arXiv subjects

Stefan Yoshi Buhmann

Publications and source records attributed to Stefan Yoshi Buhmann.

At least 55 records · Page 3Linked to original sources

arXiv2019

Dispersion forces in inhomogeneous planarly layered media: A one-dimensional model for effective polarisabilities

Dispersion forces such as van der Waals forces between two microscopic particles, the Casimir--Polder forces between a particle and a macroscopic object or the Casimir force between two dielectric objects are well studied in vacuum. However, in realistic situations the interacting objects are often embedded in an environmental medium. Such a solvent influences the induced dipole interaction. With the framework of macroscopic quantum electrodynamics, these interactions are mediated via an exchange of virtual photons. Via this method the impact of a homogeneous solvent medium can be expressed as local-field corrections leading to excess polarisabilities which have previously been derived for hard boundary conditions. In order to develop a more realistic description, we investigate on a one-dimensional analog system illustrating the influence of a continuous dielectric profile.

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arXiv2019

Symmetry-breaking in a condensate of light and its use as a quantum sensor

Bose-Einstein condensates (BECs) represent one of the very few manifestations of purely quantum effects on a macroscopic level. The vast majority of BECs achieved in the lab to date consist of bosonic atoms, which macroscopically populate the ground state once a threshold temperature has been reached. Recently, a new type of condensate was observed - the photon BEC, where light in a dye-filled cavity thermalises with dye molecules under the influence of an external driving laser, condensing to the lowest-energy mode. However, the precise relationship between the photon BEC and symmetry-breaking phenomena has not yet been investigated. Here we consider medium-induced symmetry breaking in a photon BEC and show that it can be used as a quantum sensor. The introduction of polarisable objects such as chiral molecules lifts the degeneracy between cavity modes of different polarisations. Even a tiny imbalance is imprinted on the condensate polarisation in a 'winner takes it all' effect. When used as a sensor for enantiomeric excess, the predicted sensitivity exceeds that of contemporary methods based on circular dichroism. Our results introduce a new symmetry-breaking mechanism that is independent of the external pump, and demonstrate that the photon BEC can be used for practical purposes.

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arXiv2018

A Stern-Gerlach separator of chiral enantiomers based on the Casimir-Polder potential

We propose a method to separate enantiomers using parity violation in the Casimir-Polder potential between chiral mirrors and chiral molecules. The proposed setup involves a molecular beam composed of chiral molecules passing through a planar cavity consisting of two chiral mirrors. Enantiomers of opposite handedness are deflected differently due to a chiral dependence of the Casimir-Polder potential resulting in the separation of the enantiomers. Our setup provides an alternative experimental tool for enantiomer separation, as well as shedding light on the fundamental properties of the Casimir-Polder potential.

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arXiv2018

The virtual photon approximation for three-body interatomic Coulombic decay

Interatomic Coulombic decay (ICD) is a mechanism which allows microscopic objects to rapidly exchange energy. When the two objects are distant, the energy transfer between the donor and acceptor species takes place via the exchange of a virtual photon. On the contrary, recent ab initio calculations have revealed that the presence of a third passive species can significantly enhance the ICD rate at short distances due to the effects of electronic wave function overlap and charge transfer states [Phys. Rev. Lett. 119, 083403 (2017)]. Here, we develop a virtual photon description of three-body ICD, showing that a mediator atom can have a significant influence at much larger distances. In this regime, this impact is due to the scattering of virtual photons off the mediator, allowing for simple analytical results and being manifest in a distinct geometry-dependence which includes interference effects. As a striking example, we show that in the retarded regime ICD can be substantially enhanced or suppressed depending on the position of the ICD-inactive object, even if the latter is far from both donor and acceptor species.

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arXiv2018

Distinguishing models of surface response through the self-energy of an electron

The self-energy of an electron confined between parallel surfaces with arbitrary dielectric properties is calculated. The mechanism for this effect is the surface-induced modification of the fluctuating quantised vacuum field to which the electron is coupled, thereby endowing it with a surface-dependent self-energy in broad analogy to the Casimir-Polder effect for an atom. We derive a general formula for this self-energy shift and find that its sign is different for two commonly-used models of surface response, namely the plasma model and the Drude model. We propose an experiment which could detect this difference in sign, shedding light on continuing uncertainty about the correct description of the interaction of low-frequency vacuum photons with media.

