G. Ritschel and A. Eisfeld (2014)
"Analytic Bath Correlation Functions for Ohmic and Superohmic Spectral Densities” submitted [arXiv] 
S. Möbius, S. M. Vlaming, V. A. Malyshev, J. Knoester, A. Eisfeld (2014)
"Onedimensional excitonic systems with diagonal Levy disorder: A detailed study of the absorption spectra and the disorder scaling of localization length” submitted [arXiv] 
D. Süß, A. Eisfeld and W.T. Strunz (2014)
"Hierarchy of stochastic pure states for open quantum system dynamics” submitted [arXiv] 
S.M. Vlaming, A. Eisfeld (2014)
"Tunable superradiance in porphyrin chains on insulating surfaces” submitted 
H. Zoubi, A. Eisfeld, S. Wüster (2014)
"VanderWaals stabilized Rydberg aggregates” Phys. Rev. A 89, 053426 
Abstract: Assemblies of Rydberg atoms subject to resonant dipoledipole interactions exhibit 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 dipoledipole potentials and repulsive van der Waals interactions can be combined to form stable onedimensional atom chains, akin to bound aggregates. Here the van der Waals shifts ensure a stronger homogeneous delocalization of a single excitation over the whole chain, enabling it to bind several atoms. When brought into unstable configurations, such Rydberg aggregates allow the direct monitoring of their dissociation dynamics. 
BibTeX:
@article{PhysRevA.89.053426,
title = {van der Waalsstabilized Rydberg aggregates},
author = {Zoubi, H. and Eisfeld, A. and Wüster, S.},
journal = {Phys. Rev. A},
volume = {89},
issue = {5},
pages = {053426},
numpages = {6},
year = {2014},
month = {May},
publisher = {American Physical Society},
doi = {10.1103/PhysRevA.89.053426},
url = {http://link.aps.org/doi/10.1103/PhysRevA.89.053426}
}

M. Genkin, S. Wüster, S. Möbius, A. Eisfeld, JM. Rost (2014)
"Dipoledipole induced global motion of Rydbergdressed atom clouds” J. Phys. B, 47, 095003 
Abstract: We consider two clouds of groundstate alkali atoms in two distinct hyperfine ground states. Each level is far offresonantly 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 behaviour 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 (Möbius et al 2013 Phys. Rev. A 88 012716). 
BibTeX:
@article{09534075479095003,
author={M Genkin and S Wüster and S Möbius and A Eisfeld and J M Rost},
title={Dipole–dipole induced global motion of Rydbergdressed atom clouds},
journal={Journal of Physics B: Atomic, Molecular and Optical Physics},
volume={47},
number={9},
pages={095003},
url={http://stacks.iop.org/09534075/47/i=9/a=095003},
year={2014},
abstract={We consider two clouds of groundstate alkali atoms in two distinct hyperfine ground states. Each level is far offresonantly 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 behaviour 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 (Möbius et al 2013 Phys. Rev. A 88 012716).}
}

S. Wüster, S. Möbius, M. Genkin, A. Eisfeld, J.M. Rost (2013)
"Source of entangled atom pairs on demand, using the Rydberg blockade” Phys. Rev. A 88, 063644 
Abstract: Two ultracold atom clouds, each separately in a dipoleblockade regime, realize a source of entangled atom pairs that can be ejected on demand. Entanglement generation and ejection is due to resonant dipoledipole 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. Using spatially resolved Rydberg state coupling and detection, a violation of Bell's inequalities could be measured in our setup. 
BibTeX:
@article{PhysRevA.88.063644,
title = {Source of entangled atom pairs on demand using the Rydberg blockade},
author = {W\"uster, S. and M\"obius, S. and Genkin, M. and Eisfeld, A. and Rost, J. M.},
journal = {Phys. Rev. A},
volume = {88},
issue = {6},
pages = {063644},
numpages = {8},
year = {2013},
month = {Dec},
publisher = {American Physical Society},
doi = {10.1103/PhysRevA.88.063644},
url = {http://link.aps.org/doi/10.1103/PhysRevA.88.063644}
}

J.S. Briggs and A. Eisfeld (2013)
"Quantum Dynamics Simulation with Classical Oscillators” Phys. Rev. A 88, 062104 
Abstract: In a previous paper [J. S. Briggs and A. Eisfeld, Phys. Rev. A 85, 052111 (2012)] 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 realized 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 and classical differences which hitherto have been treated more mathematically. In addition, we show that important quantum coupled phenomena, such as the LandauZener transition and the occurrence of Fano resonances can be simulated by classical oscillators. 
BibTeX:
@article{PhysRevA.88.062104,
title = {Quantum dynamics simulation with classical oscillators},
author = {Briggs, John S. and Eisfeld, Alexander},
journal = {Phys. Rev. A},
volume = {88},
issue = {6},
pages = {062104},
numpages = {12},
year = {2013},
month = {Dec},
publisher = {American Physical Society},
doi = {10.1103/PhysRevA.88.062104},
url = {http://link.aps.org/doi/10.1103/PhysRevA.88.062104}
}

R. ElGanainy, A. Eisfeld, M. Levy and D.N. Christodoulides (2013)
"Onchip nonreciprocal optical devices based on quantum inspired photonic lattices” Appl. Phys. Lett. 103, 161105 
Abstract: We propose integrated optical structures that can be used as isolators and polarization splitters based on engineered photonic lattices. Starting from optical waveguide arrays that mimic Fock space (quantum state with a welldefined particle number) representation of a noninteracting twosite Bose Hubbard Hamiltonian, we show that introducing magnetooptic nonreciprocity to these structures leads to a superior optical isolation performance. In the forward propagation direction, an input TM polarized beam experiences a perfect state transfer between the input and output waveguide channels while surface Bloch oscillations block the backward transmission between the same ports. Our analysis indicates a large isolation ratio of 75 dB after a propagation distance of 8 mm inside seven coupled waveguides. Moreover, we demonstrate that, a judicious choice of the nonreciprocity in this same geometry can lead to perfect polarization splitting. 
BibTeX:
@ARTICLE{ElEiLe13__,
author = {ElGanainy, R. and Eisfeld, A. and Levy, Miguel and Christodoulides,
D. N.},
title = {Onchip nonreciprocal optical devices based on quantum inspired
photonic lattices},
journal = {Applied Physics Letters},
year = {2013},
volume = {103},
pages = {},
number = {16},
eid = {161105},
doi = {http://dx.doi.org/10.1063/1.4824895},
url = {http://scitation.aip.org/content/aip/journal/apl/103/16/10.1063/1.4824895}
}

R. ElGanainy , A. Eisfeld and D.N. Christodoulides (2013)
"Nonclassical light in coupled optical systems: anomalous power distribution, Fock space dynamics and supersymmetry” 
M. Müller, A. Paulheim, A. Eisfeld and M. Sokolowski (2013)
"Finite size line broadening and superradiance of optical transitions in two dimensional longrange ordered molecular aggregates” J. Chem. Phys. 139, 044302 
Abstract: The width and asymmetry of the line shape of the optical transition of a sample of two dimensional (2D) molecular Jaggregates was found to be related to a finitesize effect. The 2D aggregates were domains of the ordered monolayer of the fluorescent dye molecule 3,4,9,10perylenetetracarboxylic acid dianhydride on a KCl(100) surface. Fluorescence and fluorescence excitation (FLE) spectra were measured as a function of temperature. The system shows a pronounced superradiant emission which yields additional information on the number of coherently coupled molecules participating in the emission. From calculations of the spectra within the tight binding model we find that the finite size of the 2D ordered domains of about N = 7 × 7 molecules, in combination with a Poissonian domainsize distribution, explains the line profile. Line broadening mechanisms due to site disorder or thermal effects – although not excludable straightaway – are not needed to explain the observed FLE line profile. This yields insight into the important, but so far not well understood, relation between the line profile and the size of ordered molecular aggregates. 
BibTeX:
@article{m\"{u}ller:044302,
author = {Mathias M\"{u}ller and Alexander Paulheim and Alexander Eisfeld and Moritz Sokolowski},
collaboration = {},
title = {Finite size line broadening and superradiance of optical transitions in two dimensional longrange ordered molecular aggregates},
publisher = {AIP},
year = {2013},
journal = {The Journal of Chemical Physics},
volume = {139},
number = {4},
eid = {044302},
numpages = {9},
pages = {044302},
keywords = {aggregates (materials); dyes; fluorescence; Poisson distribution; spectral line broadening; superradiance; tightbinding calculations},
url = {http://link.aip.org/link/?JCP/139/044302/1},
doi = {10.1063/1.4813521}
}

