Controlling Coherence Transfer of Dimer Interacting with Independent Environments

We study the coherence transfer between two pigments (donor and acceptor) of a dimer interacting with two independent environments. By means of a prior weak measurement on the donor and a post measurement, which is either a reversal measurement or a weak measurement, on the acceptor, we present a scheme to optimally control the transfer of the donor's coherence to the acceptor. We construct explicit relationships for the two measure-ment strengths and the evolution time, by which the coherence degree of the acceptor can approach the maximum value 0.5 at the cost of a decreased probability.     

Two-dimensional spectroscopy can distinguish between decoherence and dephasing of zero-quantum coherences

Recent experiments on a variety of photosynthetic antenna systems have revealed that coherences among electronic states persist longer than previously anticipated. In an ensemble measurement, the observed dephasing of a coherent state can occur because of either disorder across the ensemble or decoherence from interactions with the bath. Distinguishing how much such disorder affects the experimentally observed dephasing rate is paramount for understanding the role that quantum coherence may play in energy transfer through these complexes. Here, we show that two-dimensional electronic spectra can distinguish between the limiting cases of homogeneous dephasing (decoherence) and inhomogeneous dephasing by examining how the quantum beat frequency changes within a cross peak. For the antenna complex LH2 isolated from Rhodobacter sphaeroides , we find that dephasing of the coherence between the B850 and B800 rings arises predominantly from inhomogeneity. In contrast, within the Fenna-Matthews-Olson (FMO) complex from Chlorobium tepidum , dephasing of the coherence between the first two excitons appears quite homogeneous. Thus, the observed dephasing rate sets an upper bound on decoherence for the LH2 complex while establishing both an upper and lower bound for the FMO complex.     

Interferometric coherence transfer modulations in triply vibrationally enhanced four-wave mixing

Triply vibrationally enhanced (TRIVE) four-wave mixing (FWM) spectroscopy in a mixed frequency/time domain experiment contains new output coherences that isolate nonlinear pathways that involve coherence transfer. Coherence transfer occurs when a thermal bath induces coupling between two states so a quantum mechanical entanglement of a pair of quantum states evolves to entangle a new pair of quantum states. The FWM includes several equivalent coherence pathways that interfere and create a temporal modulation of the output coherence that is a signature of coherence transfer. The transfer shifts the output coherence frequency and isolates coherence transfer pathways from the stronger FWM processes that form the basis of coherent multidimensional spectroscopy. The use of coherence transfer offers the opportunity for another form of coherent multidimensional spectroscopy where cross-peaks appear because of the coherence transfer between quantum states. Since this approach is based on frequency domain methods, it requires only short-term phase coherence during the excitation process so the method is not constrained to accessing the quantum states lying within the excitation pulse bandwidth.     

Mixed frequency/time domain optical analogues of heteronuclear multidimensional NMR

Ultrafast spectroscopy is dominated by time domain methods such as pump-probe and, more recently, 2D-IR spectroscopies. In this paper, we demonstrate that a mixed frequency/time domain ultrafast four wave mixing (FWM) approach not only provides similar capabilities, but it also provides optical analogues of multiple- and zero-quantum heteronuclear nuclear magnetic resonance (NMR). The method requires phase coherence between the excitation pulses only over the dephasing time of the coherences. It uses twelve coherence pathways that include four with populations, four with zero-quantum coherences, and four with double-quantum coherences. Each pathway provides different capabilities. The population pathways correspond to those of two-dimensional (2D) time domain spectroscopies, while the double- and zero-quantum coherence pathways access the coherent dynamics of coupled quantum states. The three spectral and two temporal dimensions enable the isolation and characterization of the spectral correlations between different vibrational and/or electronic states, coherence and population relaxation rates, and coupling strengths. Quantum-level interference between the direct and free-induction decay components gives a spectral resolution that exceeds that of the excitation pulses. Appropriate parameter choices allow isolation of individual coherence pathways. The mixed frequency/time domain approach allows one to access any set of quantum states with coherent multidimensional spectroscopy.     

