ground-state atom. We show that new relevant features are obtained in the case of an initially excited atom, for example much larger values of the dynamical force with respect to the static one, allowing for an easier way to single-out and observe the dynamical Casimir-Polder effect.

quant-ph↗

arXiv2016

Impact of Casimir-Polder interaction on Poisson-spot diffraction at a dielectric sphere

Diffraction of matter-waves is an important demonstration of the fact that objects in nature possess a mixture of particle-like and wave-like properties. Unlike in the case of light diffraction, matter-waves are subject to a vacuum-mediated interaction with diffraction obstacles. Here we present a detailed account of this effect through the calculation of the attractive Casimir-Polder potential between a dielectric sphere and an atomic beam. Furthermore, we use our calculated potential to make predictions about the diffraction patterns to be observed in an ongoing experiment where a beam of indium atoms is diffracted around a silicon dioxide sphere. The result is an amplification of the on-axis bright feature which is the matter-wave analogue of the well-known `Poisson spot' from optics. Our treatment confirms that the diffraction patterns resulting from our complete account of the sphere Casimir-Polder potential are indistinguishable from those found via a large-sphere non-retarded approximation in the discussed experiments, establishing the latter as an adequate model.

quant-ph↗

arXiv2016

Casimir-Polder Shift and Decay Rate in the Presence of Nonreciprocal Media

We calculate the Casimir-Polder frequency shift and decay rate for an atom in front of a nonreciprocal medium by using macroscopic quantum electrodynamics. The results are a generalization of the respective quantities for matter with broken time-reversal symmetry which does not fulfill the Lorentz reciprocity principle. As examples, we contrast the decay rates, the resonant and nonresonant frequency shifts of a perfectly conducting (reciprocal) mirror to those of a perfectly reflecting nonreciprocal mirror. We find different power laws for the distance dependence of all quantities in the retarded and nonretarded limits. As an example of a more realistic nonreciprocal medium, we investigate a topological insulator subject to a time-symmetry breaking perturbation.

quant-ph↗

arXiv2015

Dynamical Casimir-Polder interaction between a chiral molecule and a surface

We develop a dynamical approach to study the Casimir-Polder force between a initially bare molecule and a magnetodielectric body at finite temperature. Switching on the interaction between the molecule and the field at a particular time, we study the resulting temporal evolution of the Casimir-Polder interaction. The dynamical self-dressing of the molecule and its population-induced dynamics are accounted for and discussed. In particular, we find that the Casimir-Polder force between a chiral molecule and a perfect mirror oscillates in time with a frequency related to the molecular transition frequency, and converges to the static result for large times.

quant-ph↗

arXiv2015

The Electromagnetic Green's Function for Layered Topological Insulators

The dyadic Green's function of the inhomogeneous vector Helmholtz equation describes the field pattern of a single frequency point source. It appears in the mathematical description of many areas of electromagnetism and optics including both classical and quantum, linear and nonlinear optics, dispersion forces (such as the Casimir and Casimir-Polder forces) and in the dynamics of trapped atoms and molecules. Here, we compute the Green's function for a layered topological insulator. Via the magnetoelectric effect, topological insulators are able to mix the electric, E, and magnetic induction, B, fields and, hence, one finds that the TE and TM polarizations mix on reflection from/transmission through an interface. This leads to novel field patterns close to the surface of a topological insulator.

physics.optics↗

arXiv2015

Impact of anisotropy on the interaction of an atom with a one-dimensional nano-grating

We study the interaction of an atom with a one-dimensional nano-grating within the framework of macroscopic QED, with special emphasis on possible anisotropic contributions. To this end, we first derive the scattering Green's tensor of the grating by means of a Rayleigh expansion and discuss its symmetry properties and asymptotes. We then determine the Casimir--Polder potential of an atom with the grating. In particular, we find that strong anisotropy can lead to a repulsive Casimir--Polder potential in the normal direction.

quant-ph↗

arXiv2015

Casimir-Polder interaction from exact diagonalization and state mixing near surfaces