S.M. Vlaming, V.A. Malyshev, A. Eisfeld and J. Knoester
(2013)
"Subdiffusive exciton motion in systems with heavytailed disorder" J. Chem. Phys 138, 214316 
Abstract: We study the transport of collective excitations (Frenkel excitons) in systems with static disorder in the transition energies, not limiting ourselves to Gaussian transition energy distributions. Instead, we generalize this model to the wider class of Lévy stable distributions, characterized by heavy tails. Phononassisted scattering of excitons, localized by the disorder, leads to thermally activated exciton motion. The time evolution of the second moment of the exciton distribution is shown to be sublinear, thus indicating that the exciton dynamics in such systems is not diffusive, but rather subdiffusive instead. The heavier the tail in the transition energy distribution is, the larger are the deviations from the diffusive regime. This from fluctuations of site energies larger than the exciton band width (outliers). We show that the occurrence of subdiffusive transport for heavytailed disorder distributions can be understood from the scattering rate distributions, which possess a (second) peak at zero scattering rate. 
BibTeX:
@article{vlaming:214316,
author = {S. M. Vlaming and V. A. Malyshev and A. Eisfeld and J. Knoester},
collaboration = {},
title = {Subdiffusive exciton motion in systems with heavytailed disorder},
publisher = {AIP},
year = {2013},
journal = {The Journal of Chemical Physics},
volume = {138},
number = {21},
eid = {214316},
numpages = {10},
pages = {214316},
keywords = {diffusion; excited states; fluctuations; Gaussian distribution; phononexciton interactions},
url = {http://link.aip.org/link/?JCP/138/214316/1},
doi = {10.1063/1.4808155}
}

S. Möbius, M. Genkin, A. Eisfeld S. Wüster, and J.M. Rost (2013)
"An optically resolvable Schrödinger's cat from Rydberg dressed cold atom clouds” Phys. Rev. A 87, 051602(R) 
Abstract: In Rydberg dressed ultracold gases, groundstate atoms inherit properties of a weakly admixed Rydberg state, such as sensitivity to longrange interactions. We show that through hyperfinestatedependent interactions, a pair of atom clouds can evolve into a spin and subsequently into a spatial mesoscopic superposition state: The pair is in a coherent superposition of two configurations, with cloud locations separated by micrometers. The mesoscopic nature of the state can be proven with absorption imaging, while the coherence can be revealed though recombination and interference of the split wave packets. 
BibTeX:
@article{PhysRevA.87.051602,
title = {Entangling distant atom clouds through Rydberg dressing},
author = {M\"obius, S. and Genkin, M. and Eisfeld, A. and W\"uster, S. and Rost, J. M.},
journal = {Phys. Rev. A},
volume = {87},
issue = {5},
pages = {051602},
numpages = {5},
year = {2013},
month = {May},
doi = {10.1103/PhysRevA.87.051602},
url = {http://link.aps.org/doi/10.1103/PhysRevA.87.051602},
publisher = {American Physical Society}
}

S. Möbius, M. Genkin, S. Wüster, A. Eisfeld and J.M. Rost (2013)
"Break up of Rydberg superatoms via dipoledipole interactions” PRA 88, 012716 
Abstract: We investigate resonant dipoledipole interactions between two superatoms of different angular momentum, consisting of two Rydbergblockaded atom clouds where each cloud carries initially a coherently shared single excitation. We demonstrate that the dipoledipole interaction breaks up the superatoms by removing the excitations from the clouds. The dynamics is akin to an ensemble average over systems where only one atom per cloud participates in entangled motion and excitation transfer. Our findings should thus facilitate the experimental realization of adiabatic exciton transport in Rydberg systems by replacing single sites with atom clouds. 
BibTeX:
@ARTICLE{MGeW13_012716_,
author = {M\"obius, S. and Genkin, M. and W\"uster, S. and Eisfeld, A. and
Rost, J. M.},
title = {Breakup of Rydbergblockaded atom clouds via dipoledipole interactions},
journal = {Phys. Rev. A},
year = {2013},
volume = {88},
pages = {012716},
month = {Jul},
doi = {10.1103/PhysRevA.88.012716},
publisher = {American Physical Society},
url = {http://link.aps.org/doi/10.1103/PhysRevA.88.012716}
}

S.K. Saikin, A. Eisfeld, S. Valleau and A. AspuruGuzik (2013)
"Photonics meets excitonics: natural and artificial molecular aggregates” Nanophotonics 2, 21 
Abstract: Organic molecules store the energy of absorbed light in the form of chargeneutral molecular excitations  Frenkel excitons. Usually, in amorphous organic materials, excitons are viewed as quasiparticles, localized on single molecules, which diffuse randomly through the structure. However, the picture of incoherent hopping is not applicable to some classes of molecular aggregates  assemblies of molecules that have strong near field interaction between electronic excitations in the individual subunits. Molecular aggregates can be found in nature, in photosynthetic complexes of plants and bacteria, and they also can be produced artificially in various forms including quasione dimensional chains, twodimensional films, tubes, etc. In these structures light is absorbed collectively by many molecules and the following dynamics of molecular excitation possesses coherent properties. This energy transfer mechanism, mediated by the coherent exciton dynamics, resembles the propagation of electromagnetic waves through a structured medium on the nanometer scale. The absorbed energy can be transferred resonantly over distances of hundreds of nanometers before exciton relaxation occurs. Furthermore, the spatial and energetic landscape of molecular aggregates can enable the funneling of the exciton energy to a small number of molecules either within or outside the aggregate. In this review we establish a bridge between the fields of photonics and excitonics by describing the present understanding of exciton dynamics in molecular aggregates. 
BibTeX:
@ARTICLE{SaEiVa13_21_,
author = {Saikin, Semion K. and Eisfeld, Alexander and Stéphanie Valleau and
Alán AspuruGuzik},
title = {Photonics meets excitonics: natural and artificial molecular aggregates},
year = {2013},
volume = {2},
pages = {21},
booktitle = {Nanophotonics},
issn = {21928614},
url = {http://www.degruyter.com/view/j/nanoph.2013.2.issue1/nanoph20120025/nanoph20120025.xml}
}

A. Eisfeld, S.M. Vlaming, S. Möbius, V.A. Malyshev and
J. Knoester (2012) "Reply: Excitons in molecular aggregates with Levytype disorder: Anomalous localization and exchange broadening of optical spectra" Phys. Rev. Lett. 109, 259702 
Abstract: The authors of the preceding Comment questioned our statement regarding the unconventional disorder scalings of the HWHM and the nonuniversality of the localization length (LL) distribution reported by us our Letter. We attributed these findings to originate from segmentation caused by the occurrence of outliers. We demonstrate here that our conclusions are correct and that the criticism of the Comment is a consequence of focusing on small values of the disorder strength. 
BibTeX:
@article{PhysRevLett.109.259702, title = {Eisfeld \textit{et~al.} Reply:}, author = {Eisfeld, A. and Vlaming, S. M. and M\"obius, S. and Malyshev, V. A. and Knoester, J.}, journal = {Phys. Rev. Lett.}, volume = {109}, issue = {25}, pages = {259702}, numpages = {1}, year = {2012}, month = {Dec}, doi = {10.1103/PhysRevLett.109.259702}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.109.259702}, publisher = {American Physical Society} } 
S. Valleau, A. Eisfeld and A. AspuruGuzik (2012)
"On the alternatives for bath correlators and spectral densities from mixed quantumclassical simulations” J. Chem. Phys. 137, 224103 
Abstract: We investigate on the procedure of extracting a “spectral density” from mixed QM/MM calculations and employing it in open quantum systems models. In particular, we study the connection between the energy gap correlation function extracted from ground state QM/MM and the bath spectral density used as input in open quantum system approaches. We introduce a simple model which can give intuition on when the ground state QM/MM propagation will give the correct energy gap. We also discuss the role of higher order correlators of the energygap fluctuations which can provide useful information on the bath. Further, various semiclassical corrections to the spectral density, are applied and investigated. Finally, we apply our considerations to the photosynthetic FennaMatthewsOlson complex. For this system, our results suggest the use of the Harmonic prefactor for the spectral density rather than the Standard one, which was employed in the simulations of the system carried out to date. [http://dx.doi.org/10.1063/1.4769079] 
BibTeX:
@article{valleau:224103,
author = {St\'{e}phanie Valleau and Alexander Eisfeld and Al\'{a}n AspuruGuzik},
collaboration = {},
title = {On the alternatives for bath correlators and spectral densities from mixed quantumclassical simulations},
publisher = {AIP},
year = {2012},
journal = {The Journal of Chemical Physics},
volume = {137},
number = {22},
eid = {224103},
numpages = {13},
pages = {224103},
keywords = {energy gap; ground states; molecular dynamics method; photosynthesis; quantum chemistry; quantum theory},
url = {http://link.aip.org/link/?JCP/137/224103/1},
doi = {10.1063/1.4769079}
}

