Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physics

Atoms Dressed by Virtual and Real Photons

M. Allegrini, E. Arimondo

Published Jan 8, 2024• 2 citations
Publisher

Abstract

Specific properties of quantum field theory are described by considering the combination of the system under investigation and the cloud of virtual or real particles associated with the field. Such a structure is called a “dressed system”, in contrast with the bare one in the absence of the interaction with the field. The description of the properties of such clouds in various physical situations is, today, an active research area. Here, we present the main features associated with virtual and real dressings, focusing on photon dressing. In analogy to virtual photon clouds dressing electrons in vacuum, virtual phonon clouds appear in solid-state physics. The interaction between real photons and the schematized two-level structure of an atom paves the way to flexible quantum control. Here, a unifying Floquet engineering approach is applied to describe single- and multiple-dressed atom configurations. Connections with the past and present atomic physics experiments are presented.

DOI
10.3390/physics6010005

Related papers

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

Giuseppe Compagno, G. Massimo Palma, Roberto Passante +1 more

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.

Journal of Physics B Atomic Molecular and Optical Physics•Apr 14, 1995•35 citations

Further considerations on the nature of dressed states

G. Compagno, Roberto Passante, Franco Persico

Introduction . In this final chapter we review two topics of the literature concerning dressed atoms which are of conceptual relevance and capable of shedding some light on the physical meaning and significance of atomic dressing. The first section is devoted to some recent work in connection with the quantum theory of measurement. We include the measuring apparatus in the Hamiltonian along with an appropriate apparatus-atom coupling. We argue that the theory of measurement of finite duration provides us with a tool for detecting the spectral composition of the virtual cloud surrounding an atom. In fact, we show that in a measurement of duration T on a two-level atom fully dressed by the vacuum fluctuations, as discussed in Chapters 6 and 7, the apparatus perceives the atom as dressed only by photons of frequency larger than T -1 . In the case of a two-level atom dressed by a single-mode field populated by real photons, discussed in Chapter 5, we show that if T is smaller than the inverse Rabi frequency ħΔ -1 , the atom is perceived by the apparatus as bare; on the contrary, if T is larger than ħΔ -1 , the atom is perceived as dressed. The time scales for the two cases of dressing, by vacuum fluctuations or by a real single-mode field, are very different, but the similarity of the effects indicates a common physical aspect of the two kinds of dressing.

Cambridge University Press eBooks•Aug 3, 1995•0 citations

Virtual Cloud Effects in Perturbed Atoms

G. Compagno, G. Massimo Palma, Andrea E. Santangelo

We consider the interplay between the structure and the dynamics of a quantum source and the virtual photon cloud surrounding it in two simple models: as first case we examine the structure of the virtual cloud surrounding a model of static source formed by the N = 1, N = 2 shells of the hydrogen atom when a static uniform electric field acts on the source. The distortion induced on the source is reflected on the structure of the virtual cloud. In fact in the presence of the external field new components of the virtual cloud appear. These new components extend to large distances and have the same spatial symmetry of the one induced by the static field on the source. We then consider a three level system interacting with a boson field. Its intermediate level is metastable and decays with emission of two quanta to the lowest level. Due to the interaction with the field the intermediate and the lower level are dressed by a virtual photon cloud. The effects of the dressing are taken into account by a unitary transformation. Then the two quanta decay amplitude is obtained using the resolvent technique. The dressed lineshape obtained is then compared with the one obtained in the decay between bare states In the dressed decay one of the emitted quanta has the same distribution as the virtual quanta surrounding the excited state.

