arXiv subjects

Manuel Donaire

Publications and source records attributed to Manuel Donaire.

14 recordsLinked to original sources

arXiv2026

Asymmetric Light Scattering from an Atomic System with Gain: A Quantum Analysis

We study the scattering of light by a binary system of identical atoms in which one of them is incoherently pumped. This system belongs to the kind of non-parity symmetric optical systems in which gains and losses are partially compensated. We carry out a fully quantum analysis of the directionality of the radiation scattered from the atoms when the incident light strikes the system either perpendicular or alongside the interatomic axis. By resolving the emitted field in space, the microscopic mechanisms associated with photon exchange, interference, and reciprocity become directly identifiable. We find that, generally, while the degree of asymmetry depends on the pump rate, the preferred direction for emission depends on the interatomic distance and the detuning of the probe field with respect to the resonant frequency. On physical grounds, for the case of frontal illumination, the asymmetry is the result of the interference of the photons emitted from different atoms. On the contrary, for side lighting, the asymmetry with respect to the side of incidence is caused by both the phase difference between the probe field photons that strike each atom and the interference between the photons emitted from different atoms. Further, for side lighting too, our quantum approach demonstrates that the forward scattered power depends on the side of incidence, which reveals the lack of reciprocity in the quantum optical response of the system. This result conflicts with what is obtained within a classical approach.

quant-ph↗

arXiv2025

Nonreciprocal and nonconservative forces on binary systems of identical atoms

The dynamical and radiative features of an excited system of two identical atoms are analysed. The metastability of the system, the directionality of its emission and its internal forces are studied. Closed-form expressions are derived for the time-evolution of the system, for the angular distribution of its spontaneous emission, and for its internal dipole forces, both conservative and nonconservative. The latter reveals the presence of nonreciprocal forces, which leads to a net oscillatory force upon the system. We estimate that, for a free binary system of Li Rydberg atoms, the net internal force may cause a displacement of its center of mass as large as $120 nm$ over a lifetime.

quant-ph↗

arXiv2021

Nonconservative dipole forces in a two-atom system with one atom excited

We compute the nonconservative electric dipole forces between two atoms, one of which is initially excited. These forces derive from the time variation of the longitudinal electromagnetic momentum. In contrast to the conservative van-der-Waals forces, the nonconservative forces posses components orthogonal to the interatomic axis. Thus, despite being several orders of magnitude smaller than van-der-Waals', they might be accessible experimentally for the case of two identical atoms. In addition, as with the van-der-Waals forces, the nonconservative forces present nonreciprocal constituents which result in a net force on the two-atom system. We offer an estimate of the spatial displacement caused by the nonconservative forces on a binary system of Hydrogen atoms.

quant-ph↗

arXiv2021

Resonant scattering of a single atom with gain: a wavefunction-diagrammatic approach

We characterize the optical response of a three-level atom subjected to an incoherent pump and continuously illuminated with a weak, quasi-resonant probe field. To this end, we apply a wavefunction approach based on QED Hamiltonian perturbation theory which allows for a reduction of the atomic dynamics to that of an effective two-level atom, and for an implementation of the incoherent effects that respects unitarity. Using a diagrammatic representation, we identify and classify all the radiative processes. This allows us to compute the scattered power, the spontaneous emission, and the stimulated emission, as well as the total cross sections of extinction, absorption and scattering. We find that, beside a general enhancement of the linewidth and an attenuation of the spectral amplitudes, the pump reduces the nonradiative losses and provides gains in the form of stimulated emission and incoherent radiation. For sufficiently strong pump, gains and losses compensate, resulting in the vanishing of extinction. In particular, for negligible nonradiative losses, extinction vanishes for a pumping rate of $(1+\sqrt{5})/2$ times that of the natural decay.

