Peter Nalbach

Enhanced quantum entanglement in the non-Markovian dynamics of biomolecular excitons

We show that quantum coherence of biomolecular excitons is maintained over exceedingly long times due to the constructive role of their non-Markovian protein-solvent environment. Using a numerically exact approach, we demonstrate that a slow quantum bath helps to sustain quantum entanglement of two pairs of Forster coupled excitons, in contrast to a Markovian environment. We consider the cross-over from a fast to a slow bath and from weak to strong dissipation and show that a slow bath can generate robust entanglement. This persists to surprisingly high temperatures, even higher than the excitonic gap and is absent for a Markovian bath.

Quantum coherent biomolecular energy transfer with spatially correlated fluctuations

We show that the quantum coherent transfer of excitations between biomolecular chromophores is strongly influenced by spatial correlations of environmental fluctuations. The latter are due either to propagating environmental modes or to local fluctuations with a finite localization length. A simple toy model of a single donor?acceptor pair with spatially separated chromophore sites allows one to investigate the influence of these spatial correlations on quantum coherent excitation transfer. The sound velocity of the solvent determines the wavelength of the environmental modes, which, in turn, has to be compared to the spatial distance of the chromophore sites. When the wavelength exceeds the distance between donor and acceptor sites, we find a strong suppression of decoherence. In addition, we consider two spatially separated donor?acceptor pairs under the influence of propagating environmental modes. Depending on their wavelengths fixed by the sound velocity of the solvent material, the spatial range of correlations may extend over typical interpair distances, which can lead to an increase in the decohering influence of the solvent. Surprisingly, this effect is counteracted by increasing temperature.

Iterative path-integral algorithm versus cumulant time-nonlocal master equation approach for dissipative biomolecular exciton transport

We determine the real-time quantum dynamics of a biomolecular donor–acceptor system in order to describe excitonic energy transfer in the presence of slow environmental Gaussian fluctuations. For this, we compare two different approaches. On the one hand, we use the numerically exact iterative quasi-adiabatic propagator path-integral scheme that incorporates all non-Markovian contributions. On the other, we apply the second-order cumulant time-nonlocal quantum master equation that includes non-Markovian effects. We show that both approaches yield coinciding results in the relevant crossover regime from weak to strong electronic couplings, displaying coherent as well as incoherent transitions.

Multiphonon transitions in the biomolecular energy transfer dynamics

We show that the biomolecular exciton dynamics under the influence of slow polarization fluctuations in the solvent cannot be described by lowest order, one-phonon approaches which are perturbative in the system-bath coupling. Instead, nonperturbative multiphonon transitions induced by the slow bath yield significant contributions. This is shown by comparing results for the decoherence rate of the exciton dynamics of a resumed perturbation theory with numerically exact real-time path-integral data. The exact decoherence rate for realistically slow solvent environments is significantly modified by multiphonon processes even in the weak coupling regime, while a one-phonon description is satisfactory only for fast environmental noise. Slow environments inhibit bath modes that are resonant with the exciton dynamics, thereby suppressing one-phonon transitions and enhancing multiphonon processes, which are typically not captured by lowest order perturbative treatments, such as Redfield or Lindblad approaches, even in more refined variants.

Landau-Zener transitions in a dissipative environment: numerically exact results.

We study Landau-Zener transitions in a dissipative environment by means of the numerically exact quasiadiabatic propagator path integral. It allows to cover the full range of the involved parameters. We discover a nonmonotonic dependence of the transition probability on the sweep velocity which is explained in terms of a simple phenomenological model. This feature, not captured by perturbative approaches, results from a nontrivial competition between relaxation and the external sweep.

Two-Dimensional Electronic Spectroscopy of Light-Harvesting Complex II at Ambient Temperature: A Joint Experimental and Theoretical Study

We have performed broad-band two-dimensional (2D) electronic spectroscopy of light-harvesting complex II (LHCII) at ambient temperature. We found that electronic dephasing occurs within ∼60 fs and inhomogeneous broadening is approximately 120 cm(-1). A three-dimensional global fit analysis allows us to identify several time scales in the dynamics of the 2D spectra ranging from 100 fs to ∼10 ps and to uncover the energy-transfer pathways in LHCII. In particular, the energy transfer between the chlorophyll b and chlorophyll a pools occurs within ∼1.1 ps. Retrieved 2D decay-associated spectra also uncover the spectral positions of corresponding diagonal peaks in the 2D spectra. Residuals in the decay traces exhibit periodic modulations with different oscillation periods. However, only one of them can be associated with the excitonic cross-peaks in the 2D spectrum, while the remaining ones are presumably of vibrational origin. For the interpretation of the spectroscopic data, we have applied a refined exciton model for LHCII. It reproduces the linear absorption, circular dichroism, and 2D spectra at different waiting times. Several components of the energy transport are revealed from theoretical simulations that agree well with the experimental observations.