Physica Scripta•Jan 1, 1988•7 citations

Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation

Omar Di Stefano, Roberto Stassi, Luigi Garziano +3 more

In quantum field theory, bare particles are dressed by a cloud of virtual particles to form physical particles. The virtual particles affect properties such as the mass and charge of the physical particles, and it is only these modified properties that can be measured in experiments, not the properties of the bare particles. The influence of virtual particles is prominent in the ultrastrong-coupling regime of cavity quantum electrodynamics (QED), which has recently been realised in several condensed-matter systems. In some of these systems, the effective interaction between atom-like transitions and the cavity photons can be switched on or off by external control pulses. This offers unprecedented possibilities for exploring quantum vacuum fluctuations and the relation between physical and bare particles. We consider a single three-level quantum system coupled to an optical resonator. Here we show that, by applying external electromagnetic pulses of suitable amplitude and frequency, each virtual photon dressing a physical excitation in cavity-QED systems can be converted into a physical observable photon, and back again. In this way, the hidden relationship between the bare and the physical excitations can be unravelled and becomes experimentally testable. The conversion between virtual and physical photons can be clearly pictured using Feynman diagrams with cut loops.

arXiv:1603.04984•New Journal of Physics•Apr 12, 2017•60 citations

Dressed Atoms and the Quantum Theory of Measurement

G. Compagno, Roberto Passante, Franco Persico

A gedanken experiment is discussed to measure the population of a two-level atom interacting with a single-mode radiation field. The atom-field system is taken to be in the dressed ground state. The measuring apparatus consists of a classical pointer linearly coupled to the atom. It is shown that the result of the measurement depends strongly on the duration τ on the measuring process. For τ shorter than the time of the vacuum fluctuations creating the virtual photon cloud around the atom, the pointer perceives the atom as bare. In the opposite case the atom is perceived by the pointer as dressed or partially dressed. The insight gained in the nature of the virtual photon cloud, in the meaning of the quantum detection process and in the preparation of partially dressed states, is discussed in the light of this result.

Europhysics Letters (EPL)•Jun 15, 1990•13 citations

Atoms dressed by a real e.m. field

G. Compagno, Roberto Passante, Franco Persico

Introduction . The second part of the book is dedicated to the dressed atom, and it begins with this chapter, which deals mainly with the quantum-dynamical description of an atom dressed by a real electro-magnetic field. Here the emphasis is on the adjective ‘real’, by which we mean that the field is in an excited state populated by real photons and not just by the zero-point photon background. Due to the coupling discussed in the first part of the book, atom-photon correlations are established which admix, shift and split the levels of the system atom plus radiation field. The admixed and correlated states are called dressed-atom states. In Section 5.2 we obtain the Hamiltonian for an atom in a cavity with perfectly reflecting walls. The cavity selects a discrete set of field modes, and this leads us naturally to consider the simplest possible nontrivial atom-field system: the Jaynes-Cummings model describing a two-level atom coupled to a single-mode system. In Section 5.3 we develop the theory, based on a unitary transformation, to dress a two-level atom by a mode of the cavity populated by real photons. A necessary preliminary for dealing with more complicated atom-field models is the theory of spontaneous emission in free space, which is discussed in Section 5.4 in a Wigner-Weisskopf framework.