J. Roden, W. T. Strunz, K. B. Whaley and A. Eisfeld (2012)
"Accounting for intramolecular vibrational modes in open quantum system description of molecular systems” J. Chem. Phys. 137, 204110 
Abstract: Electronicvibrational dynamics in molecular systems that interact with an environment involve a large number of degrees of freedom and are therefore often described by means of open quantum system approaches. A popular approach is to include only the electronic degrees of freedom into the system part and to couple these to a nonMarkovian bath of harmonic vibrational modes that is characterized by a spectral density. Since this bath represents both intramolecular and external vibrations, it is important to understand how to construct a spectral density that accounts for intramolecular vibrational modes that couple further to other modes. Here, we address this problem by explicitly incorporating an intramolecular vibrational mode together with the electronic degrees of freedom into the system part and using the Fano theory for a resonance coupled to a continuum to derive an “effective” bath spectral density, which describes the contribution of intramolecular modes. We compare this effective model for the intramolecular mode with the method of pseudomodes, a widely used approach in simulation of nonMarkovian dynamics. We clarify the difference between these two approaches and demonstrate that the respective resulting dynamics and optical spectra can be very different. 
BibTeX:
@ARTICLE{RoStWh12_204110_,
author = {Jan Roden and Walter T. Strunz and K. Birgitta Whaley and Alexander
Eisfeld},
title = {Accounting for intramolecular vibrational modes in open quantum
system description of molecular systems},
journal = {The Journal of Chemical Physics},
year = {2012},
volume = {137},
pages = {204110},
number = {20},
eid = {204110},
abstract = {Electronicvibrational dynamics in molecular systems that interact
with an environment involve a large number of degrees of freedom
and are therefore often described by means of open quantum system
approaches. A popular approach is to include only the electronic
degrees of freedom into the system part and to couple these to a
nonMarkovian bath of harmonic vibrational modes that is characterized
by a spectral density. Since this bath represents both intramolecular
and external vibrations, it is important to understand how to construct
a spectral density that accounts for intramolecular vibrational
modes that couple further to other modes. Here, we address this problem
by explicitly incorporating an intramolecular vibrational mode together
with the electronic degrees of freedom into the system part and using
the Fano theory for a resonance coupled to a continuum to derive
an “effective” bath spectral density, which describes the contribution
of intramolecular modes. We compare this effective model for the
intramolecular mode with the method of pseudomodes, a widely used
approach in simulation of nonMarkovian dynamics. We clarify the
difference between these two approaches and demonstrate that the
respective resulting dynamics and optical spectra can be very different.},
doi = {10.1063/1.4765329},
keywords = {Fourier transform spectra; infrared spectra; intramolecular forces;
resonant states; vibrational states},
numpages = {11},
publisher = {AIP},
url = {http://link.aip.org/link/?JCP/137/204110/1}
}

S. Mostame, P. Rebentrost, A. Eisfeld, A. J. Kerman, D. I. Tsomokos, A. AspuruGuzik (2012)
"Quantum simulator of an open quantum system using superconducting qubits: exciton transport in photosynthetic complexes” New J. Phys. 14 105013 
Abstract: Open quantum system approaches are widely used in the description of physical, chemical and biological systems. A famous example is electronic excitation transfer in the initial stage of photosynthesis, where harvested energy is transferred with remarkably high efficiency to a reaction center. This transport is affected by the motion of a structured vibrational environment, which makes simulations on a classical computer very demanding. Here we propose an analog quantum simulator of complex open system dynamics with a precisely engineered quantum environment. Our setup is based on superconducting circuits, a well established technology. As an example, we demonstrate that it is feasible to simulate exciton transport in the Fenna–Matthews–Olson photosynthetic complex. Our approach allows for a controllable singlemolecule simulation and the investigation of energy transfer pathways as well as nonMarkovian noisecorrelation effects. 
BibTeX: @ARTICLE{MoReEi12_105013_, author = {Mostame, Sarah and Patrick Rebentrost and Alexander Eisfeld and Andrew J Kerman and Dimitris I Tsomokos and Al¡n AspuruGuzik}, title = {Quantum simulator of an open quantum system using superconducting qubits: exciton transport in photosynthetic complexes}, journal = {New Journal of Physics}, year = {2012}, volume = {14}, pages = {105013}, number = {10}, abstract = {Open quantum system approaches are widely used in the description of physical, chemical and biological systems. A famous example is electronic excitation transfer in the initial stage of photosynthesis, where harvested energy is transferred with remarkably high efficiency to a reaction center. This transport is affected by the motion of a structured vibrational environment, which makes simulations on a classical computer very demanding. Here we propose an analog quantum simulator of complex open system dynamics with a precisely engineered quantum environment. Our setup is based on superconducting circuits, a well established technology. As an example, we demonstrate that it is feasible to simulate exciton transport in the FennaMatthewsOlson photosynthetic complex. Our approach allows for a controllable singlemolecule simulation and the investigation of energy transfer pathways as well as nonMarkovian noisecorrelation effects.}, url = {http://stacks.iop.org/13672630/14/i=10/a=105013} } 
Croy, A. and Eisfeld, A. (2012)
"Dynamics of a singleelectron motor EPL (Europhys Lett) 98, 68004 
Abstract: We investigate theoretically the dynamics and the charge transport properties of a rodshaped nanoscale rotor, which is driven by a similar mechanism as the nanomechanical singleelectron transistor (NEMSET) . We show that a static electric potential gradient can lead to selfexcitation of oscillatory or continuous rotational motion. We identify the relevant parameters of the device and study the dependence of the dynamics on these parameters. We discuss how the dynamics are related to the measured current through the device. Notably, in the oscillatory regime we find a negative differential conductance. The currentvoltage characteristics can be used to infer details of the surrounding environment which is responsible for damping. 
BibTeX:
@ARTICLE{CrEi12_68004_,
author = {Croy, A. and Eisfeld, A.},
title = {Dynamics of a nanoscale rotor driven by singleelectron tunneling},
journal = {EPL (Europhysics Letters)},
year = {2012},
volume = {98},
pages = {68004},
number = {6},
abstract = {We investigate theoretically the dynamics and the charge transport
properties of a rodshaped nanoscale rotor, which is driven by a
similar mechanism as the nanomechanical singleelectron transistor
(NEMSET) . We show that a static electric potential gradient can
lead to selfexcitation of oscillatory or continuous rotational motion.
We identify the relevant parameters of the device and study the dependence
of the dynamics on these parameters. We discuss how the dynamics
are related to the measured current through the device. Notably,
in the oscillatory regime we find a negative differential conductance.
The currentvoltage characteristics can be used to infer details
of the surrounding environment which is responsible for damping.}
}

