3,374 research outputs found
Adequacy of the Dicke model in cavity QED: a counter-"no-go" statement
The long-standing debate whether the phase transition in the Dicke model can
be realized with dipoles in electromagnetic fields is yet an unsettled one. The
well-known statement often referred to as the "no-go theorem", asserts that the
so-called A-square term, just in the vicinity of the critical point, becomes
relevant enough to prevent the system from undergoing a phase transition. At
variance with this common belief, in this paper we prove that the Dicke model
does give a consistent description of the interaction of light field with the
internal excitation of atoms, but in the dipole gauge of quantum
electrodynamics. The phase transition cannot be excluded by principle and a
spontaneous transverse-electric mean field may appear. We point out that the
single-mode approximation is crucial: the proper treatment has to be based on
cavity QED, wherefore we present a systematic derivation of the dipole gauge
inside a perfect Fabry-P\'erot cavity from first principles. Besides the impact
on the debate around the Dicke phase transition, such a cleanup of the
theoretical ground of cavity QED is important because currently there are many
emerging experimental approaches to reach strong or even ultrastrong coupling
between dipoles and photons, which demand a correct treatment of the Dicke
model parameters
Weakly bound molecules as sensors of new gravitylike forces
Several extensions to the Standard Model of particle physics, including light dark matter candidates and unification theories predict deviations from Newton’s law of gravitation. For macroscopic distances, the inverse-square law of gravitation is well confirmed by astrophysical observations and laboratory experiments. At micrometer and shorter length scales, however, even the state-of-the-art constraints on deviations from gravitational interaction, whether provided by neutron scattering or precise measurements of forces between macroscopic bodies, are currently many orders of magnitude larger than gravity itself. Here we show that precision spectroscopy of weakly bound molecules can be used to constrain non-Newtonian interactions between atoms. A proof-of-principle demonstration using recent data from photoassociation spectroscopy of weakly bound Yb2 molecules yields constraints on these new interactions that are already close to state-of-the-art neutron scattering experiments. At the same time, with the development of the recently proposed optical molecular clocks, the neutron scattering constraints could be surpassed by at least two orders of magnitude
Ehrenfest time in the weak dynamical localization
The quantum kicked rotor (QKR) is known to exhibit dynamical localization in
the space of its angular momentum. The present paper is devoted to the
systematic first--principal (without a regularizer) diagrammatic calculations
of the weak--localization corrections for QKR. Our particular emphasis is on
the Ehrenfest time regime -- the phenomena characteristic for the
classical--to--quantum crossover of classically chaotic systems.Comment: 27 pages, 9 figure
Time-resolved photoemission by attosecond streaking: extraction of time information
Attosecond streaking of atomic photoemission holds the promise to provide
unprecedented information on the release time of the photoelectron. We show
that attosecond streaking phase shifts indeed contain timing (or spectral
phase) information associated with the Eisenbud-Wigner-Smith time delay matrix
of quantum scattering. However, this is only accessible if the influence of the
streaking infrared (IR) field on the emission process is properly accounted
for. The IR probe field can strongly modify the observed streaking phase shift.
We show that the part of the phase shift ("time shift") due to the interaction
between the outgoing electron and the combined Coulomb and IR laser fields can
be described classically. By contrast, the strong initial-state dependence of
the streaking phase shift is only revealed through the solution of the
time-dependent Schr\"odinger equation in its full dimensionality. We find a
time delay between the hydrogenic 2s and 2p initial states in He+ exceeding
20as for a wide range of IR intensities and XUV energies
Nuclear-Astrophysics Lessons from INTEGRAL
Measurements of high-energy photons from cosmic sources of nuclear radiation
through ESA's INTEGRAL mission have advanced our knowledge: New data with high
spectral resolution showed that characteristic gamma-ray lines from radioactive
decays occur throughout the Galaxy, in its interstellar medium and from
sources. Although the number of detected sources and often the significance of
the astrophysical results remain modest, conclusions derived from this unique
astronomical window of radiation originating from nuclear processes are
important, complementing the widely-employed atomic-line based spectroscopy. We
review the results and insights obtained in the past decade from gamma-ray line
measurements of cosmic sources, in the context of their astrophysical
questions.Comment: Invited review. 30 pages, 26 figures. This is an author-created,
un-copyedited version of an article accepted for publication in Reports on
Progress in Physics. IOP Publishing Ltd is not responsible for any errors or
omissions in this version of the manuscript or any version derived from it.
The Version of Record is available online at DOI
10.1088/0034-4885/76/2/02630
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