Roberto Passante

The role of the cloud of virtual photons in the shift of the ground state energy of a hydrogen atom

Abstract The division of the hamiltonian of a hydrogen atom into three parts: atomic, radiation and interaction, together with the use of non-relativistic second-order perturbation theory, is shown to yield a physical interpretation of the energy shift of the ground state which emphasizes the role of the field due to the cloud of virtual photons which surround the atom.

Virtual Field, Causal Photon Absorption and Photodetectors

The time-dependent electric-energy density surrounding a two-level atom fixed at r = 0 is studied, the atom being taken in its ground state at t = 0 and the field having initially only one photon in a delocalized mode. The atom-field coupling includes both rotating and counterrotating terms. The energy density in the rotating wave approximation is shown to behave noncausally, while in the presence of the complete coupling it is shown to be affected only within a sphere of radius r = ct centred on the atom. It is concluded that the counterrotating terms in the atom-field coupling are essential in order to ensure causality and cannot be neglected in any accurate treatment of photon absorption. Some consequences of this conclusion on the operation of photodetectors in one-photon absorption are discussed.

Non-Hermitian Hamiltonian for a modulated Jaynes-Cummings model with PT symmetry

We consider a two-level system such as a two-level atom, interacting with a cavity field mode in the rotating wave approximation, when the atomic transition frequency or the field mode frequency is periodically driven in time. We show that in both cases, for an appropriate choice of the modulation parameters, the state amplitudes in a generic

Atoms dressed and partially dressed by the zero-point fluctuations of the electromagnetic field

n

Temperature dependence of the magnetic Casimir-Polder interaction

{-}excitation subspace obey the same equations of motion that can be obtained from a \emph{static} non-Hermitian Jaynes-Cummings Hamiltonian with

Thermal and Nonthermal Signatures of the Unruh Effect in Casimir-Polder Forces

{\mathcal PT}

van der Waals interactions between excited atoms in generic environments

symmetry, that is with an imaginary coupling constant. This gives further support to recent results showing the possible physical interest of

Enhanced resonant force between two entangled identical atoms in a photonic crystal

{\mathcal PT}

Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions

symmetric non-Hermitian Hamiltonians. We also generalize the well-known diagonalization of the Jaynes-Cummings Hamiltonian to the non-Hermitian case in terms of pseudo-bosons and pseudo-fermions, and discuss relevant mathematical and physical aspects.

Field fluctuations near a conducting plate and Casimir-Polder forces in the presence of boundary conditions

An atom or a molecule is constituted by a set of bound electric charges with dynamics governed by the laws of quantum mechanics. These charges are sources of the quantized electromagnetic field which also binds them together, and thus the effects of their interaction with the field cannot be disregarded in principle. Consequently even overall neutral atoms, on which we focus our attention, are driven by a dynamics which is inextricably related to the dynamics of the quantized electromagnetic field. One of the most prominent aspects of the atom-held interaction is the existence of a cloud of virtual photons which dresses the atom even in the lowest possible energy state of the system. In this paper we review the static as well as the dynamic aspects of the theory of the virtual cloud around the neutral atoms. We begin by reviewing various forms of the atom-field coupling as well as various models of simplified atoms which will be used in the rest of the paper. The question then arises as how to characterize quantitatively the shape of the virtual cloud, and we show that the energy density of the electromagnetic field is a physical quantity suitable for this purpose. First a perturbative approach to calculating this shape is developed and applied to several physical models of a ground-state dressed atom. The next step is to consider virtual clouds which are out of equilibrium and examine their time development. This leads to the concept of half-dressed sources, which are discussed in a different physical context both in the absence and in the presence of real photons. In particular the role of the virtual cloud in ensuring causality of the field propagating in dressing and undressing processes is emphasized. Finally, the nature of the virtual cloud is further discussed in the light of theories concerning the dynamics of an atomic pair, the quantum theory of measurement and the effects of a driving electromagnetic field.