quant-ph↗

arXiv2020

Topological thermalization via vortex formation in ultra-fast quenches

We investigate the thermalization of a two-component scalar field across a second-order phase transition under extremely fast quenches. We find that vortices start developing at the final temperature of the quench, i.e., below the critical point. Specifically, we find that vortices emerge once the fluctuating field departures from its symmetric state and evolves towards a metastable and inhomogenous configuration. The density of primordial vortices at the relaxation time is a decreasing function of the final temperature of the quench. Subsequently, vortices and antivortices annihilate at a rate that eventually determines the total thermalization time. This rate decreases if the theory contains a discrete anisotropy, which otherwise leaves the primordial vortex density unaffected. Our results thus establish a link between the topological processes involved in the vortex dynamics and the delay in the thermalization of the system.

cond-mat.stat-mech↗

arXiv2019

Acceleration of an unpolarized proton along a uniform magnetic field: Casimir momentum of leptons

It has been recently shown that a chiral molecule accelerates linearly along a spatially uniform magnetic field, as a result of the parity-time symmetry breaking induced in its QED self-interaction. In this work we extend this result to fundamental particles which present EW self-interaction, in which case parity is violated by the EW interaction itself. In particular, we demonstrate that, in a spatially uniform and adiabatically time-varying magnetic field, an unpolarized proton coupled to the leptonic vacuum acquires a kinetic momentum antiparallel to the magnetic field, whereas virtual leptons gain an equivalent $Casimir$ $momentum$ in the opposite direction. That momentum is proportional to the magnetic field and to the square of Fermi's constant. We prove that the kinetic energy of the proton is a magnetic energy which forms part of its EW self-energy.

hep-ph↗

arXiv2019

Acceleration of a proton along a uniform magnetic field: Casimir momentum of leptons

It is commonly assumed that a charged particle does not accelerate linearly along a spatially uniform magnetic field. We show that this is no longer the case if the interaction of the particle with the quantum vacuum is chiral, in which case parity and time-reversal symmetries are simultaneously broken. In particular, this is the situation of an electroweak interacting particle in the presence of a uniform magnetic field. We demonstrate first that, in a spatially uniform and adiabatically time-varying magnetic field, a proton coupled to the leptonic vacuum acquires a kinetic momentum antiparallel to the magnetic field, whereas virtual leptons gain an equivalent Casimir momentum in the opposite direction. Remarkably, leptons remain virtual throughout the process, which means that the proton acceleration is not caused by the recoil associated to the emission of any actual particle. The kinetic energy of the proton is part of its electroweak self-energy, which is provided by the source of magnetic field. In addition we find that, in a constant and uniform magnetic field, the adiabatic spin-relaxation of a single proton is accompanied by its acceleration along the magnetic field. We estimate that, at the end of the spin-polarization process, the proton reaches a velocity of the order of $μ$m/s. The latter finding may lie within the scope of experimental observations.

hep-ph↗

arXiv2016

Two-atom interaction energies with one atom in an excited state: van der Waals potentials vs. level shifts

I revisit the problem of the interaction between two dissimilar atoms with one atom in an excited state, recently addressed by the authors of Refs.[1-3], and for which precedent approaches have given conflicting results. In the first place, I discuss to what extent Refs.[1], [2] and [3] provide equivalent results. I show that the phase-shift rate of the two-atom wave function computed in Ref.[1], the van der Waals potential of the excited atom in Ref.[2] and the level shift of the excited atom in Ref.[3] possess equivalent expressions in the quasistationary approximation. In addition, I show that the level shift of the ground state atom computed in Ref.[3] is equivalent to its van der Waals potential. A diagrammatic representation of all those quantities is provided. The equivalences among them are however not generic. In particular, it is found that for the case of the interaction between two identical atoms excited, the phase-shift rate and the van der Waals potentials differ. Concerning the conflicting results of previous approaches in regards to the spatial oscillation of the interactions, I conclude in agreement with Refs.[1,3] that they refer to different physical quantities. The impacts of free-space dissipation and finite excitation rates on the dynamics of the potentials are analyzed. In contrast to Ref.[3], the oscillatory versus monotonic spatial forms of the potentials of each atom are found not to be related to the reversible versus irreversible nature of the excitation transfer involved.

