1,500 research outputs found
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
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
Tuning nonradiative lifetimes via molecular aggregation
We show that molecular aggregation can strongly influence the nonradiative
decay (NRD) lifetime of an electronic excitation. As a demonstrative example,
we consider a transition-dipole-dipole-interacting dimer whose monomers have
harmonic potential energy surfaces (PESs). Depending on the position of the NRD
channel (), we find that the NRD lifetime () can exhibit a completely different dependence on the
intermolecular-interaction strength. We observe that (i) for near
the Franck-Condon region, increases with the
interaction strength; (ii) for near the minimum of the monomer
excited PES, the intermolecular interaction has little influence on ; (iii) for near the classical turning point of the
monomer nuclear dynamics, on the other side of the minimum, decreases with the interaction strength. Our findings suggest design
principles for molecular systems where a specific fluorescence quantum yield is
desired
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
Gaussian processes for choosing laser parameters for driven, dissipative Rydberg aggregates
To facilitate quantum simulation of open quantum systems at finite
temperatures, an important ingredient is to achieve thermalization on a given
time-scale. We consider a Rydberg aggregate (an arrangement of Rydberg atoms
that interact via long-range interactions) embedded in a laser-driven atomic
environment. For the smallest aggregate (two atoms), suitable laser parameters
can be found by brute force scanning of the four tunable laser parameters. For
more atoms, however, such parameter scans are too computationally costly. Here
we apply Gaussian processes to predict the thermalization performance as a
function of the laser parameters for two-atom and four-atom aggregates. These
predictions perform remarkably well using just 1000 simulations, demonstrating
the utility of Gaussian processes in an atomic physics setting. Using this
approach, we find and present effective laser parameters for generating
thermalization, the robustness of these parameters to variation, as well as
different thermalization dynamics
Van-der-Waals stabilized Rydberg aggregates
Assemblies of Rydberg atoms subject to resonant dipole-dipole interactions
form Frenkel excitons. We show that van-der-Waals shifts can significantly
modify the exciton wave function, whenever atoms approach each other closely.
As a result, attractive excitons and repulsive van-der-Waals interactions can
be combined to form stable one-dimensional atom chains, akin to bound
aggregates. Here the van-der-Waals shifts ensure a stronger homogeneous
delocalisation of a single excitation over the whole chain, enabling it to bind
up to six atoms. When brought into unstable configurations, such Rydberg
aggregates allow the direct monitoring of their dissociation dynamics.Comment: 6 pages, 6 figure
Excitonic Wave Function Reconstruction from Near-Field Spectra Using Machine Learning Techniques
A general problem in quantum mechanics is the reconstruction of eigenstate
wave functions from measured data. In the case of molecular aggregates,
information about excitonic eigenstates is vitally important to understand
their optical and transport properties. Here we show that from spatially
resolved near field spectra it is possible to reconstruct the underlying
delocalized aggregate eigenfunctions. Although this high-dimensional nonlinear
problem defies standard numerical or analytical approaches, we have found that
it can be solved using a convolutional neural network. For both one-dimensional
and two-dimensional aggregates we find that the reconstruction is robust to
various types of disorder and noise
Pseudomodes and the corresponding transformation of the temperature-dependent bath correlation function
In open system approaches with non-Markovian environments, the process of
inserting an individual mode (denoted as "pseudomode") into the bath or
extracting it from the bath is widely employed. This procedure, however, is
typically performed on basis of the spectral density (SD) and does not
incorporate temperature. Here, we show how the - temperature-dependent - bath
correlation function (BCF) transforms in such a process. We present analytic
formulae for the transformed BCF and numerically study the differences between
factorizing initial state and global thermal (correlated) initial state of mode
and bath, respectively. We find that in the regime of strong coupling of the
mode to both system and bath, the differences in the BCFs give rise to
pronounced differences in the dynamics of the system.Comment: 12 pages, 4 figure
Hierarchy of stochastic pure states for open quantum system dynamics
We derive a hierarchy of stochastic evolution equations for pure states
(quantum trajectories) to efficiently solve open quantum system dynamics with
non-Markovian structured environments. From this hierarchy of pure states
(HOPS) the exact reduced density operator is obtained as an ensemble average.
We demonstrate the power of HOPS by applying it to the Spin-Boson model, the
calculation of absorption spectra of molecular aggregates and energy transfer
in a photosynthetic pigment-protein complex
- …