Effects of NaCl on the J-aggregation of two thiacarbocyanine dyes in aqueous solutions

The effects of NaCl on the aggregation of two typical thiacarbocyanine dyes (3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-phenyl-thiacarbocyanine triethyl ammonium salt (Dye 1) and 3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-methyl-thiacarbocyanine triethyl ammonium salt (Dye 2)) in aqueous solution have been studied by using absorption spectroscopy, fluorescence spectroscopy, and 1H- and 23Na-NMR measurements. It is found that the J-aggregation of two dyes can be promoted by the addition of NaCl and that the effective coherence length of the J-aggregate is shorter than that obtained without NaCl. Fluorescence spectra demonstrate that the fluorescence intensities of the J-aggregates of two dyes are quenched by addition of NaCl. This is consistent with the decrease of the effective coherence length of J-aggregates of the two dyes in the presence of NaCl. 1H-NMR spectra of two dyes show that the Na(+) ions penetrate into the J-aggregates and replace the counterion (triethylammonium ions) in two dyes. The measurements of the chemical shifts of 23Na nuclei provide further information about the interaction between the Na(+) ions and dye anions in the J-aggregates of the two dyes. Due to this interaction, the electrostatic repulsion between the dye anions in the J-aggregates can be reduced and thus accelerate the aggregation of the two dyes in the presence of NaCl. The apparent association constants between Na(+) ions and dye molecules in J-aggregates of Dye 1 and Dye 2 estimated from the measured chemical shifts of 23Na nuclei are about 2.38 M(-1) and 1.35 M(-1), respectively.     

Coherent spectroscopy in dissipative media: time-domain studies of channel phase and signal interferometry

We extend a recently formulated coherence spectroscopy of dissipative media [J. Chem. Phys. 122, 084502 (2005)] from the stationary excitation limit to the time domain. Our results are based on analytical and numerical solutions of the quantum Liouville equation within the Bloch framework. It is shown that the short pulse introduces a new, controllable time scale that allows better insight into the relation between the coherence signal and the phase properties of the material system. We point to the relation between the time-domain coherence spectroscopy and the method of interferometric two-photon photoemission spectroscopy, and propose a variant of the latter method, where the two time-delayed excitation pathways are distinguishable, rather than identical. In particular, we show that distinguishability of the two excitation pathways introduces the new possibility of disentangling decoherence from population relaxation.     

Ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate excited states in molecules: Population and vibrational coherence transfers

Results of a theoretical study of ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate π-electronic excited states of molecules were presented. Analytical expressions for temporal behaviors of population and vibrational coherence were derived using a simplified model to clarify the quantum mechanical interferences between the two coherently excited electronic states, which appeared in the nuclear wavepacket simulations [M. Kanno, H. Kono, Y. Fujimura, S.H. Lin, Phys. Rev. Lett 104 (2010) 108302]. The photon-polarization direction of the linearly polarized laser, which controls the populations of the two quasi-degenerate electronic states, determines constructive or destructive interference. Features of the vibrational coherence transfer between the two coupled quasi-electronic states through nonadiabatic couplings are also presented. Information on both the transition frequency and nonadiabatic coupling matrix element between the two states can be obtained by analyzing signals of two kinds of quantum beats before and after transfer through nonadiabatic coupling.     

Lasing without inversion

We present a nonlinear theory for lasing without inversion in three-level atomic systems by utilizing initial atomic coherence between two closely spaced lower levels. We discuss nondegenerate and degenerate cases for closed and open atomic systems. The equations of motion for the laser fields, and the steady state intensities of the fields are obtained. The realization of lasing without inversion is due to the absorption cancellation by the atomic coherence, i.e., the quantum interference. The quantum interference can bring about gain enhancement in the nondegenerate cases.     