The role of discrete molecular modes in the coherent exciton dynamics in FMO

We investigate the question of whether discrete molecular modes can protect quantum coherence in electronic exciton dynamics. We present numerically exact results for the electronic energy transfer dynamics in the Fenna?Matthews?Olson (FMO) molecular aggregate. In particular, we determine its single excitation subspace dynamics within an open quantum dynamics approach based on input parameters from an atomistic classical molecular dynamics modelling for FMO by Olbrich et al (2011a J. Chem. Phys. B 115 8609; 2011b J.?Chem. Phys. Lett. 2 1771). The fluctuational spectra obtained there exhibit several discrete molecular vibrational modes and a surprisingly strong continuous background induced by the solvent. The latter alone causes overdamped excitonic dynamics, although its absolute strength might be somewhat overestimated. Nevertheless, it allows us to address the principal question of whether the discrete vibrational modes are strong enough to induce additional quantum coherent exciton dynamics which would be overdamped without them. We find that including them yields only negligible effects. By this, we can rule out a scenario of quantum coherence in the FMO exciton dynamics protected by the discrete molecular mode. Only when the dynamics is coherent already without the discrete modes, its coherence might be additionally enhanced.

Ultraslow quantum dynamics in a sub-Ohmic heat bath

Freiburg Institute for Advanced Studies (FRIAS), School of Soft Matter Research,Albert-Ludwigs-Universita¨t Freiburg, Albertstr. 19, 79104 Freiburg(Dated: May 13, 2010)We show that the low-frequency modes of a sub-Ohmic bosonic heat bath generate an effectivedynamical asymmetry for an intrinsically symmetric quantum spin−1/2. An initially fully polar-ized spin first decays towards a quasiequilibrium determined by the dynamical asymmetry, therebyshowing coherent damped oscillations on the (fast) time scale of the spin splitting. On top of this,the dynamical asymmetry itself decays on an ultraslow time scale and vanishes asymptotically sincethe global equilibrium phase is symmetric. We quantitatively study the nature of the initial fastdecay to the quasiequilibrium and discuss the features of ultraslow dynamics of the quasiequilibriumitself. The dynamical asymmetry is more pronounced for smaller values of the sub-Ohmic exponentand for lower temperatures, which emphasizes the quantum many-body nature of the effect. Thesymmetry breaking is related to the dynamic crossover between coherent and overdamped relax-ation of the spin polarization and is not connected to the localization quantum phase transition.In addition to this delocalized phase, we identify a novel phase which is characterized by dampedcoherent oscillations in the localized phase. This allows for a sketch of the zero-temperature phasediagram of the sub-Ohmic spin-boson model with four distinct phases.

Memory effects in amorphous solids below 20 mK.

At temperatures below 1 K, the capacitance of a glass sample changes due to the application of a dc field in accordance with Burins dipole gap theory [J. Low Temp. Phys. 100, 309 (1995)]]. However, we now report that below 20 mK, during the first sweep cycle of the dc electric field, the capacitance is smaller by about 10(-5) compared to any subsequent sweep. Despite this overall shift, the field dependence follows the dipole gap predictions. In a subsequent sweep to higher dc fields the dielectric constant drops by about 10(-5) as soon as the applied field is higher than any field previously applied. A picture involving the dynamics of resonant pairs provides a qualitative description of this behavior.

On the origin of oscillations in two-dimensional spectra of excitonically-coupled molecular systems

We investigate an artificial molecular dimer made of two dipole coupled cyanine dye monomers in which a strong coherent coupling between electronic and vibrational degrees of freedom arises. Clear signatures of this coupling are reflected in an oscillatory time evolution of the off-diagonal vibronic cross peaks in the two-dimensional optical photon echo spectrum. We find a strong coherence component damped by fast electronic dephasing ( fs) accompanied by a much weaker component which decays on the longer time scales (ps) associated to vibrational dephasing. We find that vibronic coupling does not cause longer dephasing times of the dominant photo echo component but additional weak but long-lived components emerge.