Eisfeld, A. and Briggs, J.S. (2012)
"A classical master equation for excitonic transport under the influence of an environment" Phys Rev E 85, 046118 
Abstract: In a previous paper [Phys. Rev. E 83, 051911 (2011)] we have shown that the results of a quantummechanical calculation of electronic energy transfer (EET) over aggregates of coupled monomers can be described also by a model of interacting classical electric dipoles in a weakcoupling approximation, which we referred to as the realistic coupling approximation (RCA). The method was illustrated by EET on a simple linear chain of molecules and also by energy transfer on an arrangement of monomers corresponding to that of the FennaMatthewsOlson (FMO) complex relevant for photosynthesis. The study was limited to electronic degrees of freedom, since this is the origin of coherent EET in the quantum case. Nevertheless, more realistic models of EET require the inclusion of the decohering effects of coupling to an environment, when the molecular aggregate becomes an open quantum system. Here we consider the quantum description of EET on a linear chain and on the FMO complex, incorporating environment coupling and constructing the classical version of the same systems in the density matrix formalism. The close agreement of the exact quantum and exact classical results in the RCA is demonstrated and justiﬁed analytically in the RCA. This lends further support to the conclusion that the coherence properties of EET in the FMO complex are evident at the classical level and should not be ascribed as solely due to quantum effects. 
BibTeX:
@ARTICLE{EiBr12_046118_, author = {Eisfeld, Alexander and Briggs, John S.}, title = {Classical master equation for excitonic transport under the influence of an environment}, journal = {Phys. Rev. E}, year = {2012}, volume = {85}, pages = {046118}, month = {Apr}, doi = {10.1103/PhysRevE.85.046118}, issue = {4}, numpages = {8}, publisher = {American Physical Society}, url = {http://link.aps.org/doi/10.1103/PhysRevE.85.046118} } 
Briggs, J.S. and Eisfeld, A. (2012)
"Coherent Wavefunctions from Classical Amplitudes" Phys Rev A 85, 052111 
Abstract: In the first days of quantum mechanics Dirac pointed out an analogy between the timedependent coefficients of an expansion of the Schrödinger equation and the classical position and momentum variables solving Hamilton’s equations. Here it is shown that the analogy can be made an equivalence in that, in principle, systems of classical oscillators can be constructed whose position and momenta variables form timedependent amplitudes which are identical to the complex quantum amplitudes of the coupled wave function of an Nlevel quantum system with real coupling matrix elements. Hence classical motion can reproduce quantum coherence. 
BibTeX:
@ARTICLE{BrEi12_052111_, author = {Briggs, John S. and Eisfeld, Alexander}, title = {Coherent quantum states from classical oscillator amplitudes}, journal = {Phys. Rev. A}, year = {2012}, volume = {85}, pages = {052111}, month = {May}, doi = {10.1103/PhysRevA.85.052111}, file = {BrEi12_052111_PhysRevA.85.052111.pdf:PRA/BrEi12_052111_PhysRevA.85.052111.pdf:PDF}, issue = {5}, numpages = {10}, publisher = {American Physical Society}, timestamp = {2012.05.16}, url = {http://link.aps.org/doi/10.1103/PhysRevA.85.052111} } 
Seibt, J. and Eisfeld, A. (2012)
"Intermolecular torsional motion of a piaggregated dimer probed by twodimensional spectroscopy” J. Chem. Phys. 136, 024109 
Abstract: The energetic splitting of the two exciton states of a molecular dimer depends strongly on the relative orientation of the monomers with respect to each other. The curvature of the corresponding adiabatic potential energy surfaces can lead to torsional motion of the monomers. It has been suggested recently that this torsional motion could provide a possible relaxation mechanism for the upper state which proceeds via a crossing of the two singly excited state potentials. Another, competing, relaxation mechanism is provided by coupling to the environment, leading to direct exciton relaxation. Here we examine theoretically the combined dynamics of torsional motion and excitonic relaxation for a π aggregated dimer. Using twodimensional (2D) spectroscopy, it is shown how torsional motion through a crossing of the adiabatic excitonic potential surfaces could be distinguished from direct relaxation. For the calculations a mixed quantum/classical approach is used, where the torsional motion is treated by an Ehrenfest type of equation, while the excitonic dynamics including dephasing and direct relaxation is described by a quantum master equation. 
BibTeX:
@ARTICLE{SeEi12_024109_, author = {Seibt, Joachim and Eisfeld, Alexander}, title = {Intermolecular torsional motion of a piaggregated dimer probed by twodimensional electronic spectroscopy}, journal = {Journal of Chemical Physics}, year = {2012}, volume = {136}, pages = {024109}, number = {2}, eid = {024109}, doi = {10.1063/1.3674993}, keywords = {excited states; hyperfine structure; intermolecular mechanics; potential energy surfaces; torsion; twodimensional spectroscopy}, numpages = {15}, url = {http://link.aip.org/link/?JCP/136/024109/1} } 
Ritschel, G., Roden, J., Strunz, W.T. AspuruGuzik, A. and Eisfeld, A. (2011)
"Absence of Quantum Oscillations and the Choice of Site Energies in Electronic Excitation Transfer in the FennaMatthewsOlson Trimer” J. Phys. Chem. Lett. 2, 2912 
Abstract: Energy transfer in the photosynthetic FennaMatthewsOlson (FMO) complex of green sulfur bacteria is studied numerically taking all three subunits (monomers) of the FMO trimer and the recently found eighth bacteriochlorophyll (BChl) molecule into account. The coupling to the nonMarkovian environment is treated with a master equation derived from nonMarkovian quantum state diffusion. When the excitedstate dynamics is initialized at site eight, which is believed to play an important role in receiving excitation from the main light harvesting antenna, we see a slow exponentiallike decay of the excitation. This is in contrast to the oscillations and a relatively fast transfer that usually occurs when initialization at sites 1 or 6 is considered. We show that different sets of electronic transition energies can lead to large differences in the transfer dynamics and may cause additional suppression or enhancement of oscillations. 
BibTeX:
@ARTICLE{RiRoSt11_2912_,
author = {Ritschel, Gerhard and Roden, Jan and Strunz, Walter T. and AspuruGuzik,
Al\'{a}¡n and Eisfeld, Alexander},
title = {Absence of Quantum Oscillations and Dependence on Site Energies in
Electronic Excitation Transfer in the FennaMatthewsOlson Trimer},
journal = {The Journal of Physical Chemistry Letters},
year = {2011},
volume = {2},
pages = {29122917},
number = {22},
abstract = {Energy transfer in the photosynthetic FennaMatthewsOlson (FMO) complex
of green sulfur bacteria is studied numerically taking all three
subunits (monomers) of the FMO trimer and the recently found eighth
bacteriochlorophyll (BChl) molecule into account. The coupling to
the nonMarkovian environment is treated with a master equation derived
from nonMarkovian quantum state diffusion. When the excitedstate
dynamics is initialized at site eight, which is believed to play
an important role in receiving excitation from the main light harvesting
antenna, we see a slow exponentiallike decay of the excitation.
This is in contrast to the oscillations and a relatively fast transfer
that usually occurs when initialization at sites 1 or 6 is considered.
We show that different sets of electronic transition energies can
lead to large differences in the transfer dynamics and may cause
additional suppression or enhancement of oscillations.},
doi = {10.1021/jz201119j},
eprint = {http://pubs.acs.org/doi/pdf/10.1021/jz201119j},
url = {http://pubs.acs.org/doi/abs/10.1021/jz201119j}
}

Ritschel, G., Roden, J., Strunz, W.T. and Eisfeld, A. (2011)
"An efficient method to calculate excitation energy transfer in light harvesting systems. Application to the FMO complex." New J. Phys. 13, 113034 
Abstract: A master equation, derived from the nonMarkovian quantum state diﬀusion (NMQSD), is used to calculate excitation energy transfer in the photosynthetic FennaMatthewsOlson (FMO) pigmentprotein complex at various temperatures. This approach allows us to treat spectral densities that contain explicitly the coupling to internal vibrational modes of the chromophores. Moreover, the method is very eﬃcient, with the result that the transfer dynamics can be calculated within about one minute on a standard PC, making systematic investigations w.r.t. parameter variations tractable. After demonstrating that our approach is able to reproduce the results of the numerically exact hierarchical equations of motion (HEOM) approach, we show how the inclusion of vibrational modes inﬂuences the transfer. 
BibTeX:
@ARTICLE{RiRoSt11_113034_,
author = {Gerhard Ritschel and Jan Roden and Walter T Strunz and Alexander
Eisfeld},
title = {An efficient method to calculate excitation energy transfer in lightharvesting
systems: application to the FennaMatthewsOlson complex},
journal = {New Journal of Physics},
year = {2011},
volume = {13},
pages = {113034},
number = {11},
abstract = {A master equation derived from nonMarkovian quantum state diffusion
is used to calculate the excitation energy transfer in the photosynthetic
FennaMatthewsOlson pigmentprotein complex at various temperatures.
This approach allows us to treat spectral densities that explicitly
contain the coupling to internal vibrational modes of the chromophores.
Moreover, the method is very efficient and as a result the transfer
dynamics can be calculated within about 1 min on a standard PC, making
systematic investigations w.r.t. parameter variations tractable.
After demonstrating that our approach is able to reproduce the results
of the numerically exact hierarchical equations of motion approach,
we show how the inclusion of vibrational modes influences the transfer.},
url = {http://stacks.iop.org/13672630/13/i=11/a=113034}
}