Multiple quantum NMR excitation with a one-quantum hamiltonian

Excitation of multiple quantum coherence in dipolar coupled spin systems is usually accomplished with a two-quantum multiple pulse sequence which can be time reversed by means of a 90° phase shift. The application of such an excitation scheme to a spin system in thermal equilibrium excites only even orders of multiple quantum coherence. We demonstrate here time reversible pulse sequences that excite all orders of coherence by creating a pure one-quantum average hamiltonian. We also describe pulse schemes which can be used to create pure one- or two-quantum average hamiltonians with variable scaling between +1 and −1. These excitation schemes are relevant to the study of spin clustering by multiple quantum NMR.     

Long-Lived Coherence Originating from Electronic-Vibrational Couplings in Light-Harvesting Complexes

We theoretically investigate the evolutions of two-dimensional, third-order, nonlinear pho-ton echo rephasing spectra with population time by using an exact numerical path integral method. It is shown that for the same system, the coherence time and relaxation time of excitonic states are short, however, if the couplings of electronic and intra-pigment vibra-tional modes are considered, the coherence time and relaxation time of this vibronic states are greatly extended. It means that the couplings between electronic and vibrational modes play important roles in keeping long-lived coherence in light-harvesting complexes. Particularly, by using the method we can fix the transition path of the energy transfer in bio-molecular systems.     

Electronic coherence effects in photosynthetic light harvesting

Photosynthetic light harvesting is a paradigmatic example for quantum effects in biology. In this work, we review studies on quantum coherence effects in the LH2 antenna complex from purple bacteria to demonstrate how quantum mechanical rules play important roles in the speedup of excitation energy transfer, the stabilization of electronic excitations, and the robustness of light harvesting in photosynthesis. Subsequently, we present our recent theoretical studies on exciton dynamical localization and excitonic coherence generation in photosynthetic systems. We apply a variational-polaron approach to investigate decoherence of exciton states induced by dynamical fluctuations due to system-environment interactions. The results indicate that the dynamical localization of photoexcitations in photosynthetic complexes is significant and imperative for a complete understanding of coherence and excitation dynamics in photosynthesis. Moreover, we use a simple model to investigate quantum coherence effects in intercomplex excitation energy transfer in natural photosynthesis, with a focus on the likelihoods of generating excitonic coherences during the process. Our model simulations reveal that excitonic coherence between acceptor exciton states and transient nonlocal quantum correlation between distant pairs of chromophores can be generated through intercomplex energy transfer. Finally, we discuss the implications of these theoretical works and important open questions that remain to be answered.     

化学振荡体系相干共振的诱导和增强

采用数值模拟的方法研究了乘性和加性噪音对化学振荡体系相干共振的影响作用. 结果表明, 乘性和加性噪音同时对体系进行调制时, 所得的相干共振强于只有其中一种噪音调制的共振; 相对于加性噪音而言, 体系能更好地利用乘性噪音来增宽使体系处于振荡状态的控制参数区域; 控制参数效应和噪音效应相比, 前者对体系相干共振的产生和增强起主要的影响作用. 另外, 当噪音增到临界强度时, 乘性和加性噪音分别对体系进行调制的动力学行为差异被湮没, 说明此时乘性和加性噪音对体系动力学行为的影响趋于一致.     

Interaction of Adjacent Amino Acids

Ramachandran plots display the dihedral angles of a single protein residue. We propose a crossed torsion angle plot called SSY‐plot between two neighboring amino acids and demonstrate that a special coherence motion can exist between some very special amino acid pairs leading to spontaneous unusual structures. A 6mer was extracted from a BBA polypeptide chain which in this plot shows two diagonal domains for the Ser‐Arg pair after some induction time. Other amino acid pairs in general do not show this kind of split domain. This shows that a special pair is required for stabilizing two distinct native structures in protein folding. We suggest that the existence of these two domains corresponds to a bifurcation between two different protein structures and that the special pair is the key to producing these two structures. These two different structures are produced spontaneously without an external agent.     