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arXiv2018

Casimir-Polder Potential of a Driven Atom

We investigate theoretically the Casimir-Polder potential of an atom which is driven by a laser field close to a surface. This problem is addressed in the framework of macroscopic quantum electrodynamics using the Green's tensor formalism and we distinguish between two different approaches, a perturbative ansatz and a method based on Bloch equations. We apply our results to a concrete example, namely an atom close to a perfectly conducting mirror, and create a scenario where the tunable Casimir-Polder potential becomes similar to the respective potential of an undriven atom due to fluctuating field modes. Whereas the perturbative approach is restricted to large detunings, the ansatz based on Bloch equations is exact and yields an expression for the potential which does not exceed 1/2 of the undriven Casimir-Polder potential.

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arXiv2018

The Casimir-Polder potential in the presence of a Fock state

Atom-surface forces using excited states have a host of compelling applications, including repulsive and lateral forces. However, such states can be fragile and difficult to prepare. Here we report an explicit normal-mode based calculation of the Casimir-Polder potential between a ground-state atom and a non-dispersive surface in the presence of an external quantised field. The potential we derive shares some features with that of excited-state Casimir-Polder forces even though we consider a ground-state atom. Our work provides a physically transparent and intuitive picture of driven Casimir-Polder potentials, as well as expanding on previous investigations by providing analytic results that fully include retardation, as well as being applicable for any choice of mutual alignment of the atom's dipole moment, the external field, and the surface normal.

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arXiv2018

Purcell-Dicke effect for an atomic ensemble near a surface

The emission by an initially completely inverted atomic ensemble in the long-wavelength regime is simultaneously enhanced by both collective effects (Dicke effect) and dielectric environments (Purcell effect), thus giving rise to a combined Purcell-Dicke effect. We study this effect by treating the ensemble of N atoms as a single effective N+1-level 'Dicke atom' which couples to the environment-assisted quantum electrodynamic field. We find that an environment can indeed alter the superradiant emission dynamics, as exemplified using a perfectly conducting plate. As the emission acquires an additional anisotropy in the presence of the plate, we find an associated resonant Casimir-Polder potential for the atom that is collectively enhanced and that exhibits a superradiant burst in its dynamics. An additional tuneability of the effect is introduced by applying an external driving laser field.

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arXiv2018

Charge-Parity violating effects in Casimir-Polder potentials

We demonstrate under which conditions a violation of the charge-parity (CP) symmetry in molecules will manifest itself in the Casimir-Polder interaction of these with a magnetodielectric surface. Charge-parity violation induces a specific electric-magnetic cross-polarisability in a molecule that is not chiral, but time-reversal (T) symmetry violating. As we show, a detection of such an effect via the Casimir-Polder potential requires a material medium that is also sensitive to time-reversal, i.e., it must exhibit a non-reciprocal electromagnetic response. As simple examples of such media we consider a perfectly reflecting non-reciprocal mirror that is a special case of a perfect electromagnetic conductor as well as a Chern-Simons medium. In addition, we show that Chern-Simons and related media can induce unusual atom-surface interactions for anisotropic molecules with and without a chiral response.

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arXiv2017

Lateral Casimir-Polder forces by breaking time-reversal symmetry

We examine the lateral Casimir-Polder force acting on a circular rotating emitter near a dielectric plane surface. As the circular motion breaks time-reversal symmetry, the spontaneous emission in a direction parallel to the surface is in general anisotropic. We show that a lateral force arises which can be interpreted as a recoil force because of this asymmetric emission. The force is an oscillating function of the distance between the emitter and the surface, and the lossy character of the dielectric strongly influences the results in the near-field regime. The force exhibits also a population-induced dynamics, decaying exponentially with respect to time on timescales of the inverse of the spontaneous decay rate. We propose that this effect could be detected measuring the velocity acquired by the emitter, following different cycles of excitation and thermalisation. Our results are expressed in terms of the Green's tensor and can therefore easily be applied to more complex geometries.

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arXiv2017

Non-additivity of optical and Casimir-Polder potentials

An atom irradiated by an off-resonant laser field near a surface is expected to experience the sum of two fundamental potentials, the optical potential of the laser field and the Casimir--Polder potential of the surface. Here, we report a new non-additive potential, namely the laser-induced Casimir--Polder potential, which arises from a correlated coupling of the atom with both the laser and the quantum vacuum. We apply this result to an experimentally realizable scenario of an atomic mirror with an evanescent laser beam leaking out of a surface. We show that the non-additive term is significant for realistic experimental parameters, transforming potential barriers into potential wells, which can be used to trap atoms near surfaces.