Dispersion forces have a sizeable effect on the energy levels of highly excited Rydberg atoms when brought close to material surfaces. Rydberg atoms experience energy shifts in the GHz range at micrometer distances, suggestive of considerable state admixture. We show that despite the non-applicability of perturbation theory for Rydberg atoms near a surface, the energy shift due to the dispersion interaction can be obtained from an exact diagonalization of the interaction Hamiltonian by finding the zeros of the Pick function. Moreover, we show that contrary to intuition from single-mode approaches, surface-induced state mixing is generally suppressed even for large interaction energies. We give a tailored example where mixing is observable despite this effect.

quant-ph↗

arXiv2015

Directional spontaneous emission and lateral Casimir-Polder force on an atom close to a nanofiber

We study the spontaneous emission of an excited atom close to an optical nanofiber and the resulting scattering forces. For a suitably chosen orientation of the atomic dipole, the spontaneous emission pattern becomes asymmetric and a resonant Casimir--Polder force parallel to the fiber axis arises. For a simple model case, we show that the such a lateral force is due to the interaction of the circularly oscillating atomic dipole moment with its image inside the material. With the Casimir--Polder energy being constant in the lateral direction, the predicted lateral force does not derive from a potential in the usual way. Our results have implications for optical force measurements on a substrate as well as for laser cooling of atoms in nanophotonic traps.

quant-ph↗

arXiv2015

Casimir-Polder repulsion: Three-body effects

In this paper we study an archetypical scenario in which repulsive Casimir-Polder forces between an atom or molecule and two macroscopic bodies can be achieved. This is an extension of previous studies of the interaction between a polarizable atom and a wedge, in which repulsion occurs if the atom is sufficiently anisotropic and close enough to the symmetry plane of the wedge. A similar repulsion occurs if such an atom passes a thin cylinder or a wire. An obvious extension is to compute the interaction between such an atom and two facing wedges, which includes as a special case the interaction of an atom with a conducting screen possessing a slit, or between two parallel wires. To this end we further extend the electromagnetic multiple-scattering formalism for three-body interactions. To test this machinery we reinvestigate the interaction of a polarizable atom between two parallel conducting plates. In that case, three-body effects are shown to be small, and are dominated by three- and four-scattering terms. The atom-wedge calculation is illustrated by an analogous scalar situation, described in the Appendix. The wedge-wedge-atom geometry is difficult to analyze because this is a scale-free problem. But it is not so hard to investigate the three-body corrections to the interaction between an anisotropic atom or nanoparticle and a pair of parallel conducting cylinders, and show that the three-body effects are very small and do not affect the Casimir-Polder repulsion at large distances between the cylinders. Finally, we consider whether such highly anisotropic atoms needed for repulsion are practically realizable. Since this appears rather difficult to accomplish, it may be more feasible to observe such effects with highly anisotropic nano particles.

quant-ph↗

arXiv2013

Probing atom-surface interactions by diffraction of Bose-Einstein condensates

In this article we analyze the Casimir-Polder interaction of atoms with a solid grating and an additional repulsive interaction between the atoms and the grating in the presence of an external laser source. The combined potential landscape above the solid body is probed locally by diffraction of Bose-Einstein condensates. Measured diffraction efficiencies reveal information about the shape of the Casimir-Polder interaction and allow us to discern between models based on a pairwise-summation (Hamaker) approach and Lifshitz theory.

physics.atom-ph↗

arXiv2012

Body-assisted dispersion potentials of diamagnetic atoms

We derive Casimir-Polder and van der Waals potentials of one or two atoms with diamagnetic properties in an arbitrary environment of magnetoelectric bodies. The calculations are based on macroscopic quantum electrodynamics and leading-order perturbation theory. For the examples of an atom and a perfect mirror and two atoms in free space we show that diamagnetic dispersion potentials have the same sign as their electric counterparts, but can exhibit quite different distance dependences.

quant-ph↗

arXiv2012

Casimir-Polder Forces between Chiral Objects

The chiral component of the Casimir-Polder potential is derived within the framework of macroscopic quantum electrodynamics. It is shown to exist only if the particle and the medium are both chiral. Furthermore, the chiral component of the Casimir-Polder potential can be attractive or repulsive, depending on the chirality of the molecule and the medium. The theory is applied to a cavity geometry in the non-retarded limit with the intention of enantiomer separation. For a ground state molecule the chiral component is dominated by the electric component and thus no explicit separation will happen. If the molecule is initially in an excited state the electric component of the Casimir-Polder force can be suppressed by an appropriate choice of material and the chiral component can select the molecule based on its chirality, allowing enantiomeric separation to occur.

