529 research outputs found
Anomalous strong exchange narrowing in excitonic systems
We investigate theoretically the phenomenon of exchange narrowing in the
absorption spectrum of a chain of monomers, which are coupled via resonant
dipole-dipole interaction. The individual (uncoupled) monomers exhibit a broad
absorption line shape due to the coupling to an environment consisting of a
continuum of vibrational modes. Upon increasing the interaction between the
monomers, the absorption spectrum of the chain narrows. For a non-Markovian
environment with a Lorentzian spectral density, we find a narrowing of the peak
width (full width at half maximum (FWHM)) by a factor 1/N, where N is the
number of monomers. This is much stronger than the usual 1/sqrt{N} narrowing.
Furthermore it turns out that for a Markovian environment no exchange narrowing
at all occurs. The relation of different measures of the width (FWHM, standard
deviation) is discussed
The J- and H-bands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation
The validity of the CES approximation is investigated by comparison with
direct diagonalisation of a model vibronic Hamiltonian of identical
monomers interacting electronically. Even for quite short aggregates (N\gtrsim
6) the CES approximation is shown to give results in agreement with direct
diagonalisation, for all coupling strengths, except that of intermediate
positive coupling (the H-band region). However, previously excellent agreement
of CES calculations and measured spectra in the H-band region was obtained [A.
Eisfeld, J. S. Briggs, Chem. Phys. 324, 376]. This is shown to arise from use
of the measured monomer spectrum which includes implicitly dissipative effects
not present in the model calculation
Quantum Dynamics Simulation with Classical Oscillators
In a previous paper [J.S.Briggs and A.Eisfeld, Phys.Rev.A 85, 052111] we
showed that the time-development of the complex amplitudes of N coupled quantum
states can be mapped by the time development of positions and velocities of N
coupled classical oscillators. Here we examine to what extent this mapping can
be realised to simulate the "quantum" properties of entanglement and qubit
manipulation. By working through specific examples, e.g. of quantum gate
operation, we seek to illuminate quantum/classical differences which hitherto
have been treated more mathematically. In addition we show that important
quantum coupled phenomena, such as the Landau-Zener transition and the
occurrence of Fano resonances can be simulated by classical oscillators
On the Equivalence of Quantum and Classical Coherence in Electronic Energy Transfer
To investigate the effect of quantum coherence on electronic energy transfer,
which is the subject of current interest in photosynthesis, we solve the
problem of transport for the simplest model of an aggregate of monomers
interacting through dipole-dipole forces using both quantum and classical
dynamics. We conclude that for realistic coupling strengths quantum and
classical coherent transport are identical. This is demonstrated by numerical
calculations for a linear chain and for the photosynthetic Fenna-Matthews-Olson
(FMO) comple
Conical intersections in an ultracold gas
We find that energy surfaces of more than two atoms or molecules interacting
via dipole-dipole po- tentials generically possess conical intersections (CIs).
Typically only few atoms participate strongly in such an intersection. For the
fundamental case, a circular trimer, we show how the CI affects adiabatic
excitation transport via electronic decoherence or geometric phase
interference. These phe- nomena may be experimentally accessible if the trimer
is realized by light alkali atoms in a ring trap, whose dipole-dipole
interactions are induced by off-resonant dressing with Rydberg states. Such a
setup promises a direct probe of the full many-body density dynamics near a
conical intersection.Comment: 4 pages, 4 figures, replacement to add archive referenc
Excitation transport through Rydberg dressing
We show how to create long range interactions between alkali-atoms in
different hyper-fine ground states, allowing coherent electronic quantum state
migration. The scheme uses off resonant dressing with atomic Rydberg states,
exploiting the dipole-dipole excitation transfer that is possible between
those. Actual population in the Rydberg state is kept small. Dressing offers
large advantages over the direct use of Rydberg levels: It reduces ionisation
probabilities and provides an additional tuning parameter for life-times and
interaction-strengths. We present an effective Hamiltonian for the ground-state
manifold and show that it correctly describes the full multi-state dynamics for
up to 5 atoms.Comment: 22 pages + 6 pages appendices, 8 figures, replaced with revised
version, added journal referenc
Newton's cradle and entanglement transport in a flexible Rydberg chain
In a regular, flexible chain of Rydberg atoms, a single electronic excitation
localizes on two atoms that are in closer mutual proximity than all others. We
show how the interplay between excitonic and atomic motion causes electronic
excitation and diatomic proximity to propagate through the Rydberg chain as a
combined pulse. In this manner entanglement is transferred adiabatically along
the chain, reminiscent of momentum transfer in Newton's cradle.Comment: 4 pages, 3 figures. Revised versio
Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra
We predict the existence of exchange broadening of optical lineshapes in
disordered molecular aggregates and a nonuniversal disorder scaling of the
localization characteristics of the collective electronic excitations
(excitons). These phenomena occur for heavy-tailed L\'evy disorder
distributions with divergent second moments - distributions that play a role in
many branches of physics. Our results sharply contrast with aggregate models
commonly analyzed, where the second moment is finite. They bear a relevance for
other types of collective excitations as well
Dipole-dipole induced global motion of Rydberg-dressed atom clouds
We consider two clouds of ground state alkali atoms in two distinct hyperfine
ground states. Each level is far off-resonantly coupled to a Rydberg state,
which leads to dressed ground states with a weak admixture of the Rydberg state
properties. Due to this admixture, for a proper choice of the Rydberg states,
the atoms experience resonant dipole-dipole interactions that induce mechanical
forces acting on all atoms within both clouds. This behavior is in contrast to
the dynamics predicted for bare dipole-dipole interactions between Rydberg
superatoms, where only a single atom per cloud is subject to dipole-dipole
induced motion [Phys. Rev. A {\bf 88} 012716 (2013)].Comment: 15 pages, 2 figure
Source of entangled atom pairs on demand, using the Rydberg blockade
Two ultracold atom clouds, each separately in a dipole-blockade regime,
realize a source of entangled atom pairs that can be ejected on demand.
Entanglement generation and ejection is due to resonant dipole-dipole
interactions, while van-der-Waals interactions are predominantly responsible
for the blockade that ensures the ejection of a single atom per cloud. A source
of entangled atoms using these effects can operate with a 10 kHz repetition
rate producing ejected atoms with velocities of about 0.5 m/s.Comment: 7 pages, 4 figure
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