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arXiv2017

Correct zero-point energy-momentum of the electromagnetic field in media for studying the Casimir force

We address the long-standing controversy regarding the correct description of the electromagnetic energy-momentum tensor in media and its consequences for the Casimir force. The latter being due to the zero-point momentum of the electromagnetic field, competing approaches based on the Abraham or Maxwell stress tensor lead to different predictions. We consider the test scenario of two colloidal spherical particles submerged in a dielectric medium and use three criteria to distinguish the two approaches: we show that the Abraham stress tensor, and not the Maxwell stress tensor, leads to a Casimir force that is form-equivalent to Casimir-Polder and van der Waals forces, obeys duality as a fundamental symmetry and is consistent with microscopic many-body calculations.

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arXiv2017

Casimir effect for perfect electromagnetic conductors (PEMCs): A sum rule for attractive/repulsive forces

We discuss the Casimir effect for boundary conditions involving perfect electromagnetic conductors (PEMCs). Based on the corresponding reciprocal Green's tensor we construct the Green's tensor for two perfectly reflecting plates with magnetoelectric coupling (non-reciprocal media) within the framework of macroscopic quantum electrodynamics. We calculate the Casimir force between two PEMC plates in terms of the PEMC parameter M and the duality transformation angle $θ$ resulting in a universal analytic expression that connects the attractive Casimir force with the repulsive Boyer force. We relate the results to the duality symmetry of electromagnetism.

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arXiv2017

Cavity-QED interactions of two correlated atoms

We consider the resonant van der Waals interaction between two correlated identical two-level atoms (at least one of which being excited) within the framework of macroscopic cavity quantum electrodynamics in linear, dispersing and absorbing media. The interaction of both atoms with the body-assisted electromagnetic field of the cavity is assumed to be strong. Our time-independent evaluation is based on an extended Jaynes-Cummings model. For a system prepared in a superposition of its dressed states, we derive the general form of the van der Waals forces, using a Lorentzian single mode approximation. We demonstrate the applicability of this approach by considering the case of a planar cavity and showing the position-dependence of Rabi oscillations. We also show that in the limiting case of weak coupling, our results reproduce the perturbative ones, for the case where the field is initially in vacuum state while the atomic state is in a superposition of two correlated states sharing one excitation.

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arXiv2017

Casimir-Lifshitz Force for Nonreciprocal Media and Applications to Photonic Topological Insulators

Based on the theory of macroscopic quantum electrodynamics, we generalize the expression of the Casimir force for nonreciprocal media. The essential ingredient of this result is the Green's tensor between two nonreciprocal semi-infinite slabs including a reflexion matrix with four coefficients that mixes optical polarizations. This Green's tensor does not obey Lorentz's reciprocity and thus violates time-reversal symmetry. The general result for the Casimir force is analyzed in the retarded and nonretarded limits, concentrating on the influences arising from reflections with or without change of polarization. In a second step we apply our general result to a photonic topological insulator whose nonreciprocity stems from an anisotropic permittivity tensor, namely InSb. We show that there is a regime for the distance between the slabs where the magnitude of the Casimir force is tunable by an external magnetic field. Furthermore the strength of this tuning depends on the orientation of the magnetic field with respect to the slab surfaces.

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arXiv2017

Manipulating the Coulomb interaction: A Green's function perspective

We present a unified framework for studying Coulomb interactions in arbitrary environments using macroscopic quantum electrodynamics on the basis of the electromagnetic Green's function. Our theory can be used to derive the Coulomb potential of a single charged particle as well as that between two charges in the presence of media, bodies and interfaces of arbitrary shapes. To demonstrate this, we reproduce the well-known screened Coulomb force, account for local-field effects and consider new cases such as a dielectric cavity and a conducting plate with a hole.

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arXiv2017

Enhanced chiral discriminatory van der Waals interactions mediated by chiral surfaces

We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic and chiral response to non-trivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waals interaction is discriminatory with respect to enantiomers of different handedness and could be used to separate enantiomers. We also suggest a specific geometric configuration where the electric contribution to the van der Waals interaction is zero, making the chiral component the dominant effect.

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arXiv2017

Van der Waals interactions between excited atoms in generic environments

We consider the the van der Waals force involving excited atoms in general environments, constituted by magnetodielectric bodies. We develop a dynamical approach studying the dynamics of the atoms and the field, mutually coupled. When only one atom is excited, our dynamical theory suggests that for large distances the van der Waals force acting on the ground-state atom is monotonic, while the force acting in the excited atom is spatially oscillating. We show how this latter force can be related to the known oscillating Casimir--Polder force on an excited atom near a (ground-state) body. Our force also reveals a population-induced dynamics: for times much larger that the atomic lifetime the atoms will decay to their ground-states leading to the van der Waals interaction between ground-state atoms.

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