quant-ph↗

arXiv2012

Casimir-Polder interaction of fullerene molecules with surfaces

We calculate the thermal Casimir--Polder potential of C60 and C70 fullerene molecules near gold and silicon nitride surfaces, motivated by their relevance for molecular matter wave interference experiments. We obtain the coefficients governing the asymptotic power laws of the interaction in the thermal, retarded and nonretarded distance regimes and evaluate the full potential numerically. The interaction is found to be dominated by electronic transitions, and hence independent of the internal temperature of the molecules. The contributions from phonon transitions, which are affected by the molecular temperature, give rise to only a small correction. Moreover, we find that the sizeable molecular line widths of thermal fullerenes may modify the nonretarded interaction, depending on the model used. Detailed measurements of the nonretarded potential of fullerene thus allow one to distinguish between different theories of incorporating damping.

quant-ph↗

arXiv2012

Casimir-Polder energy level shifts of an out-of-equilibrium particle near a microsphere

Rydberg atoms and beams of ultracold polar molecules have become highly useful experimental tools in recent years. There is therefore a need for accessible calculations of interaction potentials between such particles and nearby surfaces and structures, bearing in mind that the particles are far out of thermal equilibrium with their environment and that their interaction is predominantly non-retarded. Based on a new perturbative expansion with respect to the inverse speed of light and the inverse conductivity, we derive a simple, closed-form expression for the interaction potential (i.e., the particle energy level shifts) of a particle and a metallic sphere that is is accurate at better than 1% level for typical experimental set-ups at room temperature and above, and off by no more than a few percent at any temperature including zero. Our result illuminates the influence of retardation and imperfect conductivity and the interplay of these effects with geometry. The method developed for the present study may be applied to other, more complex geometries.

quant-ph↗

arXiv2011

Macroscopic quantum electrodynamics and duality in non-local and Onsager-violating media

We formulate macroscopic quantum electrodynamics for the most general linear, absorbing media. In particular, Onsager reciprocity is not assumed to hold. For media with a non-local response, the field quantisation is based on the conductivity tensor and the Green tensor for the electromagnetic field. For a local medium response, we introduce the permittivity, permeability and magnetoelectric susceptibilities to obtain an explicitly duality-invariant scheme. We find that duality invariance only holds as a continuous symmetry when non-reciprocal responses are allowed for.

quant-ph↗

arXiv2011

Temperature-independent Casimir-Polder forces in arbitrary geometries

We show that the Casimir-Polder potential of a particle in an energy eigenstate at nonretarded distance from a well-conducting body of arbitrary shape is independent of the environment temperature. This is true even when the thermal photon numbers at the relevant atomic transition energies are large. A compact expression is obtained for the temperature-independent potential, which can greatly simplify calculations in nontrivial geometries for experimentally relevant systems such as Rydberg atoms and polar molecules. We give criteria for the validity of our temperature-independent result. They are illustrated by numerical studies of a particle near a gold sphere or inside a gold cylindrical cavity.

quant-ph↗

arXiv2010

Casimir-Polder potential and transition rate in resonating cylindrical cavities

We consider the Casimir-Polder potential of particles placed inside a metallic cylindrical cavity at finite temperatures, taking account of thermal non-equilibrium effects. In particular, we study how the resonant (thermal non-equilibrium) potential and transition rates can be enhanced by fine-tuning the radius of the cavity to match the transition wavelength of the dominant transition of the particle. Numerical calculations show that the CP potential acting atoms prepared in low-lying Rydberg states can be enhanced beyond 30 kHz, which is within the range of observability of modern experiments. Because the magnitude of the resonance peaks depend sensitively on the low frequency dissipation of the cavity metal, experiments in this set-up could be a critical test of the disputed thermal correction to the Casimir force between metal plates.

quant-ph↗