Möbius, S., Wüster, S., Ates, C., Eisfeld, A. and Rost, J.M. (2011)
”Adiabatic entanglement transport in Rydberg aggregates” J.Phys.B 44, 184011 
Abstract: We consider the interplay between excitonic and atomic motion in a regular, flexible chain of Rydberg atoms, extending our recent results on entanglement transport in Rydberg chains (Wuester et al 2010 Phys. Rev. Lett. 105 053004). In such a Rydberg chain, similar to molecular aggregates, an electronic excitation is delocalized due to longrange dipoledipole interactions among the atoms. The transport of an exciton that is initially trapped by a chain dislocation is strongly coupled to nuclear dynamics, forming a localized pulse of combined excitation and displacement. This pulse transfers entanglement between dislocated atoms adiabatically along the chain. Details about the interaction and the preparation of the initial state are discussed. We also present evidence that the quantum dynamics of this complex manybody problem can be accurately described by selected quantumâclassical methods, which greatly simplify investigations of excitation transport in flexible chains. 
BibTeX:
@ARTICLE{MoeWueAt11_184011_, author = {Möbius, S. and Wüster, S. and Ates, C. and Eisfeld, A. and Rost, J.M.}, title = {Adiabatic entanglement transport in Rydberg aggregates}, journal = {Journal of Physics B: Atomic, Molecular and Optical Physics}, year = {2011}, volume = {44}, pages = {184011}, number = {18}, abstract = {We consider the interplay between excitonic and atomic motion in a regular, flexible chain of Rydberg atoms, extending our recent results on entanglement transport in Rydberg chains (Wuester et al 2010 Phys. Rev. Lett. 105 053004). In such a Rydberg chain, similar to molecular aggregates, an electronic excitation is delocalized due to longrange dipoledipole interactions among the atoms. The transport of an exciton that is initially trapped by a chain dislocation is strongly coupled to nuclear dynamics, forming a localized pulse of combined excitation and displacement. This pulse transfers entanglement between dislocated atoms adiabatically along the chain. Details about the interaction and the preparation of the initial state are discussed. We also present evidence that the quantum dynamics of this complex manybody problem can be accurately described by selected quantumâclassical methods, which greatly simplify investigations of excitation transport in flexible chains.}, url = {http://stacks.iop.org/09534075/44/i=18/a=184011} } 
Wüster, S., Ates, C., Eisfeld, A., and Rost, J.M. (2011)
"Excitation transport through Rydberg dressing" New J Phys 13, 073044 
Abstract: We show how to create longrange interactions between alkali atoms in different hyperfine ground states, with the goal of coherent quantum transport. The scheme uses offresonant dressing with atomic Rydberg states. We demonstrate coherent migration of electronic excitation through dressed dipoledipole interaction by full solutions of models with four essential states per atom and give the structure of the spectrum of dressed states for a dimer. In addition, we present an effective (perturbative) Hamiltonian for the groundstate manifold and show that it correctly describes the full multistate dynamics. We discuss excitation transport in detail for a chain of five atoms. In the presented scheme, the actual population in the Rydberg state is kept small. Dressing offers many advantages over the direct use of Rydberg levels: it reduces ionization probabilities and provides an additional tuning parameter for lifetimes and interaction strengths. 
BibTeX:
@ARTICLE{WAtEi11_073044_, author = {Wüster, S. and Ates, C. and Eisfeld, A. and Rost, J.M.}, title = {Excitation transport through Rydberg dressing}, journal = {New Journal of Physics}, year = {2011}, volume = {13}, pages = {073044}, number = {7}, abstract = {We show how to create longrange interactions between alkali atoms in different hyperfine ground states, with the goal of coherent quantum transport. The scheme uses offresonant dressing with atomic Rydberg states. We demonstrate coherent migration of electronic excitation through dressed dipoledipole interaction by full solutions of models with four essential states per atom and give the structure of the spectrum of dressed states for a dimer. In addition, we present an effective (perturbative) Hamiltonian for the groundstate manifold and show that it correctly describes the full multistate dynamics. We discuss excitation transport in detail for a chain of five atoms. In the presented scheme, the actual population in the Rydberg state is kept small. Dressing offers many advantages over the direct use of Rydberg levels: it reduces ionization probabilities and provides an additional tuning parameter for lifetimes and interaction strengths.}, url = {http://stacks.iop.org/13672630/13/i=7/a=073044} } 
Briggs, J.S. and Eisfeld, A. (2011) "Equivalence of quantum and classical coherence in electronic energy transfer" Phys. Rev. E 83, 051911 
Abstract: 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 dipoledipole 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 FennaMatthewsOlson complex. 
BibTeX:
@ARTICLE{BrEi11_051911_, author = {Briggs, John S. and Eisfeld, Alexander }, title = {Equivalence of quantum and classical coherence in electronic energy transfer}, journal = {Phys. Rev. E}, year = {2011}, volume = {83}, pages = {051911}, number = {5}, month = {May}, abstract = {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 dipoledipole 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 FennaMatthewsOlson complex.}, doi = {10.1103/PhysRevE.83.051911} numpages = {4}, publisher = {American Physical Society} } 
Eisfeld, A., Schulz, G. and Briggs, J.S. (2011) "The influence of geometry on the vibronic spectra of quantum aggregates" J. Lumin. 131, 2555 
Abstract: A study is presented of the localisation of excitonic states on extended molecular aggregates composed of identical monomers arising, not from disorder due to statistical energy shifts of the monomers, induced by environmental interactions (Anderson localisation), but rather simply due to changes in the orientation and geometrical arrangement of the transition dipoles. It is shown further that such small changes nevertheless can have a drastic effect on the shape of the vibronic spectrum of the aggregate. The vibronic spectra are calculated using the [`]coherent exciton scattering' (CES) approximation whose derivation we generalise to be applicable to aggregates of arbitrary size and geometry. 
BibTeX:
@ARTICLE{EiScBr11_2555_, author = {Eisfeld, Alexander and Schulz, Georg and Briggs, John}, title = {The influence of geometry on the vibronic spectra of quantum aggregates}, journal = {Journal of Luminescence}, year = {2011}, volume = {131}, pages = {2555  2564}, number = {12}, abstract = {A study is presented of the localisation of excitonic states on extended molecular aggregates composed of identical monomers arising, not from disorder due to statistical energy shifts of the monomers, induced by environmental interactions (Anderson localisation), but rather simply due to changes in the orientation and geometrical arrangement of the transition dipoles. It is shown further that such small changes nevertheless can have a drastic effect on the shape of the vibronic spectrum of the aggregate. The vibronic spectra are calculated using the [`]coherent exciton scattering' (CES) approximation whose derivation we generalise to be applicable to aggregates of arbitrary size and geometry.}, doi = {DOI: 10.1016/j.jlumin.2011.06.043}, issn = {00222313}, keywords = {Quantum aggregates}, url = {http://www.sciencedirect.com/science/article/pii/S0022231311003917} } 
Wüster, S., Eisfeld, A. and Rost, J.M. (2011) "Conical intersections in an ultracold gas" Phys. Rev. Lett 106, 153002 
Abstract: We find that energy surfaces of more than two atoms or molecules interacting via transition dipoledipole potentials 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 phenomena may be experimentally accessible if the trimer is realized by light alkali atoms in a ring trap, whose interactions are induced by offresonant dressing with Rydberg states. Such a setup promises a direct probe of the full manybody density dynamics near a CI. 
BibTeX:
@ARTICLE{WEiRo11_153002_, author = {Wüster, S. and Eisfeld, A. and Rost, J.M.}, title = {Conical intersections in an ultracold gas}, journal = {Phys. Rev. Lett.}, year = {2011}, volume = {106}, pages = {153002}, abstract = {We find that energy surfaces of more than two atoms or molecules interacting via transition dipoledipole potentials 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 phenomena may be experimentally accessible if the trimer is realized by light alkali atoms in a ring trap, whose interactions are induced by offresonant dressing with Rydberg states. Such a setup promises a direct probe of the full manybody density dynamics near a CI.}, doi = {10.1103/PhysRevLett.106.153002}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.106.153002} } 
Eisfeld, A. (2011) "Phase directed excitonic transport and its limitations due to environmental influence" Chem. Phys. 379, 33 
Abstract: We investigate theoretically the transfer of excitation along a one dimensional chain of monomers for a situation in which initially the excitation is coherently shared by two monomers. We show that depending on the relative phase between the two monomers strong directionality of the energy transfer is possible. It is also investigated how dephasing, induced by an environment, influences this directed transport. 
BibTeX:
@ARTICLE{Ei11_33_, author = {Eisfeld, Alexander }, title = {Phase directed excitonic transport and its limitations due to environmental influence}, journal = {Chemical Physics}, year = {2011}, volume = {379}, pages = {33  38}, number = {13}, abstract = {We investigate theoretically the transfer of excitation along a one dimensional chain of monomers for a situation in which initially the excitation is shared coherently by two monomers. We show that depending on the relative phase between the two monomers strong directionality of the energy transfer is possible. It is investigated how dephasing, induced by an environment, influences this directed transport.}, doi = {DOI: 10.1016/j.chemphys.2010.10.013}, url = {http://www.sciencedirect.com/science/article/B6TFM51D16211/2/3e9e9604882672eaa085582b18744e63} } 
Roden, J., Eisfeld, A., Dvorak, M., Bünermann, O. &
Stienkemeier, F. (2011) "Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets" J. Chem. Phys. 134, 054907 
Abstract: Oligomers of the organic semiconductor PTCDA are studied by means of helium nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasi continuous spectra, resembling the broad oligomer spectra. 
BibTeX:
@ARTICLE{RoEiDv11_054907_, author = {Roden, Jan and Eisfeld, Alexander and Dvo\V{r}\'ak, Matthieu and Bünermann, Oliver and Stienkemeier, Frank}, title = {Vibronic line shapes of {PTCDA} oligomers in helium nanodroplets}, journal = {Journal of Chemical Physics}, year = {2011}, volume = {134}, pages = {054907}, number = {5}, eid = {054907}, abstract = {Oligomers of the organic semiconductor 3,4,9,10perylenetetracarboxylicdianhydride, C24H8O6 (PTCDA) are studied by means of helium nanodroplet isolation spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasicontinuous spectra, resembling the broad oligomer spectra.}, doi = {10.1063/1.3526749}, keywords = {drops; excitons; helium; organic semiconductors; spectral line breadth; vibrational modes; vibrational states; vibronic states}, numpages = {12}, publisher = {AIP} url = {http://link.aip.org/link/?JCP/134/054907/1} } 
Genkin, M. and Eisfeld (2011) "Robustness of spatial Penningtrap modes against environmentassisted entanglement" J. Phys. B 44, 035502 
Abstract: The separability of the spatial modes of a charged particle in a Penning trap in the presence of an environment is studied by means of the positive partial transpose criterion. Assuming a weak Markovian environment, described by linear Lindblad operators, our results strongly suggest that the environmental coupling of the axial and cyclotron degrees of freedom does not lead to entanglement at experimentally realistic temperatures. We therefore argue that, apart from unavoidable decoherence, the presence of such an environment does not alter the effectiveness of recently suggested quantum information protocols in Penning traps, which are based on the combination of a spatial mode with the spin of the particle. 
BibTeX:
@ARTICLE{GeEi11_035502_, author = {Genkin, M. and Eisfeld, A.}, title = {Robustness of spatial Penningtrap modes against environmentassisted entanglement}, journal = {J. Phys. B}, year = {2011}, volume = {44}, pages = {035502}, number = {3}, abstract = {The separability of the spatial modes of a charged particle in a Penning trap in the presence of an environment is studied by means of the positive partial transpose criterion. Assuming a weak Markovian environment, described by linear Lindblad operators, our results strongly suggest that the environmental coupling of the axial and cyclotron degrees of freedom does not lead to entanglement at experimentally realistic temperatures. We therefore argue that, apart from unavoidable decoherence, the presence of such an environment does not alter the effectiveness of recently suggested quantum information protocols in Penning traps, which are based on the combination of a spatial mode with the spin of the particle.}, doi = {10.1088/09534075/44/3/035502}, url = {http://stacks.iop.org/09534075/44/i=3/a=035502} } 
Roden, J., Strunz, W. T. & Eisfeld,
A. (2011) "NonMarkovian quantum state diffusion for absorption spectra of molecular aggregates" J. Chem. Phys. 134, 034902 ; doi:10.1063/1.3512979 
Abstract: In many molecular systems one encounters the situation where electronic excitations couple to a quasicontinuum of phonon modes. That continuum may be highly structured e.g.\ due to some weakly damped high frequency modes. To handle such a situation, an approach combining the nonMarkovian quantum state diffusion (NMQSD) description of open quantum systems with an efficient but abstract approximation was recently applied to calculate energy transfer and absorption spectra of molecular aggregates~[Roden, Eisfeld, Wolff, Strunz, PRL 103 (2009) 058301]. To explore the validity of the used approximation for such complicated systems, in the present work we compare the calculated (approximative) absorption spectra with exact results. These are obtained from the method of pseudomodes, which we show to be capable of determining the exact spectra for small aggregates and a few pseudomodes. It turns out that in the cases considered, the results of the two approaches mostly agree quite well. The advantages and disadvantages of the two approaches are discussed. 
BibTeX:
@ARTICLE{RoStEi10_PM_, author = {Roden, Jan and Walter T. Strunz and Alexander Eisfeld and }, title = {NonMarkovian quantum state diffusion for absorption spectra of molecular aggregates}, journal = { J. Chem. Phys.}, year = {2011}, volume = {134}, pages = {034902}, url = {http://link.aip.org/link/?JCP/134/034902/1} doi = {10.1063/1.3512979}, } 
Roden, J., Eisfeld, A. (2011) "Anomalous strong exchange narrowing in excitonic systems" J. Chem. Phys. 134, 034901 
Abstract: We theoretically investigate the phenomenon of exchange narrowing in the absorption spectrum of a chain of monomers, that are coupled via resonant dipoledipole 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 nonMarkovian environment with a Lorentzian spectral density, we ﬁnd a narrowing of the peak width 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. 
Eisfeld, A., Vlaming, S. M., Malyshev, V. A. &
Knoester, J. (2010) "Excitons in molecular aggregates with Levytype disorder: Anomalous localization and exchange broadening of optical spectra" Phys. Rev. Lett. 105, 137402 
Abstract: 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 heavytailed 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. 
BibTeX:
@ARTICLE{EiVlMa10_137402_, author = {Eisfeld, A. and Vlaming, S. M. and Malyshev, V. A. and Knoester, J. }, title = {Excitons in Molecular Aggregates with L\'evyType Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra}, journal = {Phys. Rev. Lett.}, year = {2010}, volume = {105}, pages = {137402}, number = {13}, month = {Sep}, doi = {10.1103/PhysRevLett.105.137402}, numpages = {4}, publisher = {American Physical Society} } 
Wüster, S., Ates, C., Eisfeld, A. & Rost,
J.M. (2010) "Newton's cradle and entanglement transport in a flexible Rydberg chain" Phys. Rev. Lett. 105, 053004 
Abstract: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. 
BibTeX:
@ARTICLE{WAtEi10_053004_, author = {{Wüster, S. and Ates, C. and Eisfeld, A. and Rost, J. M.}}, title = {Newton's Cradle and Entanglement Transport in a Flexible Rydberg Chain}, journal = {Phys. Rev. Lett.}, year = {2010}, volume = {105}, pages = {053004}, number = {5}, month = {Jul}, abstract = {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.}, doi = {10.1103/PhysRevLett.105.053004}, numpages = {4}, publisher = {American Physical Society} } 