Cambridge University Press eBooks•Aug 3, 1995•1 citation
Specific properties of quantum field theory are described by considering the combination of the system under investigation and the cloud of virtual or real particles associated with the field. Such a structure is called a “dressed system”, in contrast with the bare one in the absence of the interaction with the field. The description of the properties of such clouds in various physical situations is, today, an active research area. Here, we present the main features associated with virtual and real dressings, focusing on photon dressing. In analogy to virtual photon clouds dressing electrons in vacuum, virtual phonon clouds appear in solid-state physics. The interaction between real photons and the schematized two-level structure of an atom paves the way to flexible quantum control. Here, a unifying Floquet engineering approach is applied to describe single- and multiple-dressed atom configurations. Connections with the past and present atomic physics experiments are presented.
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.
Introduction . In this final chapter we review two topics of the literature concerning dressed atoms which are of conceptual relevance and capable of shedding some light on the physical meaning and significance of atomic dressing. The first section is devoted to some recent work in connection with the quantum theory of measurement. We include the measuring apparatus in the Hamiltonian along with an appropriate apparatus-atom coupling. We argue that the theory of measurement of finite duration provides us with a tool for detecting the spectral composition of the virtual cloud surrounding an atom. In fact, we show that in a measurement of duration T on a two-level atom fully dressed by the vacuum fluctuations, as discussed in Chapters 6 and 7, the apparatus perceives the atom as dressed only by photons of frequency larger than T -1 . In the case of a two-level atom dressed by a single-mode field populated by real photons, discussed in Chapter 5, we show that if T is smaller than the inverse Rabi frequency ħΔ -1 , the atom is perceived by the apparatus as bare; on the contrary, if T is larger than ħΔ -1 , the atom is perceived as dressed. The time scales for the two cases of dressing, by vacuum fluctuations or by a real single-mode field, are very different, but the similarity of the effects indicates a common physical aspect of the two kinds of dressing.
We consider the interplay between the structure and the dynamics of a quantum source and the virtual photon cloud surrounding it in two simple models: as first case we examine the structure of the virtual cloud surrounding a model of static source formed by the N = 1, N = 2 shells of the hydrogen atom when a static uniform electric field acts on the source. The distortion induced on the source is reflected on the structure of the virtual cloud. In fact in the presence of the external field new components of the virtual cloud appear. These new components extend to large distances and have the same spatial symmetry of the one induced by the static field on the source. We then consider a three level system interacting with a boson field. Its intermediate level is metastable and decays with emission of two quanta to the lowest level. Due to the interaction with the field the intermediate and the lower level are dressed by a virtual photon cloud. The effects of the dressing are taken into account by a unitary transformation. Then the two quanta decay amplitude is obtained using the resolvent technique. The dressed lineshape obtained is then compared with the one obtained in the decay between bare states In the dressed decay one of the emitted quanta has the same distribution as the virtual quanta surrounding the excited state.
In quantum field theory, bare particles are dressed by a cloud of virtual particles to form physical particles. The virtual particles affect properties such as the mass and charge of the physical particles, and it is only these modified properties that can be measured in experiments, not the properties of the bare particles. The influence of virtual particles is prominent in the ultrastrong-coupling regime of cavity quantum electrodynamics (QED), which has recently been realised in several condensed-matter systems. In some of these systems, the effective interaction between atom-like transitions and the cavity photons can be switched on or off by external control pulses. This offers unprecedented possibilities for exploring quantum vacuum fluctuations and the relation between physical and bare particles. We consider a single three-level quantum system coupled to an optical resonator. Here we show that, by applying external electromagnetic pulses of suitable amplitude and frequency, each virtual photon dressing a physical excitation in cavity-QED systems can be converted into a physical observable photon, and back again. In this way, the hidden relationship between the bare and the physical excitations can be unravelled and becomes experimentally testable. The conversion between virtual and physical photons can be clearly pictured using Feynman diagrams with cut loops.
A gedanken experiment is discussed to measure the population of a two-level atom interacting with a single-mode radiation field. The atom-field system is taken to be in the dressed ground state. The measuring apparatus consists of a classical pointer linearly coupled to the atom. It is shown that the result of the measurement depends strongly on the duration τ on the measuring process. For τ shorter than the time of the vacuum fluctuations creating the virtual photon cloud around the atom, the pointer perceives the atom as bare. In the opposite case the atom is perceived by the pointer as dressed or partially dressed. The insight gained in the nature of the virtual photon cloud, in the meaning of the quantum detection process and in the preparation of partially dressed states, is discussed in the light of this result.
Introduction . The second part of the book is dedicated to the dressed atom, and it begins with this chapter, which deals mainly with the quantum-dynamical description of an atom dressed by a real electro-magnetic field. Here the emphasis is on the adjective ‘real’, by which we mean that the field is in an excited state populated by real photons and not just by the zero-point photon background. Due to the coupling discussed in the first part of the book, atom-photon correlations are established which admix, shift and split the levels of the system atom plus radiation field. The admixed and correlated states are called dressed-atom states. In Section 5.2 we obtain the Hamiltonian for an atom in a cavity with perfectly reflecting walls. The cavity selects a discrete set of field modes, and this leads us naturally to consider the simplest possible nontrivial atom-field system: the Jaynes-Cummings model describing a two-level atom coupled to a single-mode system. In Section 5.3 we develop the theory, based on a unitary transformation, to dress a two-level atom by a mode of the cavity populated by real photons. A necessary preliminary for dealing with more complicated atom-field models is the theory of spontaneous emission in free space, which is discussed in Section 5.4 in a Wigner-Weisskopf framework.