arXiv subjects

Robert Bennett

Publications and source records attributed to Robert Bennett.

At least 37 records · Page 2Linked to original sources

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

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.

quant-ph↗

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.

quant-ph↗

arXiv2018

Revealing short-range non-Newtonian gravity through Casimir-Polder shielding

We carry out a realistic, yet simple, calculation of the Casimir-Polder interaction in the presence of a metallic shield in order to aid the design of experiments to test non-Newtonian gravity. In particular, we consider a rubidium atom near a movable silicon slab with a gold film in between. We show that by moving the slab to various distances and making precise measurements of the force exerted on the atom, one could in principle discern the existence of short-range modifications to Newtonian gravity. This avoids the need for a patterned surface where calculations are much harder and for which the probe must be moved laterally at a fixed distance. We also briefly discuss the case where an atomic cloud undergoes Bloch oscillations within an optical lattice created by reflecting a laser off the shield. We find that our scheme has the potential to improve current constraints if relatively modest improvements in atom localisation in optical lattices are made.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

arXiv2017

Direct measurement of short-range forces with a levitated nanoparticle

Short-range forces have important real-world relevance across a range of settings in the nano world, from colloids and possibly for protein folding to nano-mechanical devices, but also for detection of weak long-range forces, such as gravity, at short distances and of candidates to solve the problem of dark energy. Short-range forces, such as Casimir-Polder or van der Waals are in general difficult to calculate as a consequence of their non-additive nature, and challenging to measure due to their small magnitude - especially for charged particles where dispersion forces are normally many orders of magnitude smaller than electrostatic image forces. Therefore short-range forces have represented a continuing theoretical and experimental challenge over the last half-century. Here we report on experiments with a single glass nanoparticle levitated in close proximity to a neutral silicon surface in vacuum, which allow for direct measurement of short-range forces in a new distance and sensitivity regime - outperforming existing force microscopies.

cond-mat.mes-hall↗

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.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

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

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↗

arXiv2015

A physically-motivated quantisation of the electromagnetic field

The notion that the electromagnetic field is quantised is usually inferred from observations such as the photoelectric effect and the black-body spectrum. However accounts of the quantisation of this field are usually mathematically motivated and begin by introducing a vector potential, followed by the imposition of a gauge that allows the manipulation of the solutions of Maxwell's equations into a form that is amenable for the machinery of canonical quantisation. By contrast, here we quantise the electromagnetic field in a less mathematically and more physically-motivated way. Starting from a direct description of what one sees in experiments, we show that the usual expressions of the electric and magnetic field observables follow from Heisenberg's equation of motion. In our treatment, there is no need to invoke the vector potential in a specific gauge and we avoid the commonly-used notion of a fictitious cavity that applies boundary conditions to the field.

quant-ph↗

arXiv2015

A generalized bag-like boundary condition for fields with arbitrary spin

Boundary conditions for the Maxwell and Dirac fields at material surfaces are widely-used and physically well-motivated, but do not appear to have been generalised to deal with higher spin fields. As a result there is no clear prescription as to which boundary conditions should be selected in order to obtain physically-relevant results pertaining to confined higher spin fields. This lack of understanding is significant given that boundary-dependent phenomena are ubiquitous across physics, a prominent example being the Casimir effect. Here, we use the two-spinor calculus formalism to present a unified treatment of boundary conditions routinely employed in the treatment of spin-1/2 and spin-1 fields. We then use this unification to obtain a boundary condition that can be applied to massless fields of any spin, including the spin-2 graviton, and its supersymmetric partner the spin-3/2 gravitino.

hep-th↗

arXiv2015

Spontaneous decay rate and Casimir-Polder potential of an atom near a lithographed surface

Radiative corrections to an atom are calculated near a half-space that has arbitrarily-shaped small depositions upon its surface. The method is based on calculation of the classical Green's function of the macroscopic Maxwell equations near an arbitrarily perturbed half-space using a Born series expansion about the bare half-space Green's function. The formalism of macroscopic quantum electrodynamics is used to carry this over into the quantum picture. The broad utility of the calculated Green's function is demonstrated by using it to calculate two quantities --- the spontaneous decay rate of an atom near a sharp surface feature, and the Casimir-Polder potential of a finite grating deposited on a substrate. Qualitatively new behaviour is found in both cases, most notably in the latter where it is observed that the periodicity of the Casimir-Polder potential persists even outside the immediate vicinity of the grating.

quant-ph↗

arXiv2014

The Casimir effect for fields with arbitrary spin

The Casimir force between two perfectly reflecting parallel plates is considered. In a recent paper we presented generalised physical boundary conditions describing perfectly reflecting parallel plates. These boundary conditions are applicable to a field possessing any spin, and include the well-known spin-1/2 and spin-1 boundary conditions as special cases. Here we use these general boundary conditions to show that the allowed values of energy-momentum turn out to be the same for any massless fermionic field and the same for any massless bosonic field. As a result one expects to obtain only two possible Casimir forces, one associated with fermions and the other with bosons. We explicitly verify that this is the case for the fields up to spin-2. A significant implication of our work is that periodic boundary conditions cannot be applied to a fermionic field confined between two parallel plates.

quant-ph↗