1,892 research outputs found
Decays in Quantum Hierarchical Models
We study the dynamics of a simple model for quantum decay, where a single
state is coupled to a set of discrete states, the pseudo continuum, each
coupled to a real continuum of states. We find that for constant matrix
elements between the single state and the pseudo continuum the decay occurs via
one state in a certain region of the parameters, involving the Dicke and
quantum Zeno effects. When the matrix elements are random several cases are
identified. For a pseudo continuum with small bandwidth there are weakly damped
oscillations in the probability to be in the initial single state. For
intermediate bandwidth one finds mesoscopic fluctuations in the probability
with amplitude inversely proportional to the square root of the volume of the
pseudo continuum space. They last for a long time compared to the non-random
case
Equivalence of two mathematical forms for the bound angular momentum of the electromagnetic field
It is shown that the mathematical form, obtained in a recent paper, for the
angular momentum of the electromagnetic field in the vicinity of electric
charge is equivalent to another form obtained previously by Cohen-Tannoudji,
Dupont-Roc and Gilbert. In this version of the paper an improved derivation is
given.Comment: 4 pages pdf, simpler derivatio
Absorption and emission spectroscopies of homogeneous and inhomogeneously broadened multilevel systems in strong light fields
A method is introduced to calc., for a model set of mol. levels, the spectral line shapes expected for a variety of conventional laser expts. including absorption, hole burning, fluorescence line narrowing, and Raman scattering. The method allows the incident laser field to have arbitrary intensity. Furthermore, the effects of model gaussian or lorenzian inhomogeneous distributions are readily incorporated. Earlier results for a 2-level system are easily obtained and new results are presented for inhomogeneously broadened 2- and 3-level systems, and for the effects of pure dephasing on the strong field spectra. The differences between fluorescence and Raman in strong fields, and the effect of strong fields on the spontaneous emission of inhomogeneously broadened transitions were described. Some predictions are made regarding line narrowing expts. in the strong-field limit
On the entanglement of a quantum field with a dispersive medium
In this Letter we study the entanglement of a quantum radiation field
interacting with a dielectric medium. In particular, we describe the quantum
mixed state of a field interacting with a dielectric through plasma and Drude
models and show that these generate very different entanglement behavior, as
manifested in the entanglement entropy of the field. We also present a formula
for a "Casimir" entanglement entropy, i.e., the distance dependence of the
field entropy. Finally, we study a toy model of the interaction between two
plates. In this model, the field entanglement entropy is divergent; however, as
in the Casimir effect, its distance-dependent part is finite, and the field
matter entanglement is reduced when the objects are far.Comment: Final published PRL versio
Decoherence of Rabi oscillations of electronic spin states in a double quantum dot
We study the role of charge fluctuations in the decoherence of Rabi
oscillations between spin states |>, |> of two electrons in a double dot structure. We consider the effects
of fluctuations in energy and in the quantum state of the system, both in the
classical and quantum limit. The role of state fluctuations is shown to be of
leading order at sufficiently high temperature, applicable to actual
experiments. At low temperature the low frequency energy fluctuations are the
only dominant contribution.Comment: 5 pages, 2 figures; v2: (extended version of the published article)
added details of calculations, modified fig. 2, improved "readability
Detecting entanglement of two electron spin qubits with witness operators
We propose a scheme for detecting entanglement between two electron spin
qubits in a double quantum dot using an entanglement witness operator. We first
calculate the optimal configuration of the two electron spins, defined as the
position in the energy level spectrum where, averaged over the nuclear spin
distribution, 1) the probability to have two separated electrons, and 2) the
degree of entanglement of the quantum state quantified by the concurrence are
both large. Using a density matrix approach, we then calculate the evolution of
the expectation value of the witness operator for the two-spin singlet state,
taking into account the effect of decoherence due to quantum charge
fluctuations modeled as a boson bath. We find that, for large interdot
coupling, it is possible to obtain a highly entangled and robust ground state.Comment: 4 pages, 3 figure
Bose-Einstein condensates in RF-dressed adiabatic potentials
Bose-Einstein condensates of Rb atoms are transferred into
radio-frequency (RF) induced adiabatic potentials and the properties of the
corresponding dressed states are explored. We report on measurements of the
spin composition of dressed condensates. We also show that adiabatic potentials
can be used to trap atom gases in novel geometries, including suspending a
cigar-shaped cloud above a curved sheet of atoms
Entanglement spectroscopy of a driven solid-state qubit and its detector
We study the asymptotic dynamics of a driven quantum two level system coupled
via a quantum detector to the environment. We find multi-photon resonances
which are due to the entanglement of the qubit and the detector. Different
regimes are studied by employing a perturbative Floquet-Born-Markov approach
for the qubit+detector system, as well as non-perturbative real-time path
integral schemes for the driven spin-boson system. We find analytical results
for the resonances, including the red and the blue sidebands. They agree well
with those of exact ab-initio calculations.Comment: 4 pages, 4 figure
Balanced homodyne detectors in QFT
Within the dipole approximation we describe the interaction of a photodiode
with the quantum electric field. The diode is modelled by an electron in a
bound state which upon interaction, treated perturbatively in the paper, can
get excited to one of the scattering states. We furthermore analyze a balanced
homodyne detector (BHD) with a local oscillator (LO) consisting of two
photodiodes illuminated by a monochromatic coherent state. We show, that to the
leading order the BHD's output measures the expectation value of the quantum
electric field, in the state without the LO, restricted to the frequency of the
LO. The square of the output measures the two-point function of the quantum
field. This shows that the BHDs provide tools for measurements of sub-vacuum
(negative) expectation values of the squares quantum fields and thus for test
of Quantum Energy Inequality - like bounds, or other QFT effects under the
influence of external conditions.Comment: Revised version with minor mistakes remove
Noncovariant gauge fixing in the quantum Dirac field theory of atoms and molecules
Starting from the Weyl gauge formulation of quantum electrodynamics (QED),
the formalism of quantum-mechanical gauge fixing is extended using techniques
from nonrelativistic QED. This involves expressing the redundant gauge degrees
of freedom through an arbitrary functional of the gauge-invariant transverse
degrees of freedom. Particular choices of functional can be made to yield the
Coulomb gauge and Poincar\'{e} gauge representations. The Hamiltonian we derive
therefore serves as a good starting point for the description of atoms and
molecules by means of a relativistic Dirac field. We discuss important
implications for the ontology of noncovariant canonical QED due to the gauge
freedom that remains present in our formulation.Comment: 8 pages, 0 figure
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