quant-ph↗

arXiv2016

Velocity-dependent dipole forces on an excited atom

We present a time-dependent calculation of the velocity-dependent forces which act on an excited atomic dipole in relative motion with respect to ground state atoms of a different kind. Both, its interaction with a single atom and with a dilute atomic plate are evaluated. In either case, the total force consists of a conservative van der Waals component and a non-conservative Rontgen component. On physical grounds, the former corresponds to the velocity-dependent recoil experienced by the excited atom in the processes of absorption and emission of the photons that it exchanges with the ground state atoms on a periodic basis. The latter corresponds to the time-variation of the Rontgen momentum, which is also mediated by the periodic exchange of quasi-resonant photons. We find that, at leading order, all these interactions are linear in the velocity. In the non-retarded regime the van der Waals force dominates, being antiparallel to the velocity. On the contrary, in the retarded regime the velocity-dependent forces oscillate in space, van der Waals and Rontgen forces are of the same order in the atom-atom interaction, and the Rontgen component dominates in the atom-surface interaction.

quant-ph↗

arXiv2015

Quasi-resonant van der Waals interaction between non-identical atoms

We present a time-dependent quantum calculation of the van der Waals interaction between a pair of dissimilar atoms, one of which is initially excited while the other one is in its ground state. For small detuning, the interaction is predominantly mediated at all distances by the exchange of doubly resonant photons between the two atoms. We find that it presents both temporal and spatial oscillations. Spatially oscillating terms depend on the resonant frequencies of both atoms, while the frequency of the time oscillations is given by their detuning. We analyse the physical content of our findings and discuss to what extent previous conflicting stationary approaches provide compatible results. A proper account of causality is found essential in order to obtain the correct result.

quant-ph↗

arXiv2015

On the coherent effect of vacuum fluctuations on driven atoms

We study the coherent effect of the Casimir-Polder interaction on the oscillations of two-photon driven atoms. We find that, for oscillations between two degenerate states in lambda-configuration, shifts on the Rabi frequency may be induced by non-additive level shifts. For oscillations between two Rydberg states in ladder-configuration, shifts on the Rabi frequency may be induced by the effective renormalization of the laser interaction.

quant-ph↗

arXiv2015

Transfer of linear momentum from the quantum vacuum to a magnetochiral molecule

In a recent publication [Phys. Rev. Lett. 111, 143602] we have shown using a QED approach that, in the presence of a magnetic field, the quantum vacuum coupled to a chiral molecule provides a kinetic momentum directed along the magnetic field. Here we explain the physical mechanisms which operate in the transfer of momentum from the vacuum to the molecule. We show that the variation of the molecular kinetic energy originates from the magnetic energy associated with the vacuum correction to the magnetization of the molecule. We carry out a semiclassical calculation of the vacuum momentum and compare the result with the QED calculation.

quant-ph↗

arXiv2013

Casimir Momentum of a Chiral Molecule in a Magnetic Field

In a classical description, a neutral, polarizable object acquires a kinetic momentum when exposed to crossed electric and magnetic fields. In the presence of only a magnetic field no such momentum exists classically, although it is symmetry-allowed for an object lacking mirror symmetry. We perform a full QED calculation to show that the quantum vacuum coupled to a chiral molecule provides a kinetic "Casimir" momentum directed along the magnetic field, proportional to its rotatory power and the fine structure constant.

quant-ph↗

arXiv2000

Sigma Model Corrections to the Confining Background

Sigma model ($α^{\prime}$) corrections to the confining string background are obtained. The main result is that the Poincaré invariant ansatz is maintained. Physical conditions for the dissapearance of the naked singularity are discussed.

hep-th↗