Roden, J., Strunz, W. T. & Eisfeld,
A. (2010) "Spectral properties of molecular oligomers. A nonMarkovian quantum state diffusion approach." International Journal of Modern Physics B 24, 5060 
Abstract:Absorption spectra of small molecular aggregates (oligomers) are considered. The dipoledipole interaction between the monomers leads to shifts of the oligomer spectra with respect to the monomer absorption. The lineshapes of monomer as well as oligomer absorption depend strongly on the coupling to vibrational modes. Using a recently developed approach [Roden et.~al, PRL,] we investigate the length depends of spectra of onedimensional aggregates for various values of the interaction strength between the monomers. It is demonstrated, that the present approach is well suited to describe the occurrence of the J and Hband. 
BibTeX:
@ARTICLE{RoStEi10__, author = {Roden, Jan and Walter T. Strunz and Alexander Eisfeld and }, title = {Spectral properties of molecular oligomers. A nonMarkovian quantum state diffusion approach.}, journal = {International Journal of Modern Physics B}, year = {2010}, volume = {}, pages = {}, } 
Roden, J., Eisfeld, A., Wolff, W. & Strunz, W. T. (2009) "Influence of Complex ExcitonPhonon Coupling on Optical Absorption and Energy Transfer of Quantum Aggregates" Phys. Rev. Lett.. Vol. 103, pp. 058301. 
Abstract: We present a theory that efficiently describes the quantum dynamics of an electronic excitation that is coupled to a continuous, highly structured phonon environment. Based on a stochastic approach to nonMarkovian open quantum systems, we develop a dynamical framework that allows us to handle realistic systems where a fully quantum treatment is desired yet the usual approximation schemes fail. The capability of the method is demonstrated by calculating spectra and energy transfer dynamics of mesoscopic molecular aggregates, elucidating the transition from fully coherent to incoherent transfer. 
BibTeX:
@ARTICLE{RoEiWo09_058301_, author = {Roden, Jan and Alexander Eisfeld and Wolfgang Wolff and Walter T. Strunz}, title = {Influence of Complex ExcitonPhonon Coupling on Optical Absorption and Energy Transfer of Quantum Aggregates}, journal = {Physical Review Letters}, year = {2009}, volume = {103}, pages = {058301}, number = {5}, eid = {058301}, doi = {10.1103/PhysRevLett.103.058301}, numpages = {4}, url = {http://link.aps.org/abstract/PRL/v103/e058301} } 
Roden, J., Schulz, G., Eisfeld, A. & Briggs, J. S. (2009) "Electronic energy transfer on a vibronically coupled quantum aggregate" J. Chem. Phys.. Vol. 131, pp. 044909. 
Abstract:We examine the transfer of electronic excitation (an exciton) along a chain of electronically coupled monomers possessing internal vibronic structure and which also interact with degrees of freedom of the surrounding environment. Using a combination of analytical and numerical methods, we calculate the time evolution operator or timedependent Green's function of the system and thereby isolate the physical parameters influencing the electronic excitation transport. Quite generally, we show that coupling to vibrations slows down and inhibits migration of electronic excitation due to dephasing effects on the coherent transfer present without vibrations. In particular, coupling to a continuous spectrum of environment states leads to a complete halting of transfer, i.e., a trapping of the exciton. 
BibTeX:
@ARTICLE{RoScEi09_044909_, author = {Jan Roden and Georg Schulz and Alexander Eisfeld and John Briggs}, title = {Electronic energy transfer on a vibronically coupled quantum aggregate}, journal = {The Journal of Chemical Physics}, year = {2009}, volume = {131}, pages = {044909}, number = {4}, eid = {044909}, doi = {10.1063/1.3176513}, numpages = {17}, url = {http://link.aip.org/link/?JCP/131/044909/1} } 
Eisfeld, A., Seibt, J. & Engel, V. (2008) "On the inversion of geometric parameters from absorption and circular dichroism spectroscopy of molecular dimers" Chem. Phys. Lett.. Vol. 467, pp. 186. 
Abstract: Absorption spectra of molecular dimers usually exhibit complex lineshapes. In an approximate treatment the excited dimer states are described as a twolevel system, leading to two bands separated by twice the electronic coupling element which is responsible for the splitting. The band intensities are related to the relative orientation of the monomer transition dipolemoments. We show, considering absorption and circular dichroism spectra, that extracting geometric parameters using the twolevel description can lead to substantial errors when internal monomer vibrations become relevant. The application of sumrules offers a more accurate approach to invert the data. 
BibTeX:
@article{EiSeEn08_186_, author = {Alexander Eisfeld and Joachim Seibt and Volker Engel}, title = {On the inversion of geometric parameters from absorption and circular dichroism spectroscopy of molecular dimers}, journal = {Chemical Physics Letters}, year = {2008}, volume = {467}, number = {13}, pages = {186  190}, url = {http://www.sciencedirect.com/science/article/B6TFN4TVTJS91/2/ad9763c7ff685b6d26bdc54d312dacfa}, doi = {DOI: 10.1016/j.cplett.2008.10.087} } 
Roden, J., Eisfeld, A. & Briggs, J. S. (2008) "The J and Hbands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation" Chemical Physics. Vol. 352, pp. 258. 
Abstract: The validity of the CES approximation is investigated by comparison with direct diagonalisation of a model vibronic Hamiltonian of N identical monomers interacting electronically. Even for quite short aggregates (N[greater, approximate]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 Hband region). However, previously excellent agreement of CES calculations and measured spectra in the Hband region was obtained [A. Eisfeld, J.S. Briggs, Chem. Phys. 324 (2006) 376]. This is shown to arise from use of the measured monomer spectrum which includes implicitly dissipative effects not present in the model calculation. 
BibTeX:
@article{RoEiBr08_258_, author = {Roden, J. and Eisfeld, A. and Briggs, J.S.}, title = {The J and Hbands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation}, journal = {Chemical Physics}, year = {2008}, volume = {352}, number = {13}, pages = {258266}, url = {http://www.sciencedirect.com/science/article/B6TFM4SXYG281/2/70568d84c97e9f1aabc048c27e7207d2} } 
Ates, C., Eisfeld, A. & Rost, J.M. (2008) "Motion of Rydberg atoms induced by resonant dipoledipole interactions" New Journal of Physics. Vol. 10, pp. 045030. 
Abstract: We show that nuclear motion of Rydberg atoms can be induced by resonant dipoledipole interactions that trigger the energy transfer between two energetically close Rydberg states. How and if the atoms move depends on their initial arrangement as well as on the initial electronic excitation. Using a mixed quantum/classical propagation scheme, we obtain the trajectories and kinetic energies of atoms, initially arranged in a regular chain and prepared in excitonic eigenstates. The influence of the offdiagonal disorder on the motion of the atoms is examined and it is shown that irregularity in the arrangement of the atoms can lead to an acceleration of the nuclear dynamics. 
BibTeX:
@article{Ates_Eisfeld_Rost_2008_NJoP_045030, author = {Ates, C. and Eisfeld, A. and Rost, J. M.}, title = {Motion of Rydberg atoms induced by resonant dipoledipole interactions}, journal = {New Journal of Physics}, year = {2008}, volume = {10}, number = {4}, pages = {045030}, url = {http://stacks.iop.org/13672630/10/045030} } 
Walczak, P., Eisfeld, A. & Briggs, J.S. (2008) "Exchange narrowing of the Jband of molecular dye aggregates" J. Chem. Phys.. Vol. 128, pp. 044505. 
Abstract: The exchange narrowing of the J band of certain dye monomers upon aggregation in solution has been known since the 1930s. Here, we analyze the theoretical explanations put forward to account for these narrow absorption bands. Although the theories range from models of identical monomers interacting with vibrations to the opposite of rigid monomers with statistically distributed electronic site energies, all approaches exhibit exchange narrowing. However, we show that the origins of the narrowing are different. A uniﬁed theory incorporating the two approaches is presented in which features of both narrowing mechanisms are evident. 
BibTeX:
@article{Walczak_Eisfeld_Briggs_2008_JCP_044505, author = {Walczak, P.B. and Eisfeld, A. and Briggs, J. S.}, title = {Exchange narrowing of the Jband of molecular dye aggregates}, journal = {J. Chem. Phys.}, year = {2008}, volume = {128}, pages = {044505} } 
Eisfeld, A. (2007) "A simple method to obtain information on the conformation of dipoledipole coupled dimers" Chemical Physics Letters. Vol. 445, pp. 321324. 
Abstract: A dimer is considered where the two monomers are coupled by dipoledipole interactions. Upon electronic excitation the interaction leads to a drastic change of the dimer absorption spectrum compared to that of the noninteracting monomers. The method presented here uses sum rules, to obtain information on the interaction strength between the monomers and the conformation of the dimer directly from the measured spectra. Comparison with previous results [A. Eisfeld, L. Braun, W.T. Strunz, J.S. Briggs, J. Beck, V. Engel, J. Chem. Phys. 122 (2005) 134103], obtained by diagonalisation of the dimer Hamiltonian, shows remarkable agreement. 
BibTeX:
@article{Eisfeld_2007_CPL_321, author = {Eisfeld, A.}, title = {A simple method to obtain information on the conformation of dipoledipole coupled dimers}, journal = {Chemical Physics Letters}, year = {2007}, volume = {445}, number = {46}, pages = {321324}, url = {http://www.sciencedirect.com/science/article/B6TFN4PC3SHM9/2/27088059f6e1d2e53d7738f754fddef8} } 
Eisfeld, A. & Briggs, J.S. (2007) "The Shape of the Jband of Pseudoisocyanine" Chem. Phys. Lett.. Vol. 446, pp. 354358. 
Abstract: We show that, within the coherent exciton scattering (CES) approximation, both the measured exponential bandtail and the detailed shape of the Jband region of the absorption spectrum of pseudoisocyanine aggregates can be reproduced with high accuracy, using only a single free parameter. Our numerical studies conﬁrm analytical results that the Jband width depends crucially on the monomer absorption strength in the energy region where the Jband peak occurs. 
BibTeX:
@article{Eisfeld_Briggs_2007_CPL_354, author = {Eisfeld, A. and Briggs, J. S.}, title = {The Shape of the Jband of Pseudoisocyanine}, journal = {Chem. Phys. Lett.}, year = {2007}, volume = {446}, pages = {354358}, url = {http://dx.doi.org/10.1016/j.cplett.2007.07.110}, doi = {http://dx.doi.org/10.1016/j.cplett.2007.07.110} } 
Eisfeld, A., Kniprath, R. & Briggs, J. (2007) "Theory of the absorption and circular dichroism spectra of helical molecular aggregates" J. Chem. Phys.. Vol. 126, pp. 104904. 
Abstract: A theory of the electronic circular dichroism CD and optical rotatory dispersion ORD of inﬁnite aggregates exhibiting cylindrical symmetry is presented in which, to the authors’ knowledge, for the ﬁrst time vibrational structure is included explicitly. It is shown that, with the coherent exciton scattering approximation in the Green function approach, the detailed vibrational structure of the aggregate absorption. CD and ORD bands can be calculated from a knowledge of the electronic coupling and the monomer absorption line shape alone. Detailed model calculations for a single helix are made and the results are used to expose the origin of different spectral features. A good reproduction of experimental Jaggregate spectra is obtained, using the same electronic interaction to ﬁt both absorption and CD spectral line shapes. The theory allows some prediction of aggregate geometry to be made, but it is shown that an unambiguous geometrical assignment can only be made where experimental spectra for light of different propagation directions with respect to the cylinder axis are available. 
BibTeX:
@article{Eisfeld_Kniprath_Briggs_2007_JCP_104904, author = {Eisfeld, A. and Kniprath, R. and Briggs, J.S.}, title = {Theory of the absorption and circular dichroism spectra of helical molecular aggregates}, journal = {J. Chem. Phys.}, year = {2007}, volume = {126}, pages = {104904} } 
Eisfeld, A. & Briggs, J.S. (2006) "The J and Hbands of organic dye aggregates " Chem. Phys.. Vol. 324, pp. 376384. 
Abstract: Certain molecular aggregates consisting of organic dyes are remarkable in exhibiting an intense and very narrow absorption peak, known as a Jband, which is redshifted away from the region of monomer absorption. Apart from those dyes showing the Jband on aggregation, there are also dyes where the absorption maximum is shifted to higher energies. The width of the resulting absorption band (called an Hband) is comparable to that of the monomeric dyes and shows a complicated vibrational structure. Following our analysis of the Jband spectra of polymer aggregates using the CES approximation [A. Eisfeld, J.S. Briggs, Chem. Phys. 281 (2002) 61], a theory that includes vibrations explicitly, we show that the same approximation can account for measured Hband spectra. Using simple analytical forms of the monomer spectrum the origin of the widely different shapes of H and Jbands is explained within the CES approximation. 
BibTeX:
@article{Eisfeld_Briggs_2006_CP_376, author = {A. Eisfeld and J. S. Briggs}, title = {The J and Hbands of organic dye aggregates }, journal = {Chem. Phys.}, year = {2006}, volume = {324}, pages = {376384}, doi = {http://dx.doi.org/10.1016/j.chemphys.2005.11.015} } 
Eisfeld, A. & Briggs, J.S. (2006) "Absorption Spectra of Quantum Aggregates Interacting via LongRange Forces" Phys. Rev. Lett.. Vol. 96, pp. 113003. 
Abstract: We present a simple formula by which the shape of the absorption spectrum of an aggregate of quantum "monomers" (cold atoms, molecules, quantum dots, nanoparticles, etc.) interacting via dipoledipole forces can be calculated from the averaged spectrum of the quantum monomer itself. Spectral broadening, due to a wide variety of causes, is included explicitly so that the formula is applicable not only to the idealization of a discrete spectrum but also to the practical situation of a continuously broadened spectrum. In simple cases, analytic results are obtained showing the strong dependence of the aggregate spectrum on the precise nature of the broadening of the quantum monomer spectrum. The formula is compared with results of exact diagonalization of model aggregate Hamiltonians and with experiment. 
BibTeX:
@article{Eisfeld_Briggs_2006_PRL_113003, author = {A. Eisfeld and J. S. Briggs}, title = {Absorption Spectra of Quantum Aggregates Interacting via LongRange Forces}, journal = {Phys. Rev. Lett.}, year = {2006}, volume = {96}, pages = {113003} } 
Eisfeld, A., Braun, L., Strunz, W.T., Briggs, J.S., Beck, J. & Engel, V. (2005) "Vibronic energies and spectra of molecular dimers" J. Chem. Phys.. Vol. 122, pp. 134103. 
Abstract: We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipoledipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the JaynesCummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the timedependent Schrodinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra. (C) 2005 American Institute of Physics. 
BibTeX:
@article{Eisfeld_Braun_Strunz_EtAl_2005_JCP_134103, author = {Eisfeld, A. and Braun, L. and Strunz, W. T. and Briggs, J. S. and Beck, J. and Engel, V.}, title = {Vibronic energies and spectra of molecular dimers}, journal = {J. Chem. Phys.}, year = {2005}, volume = {122}, number = {13}, pages = {134103}, url = {http://dx.doi.org/10.1063/1.1861883}, doi = {http://dx.doi.org/10.1063/1.1861883} } 
Eisfeld, A. & Briggs, J.S. (2002) "The Jband of organic dyes: lineshape and coherence length" Chem. Phys.. Vol. 281, pp. 6170. 
Abstract: Selforganised Jaggregates of dye molecules, known for over 60 years, are emerging as remarkably versatile quantum systems with applications in photography, optoelectronics, solar cells, photobiology and as supramolecular fibres. Recently there has been much effort to achieve quantum entanglement and coherence on the nanoscale in atom traps and quantum dot aggregates (for use in quantum computing). We point out that the excitonic state of the Jaggregate is a textbook case of mesoscopic quantum coherence and entanglement. The establishment of coherence can literally be seen since the dye changes colour dramatically on aggregation due to strong shifts in the absorption spectrum. Here we reproduce in a simple theory the shifts and shapes of optical absorption spectra upon aggregation to a polymer and calculate the coherence length of quantum entanglement of monomer wavefunctions. (C) 2002 Published by Elsevier Science B.V.. 
BibTeX:
@article{Eisfeld_Briggs_2002_CP_61, author = {Eisfeld, A. and Briggs, J. S.}, title = {The Jband of organic dyes: lineshape and coherence length}, journal = {Chem. Phys.}, year = {2002}, volume = {281}, number = {1}, pages = {6170} } 
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