Population and coherence dynamics in light harvesting complex II (LH2)

The electronic excitation population and coherence dynamics in the chromophores of the photosynthetic light harvesting complex 2 (LH2) B850 ring from purple bacteria (Rhodopseudomonas acidophila) have been studied theoretically at both physiological and cryogenic temperatures. Similar to the well-studied Fenna-Matthews-Olson (FMO) protein, oscillations of the excitation population and coherence in the site basis are observed in LH2 by using a scaled hierarchical equation of motion approach. However, this oscillation time (300 fs) is much shorter compared to the FMO protein (650 fs) at cryogenic temperature. Both environment and high temperature are found to enhance the propagation speed of the exciton wave packet yet they shorten the coherence time and suppress the oscillation amplitude of coherence and the population. Our calculations show that a long-lived coherence between chromophore electronic excited states can exist in such a noisy biological environment.     

Coherence resonance and bi-resonance by time-periodic coupling strength in Hodgkin-Huxley neuron networks

We study the effect of time-periodic coupling strength on the spiking coherence of Newman-Watts networks of Hodgkin-Huxley(HH) neurons with non-Gaussian noise.It is found that the spiking can exhibit coherence resonance(CR) when the extent of deviation of non-Gaussian noise from Gaussian noise and the amplitude of the coupling strength are varied.In particular,coherence bi-resonance(CBR) is observed when the frequency of the coupling strength is varied,and the CBR is always observed when the frequency is equal to,or a multiple of,the spiking period,manifesting as the locking between the frequencies of the spiking and the coupling strength.The results show that a time-periodic coupling strength may play a more constructive and efficient role in enhancing the spiking coherence of the neuronal networks than a constant coupling strength.These findings provide insight into the role of time-periodic coupling strength for enhancing the time precision of information processing in neuronal networks.     

Spectral quantum beating in mixed frequency/time-domain coherent multidimensional spectroscopy

Coherent multidimensional spectroscopy performed in the mixed frequency/time domain exhibits both temporal and spectral quantum beating when two quantum states are simultaneously excited. The excitation of both quantum states can occur because either the spectral width of the states or the excitation pulse exceeds the frequency separation of the quantum states. The quantum beating appears as a line that broadens and splits into two peaks and then recombines as the time delay between excitation pulses increases. The splitting depends on the spectral width of the excitation pulses. We observe the spectral quantum beating between the two nearly degenerate asymmetric carbonyl stretch modes in a nickel tricarbonyl chelate using the nonrephasing, ground state bleaching coherence pathway in triply vibrationally enhanced four-wave mixing as the time delay between the first two excitation pulses changes.     

Signatures of correlated excitonic dynamics in two-dimensional spectroscopy of the Fenna-Matthew-Olson photosynthetic complex

Long-lived excitonic coherence in photosynthetic proteins has become an exciting area of research because it may provide design principles for enhancing the efficiency of energy transfer in a broad range of materials. In this publication, we provide new evidence that long-lived excitonic coherence in the Fenna-Mathew-Olson pigment-protein (FMO) complex is consistent with the assumption of cross correlation in the site basis, indicating that each site shares bath fluctuations. We analyze the structure and character of the beating crosspeak between the two lowest energy excitons in two-dimensional (2D) electronic spectra of the FMO Complex. To isolate this dynamic signature, we use the two-dimensional linear prediction Z-transform as a platform for filtering coherent beating signatures within 2D spectra. By separating signals into components in frequency and decay rate representations, we are able to improve resolution and isolate specific coherences. This strategy permits analysis of the shape, position, character, and phase of these features. Simulations of the crosspeak between excitons 1 and 2 in FMO under different regimes of cross correlation verify that statistically independent site fluctuations do not account for the elongation and persistence of the dynamic crosspeak. To reproduce the experimental results, we invoke near complete correlation in the fluctuations experienced by the sites associated with excitons 1 and 2. This model contradicts ab initio quantum mechanic∕molecular mechanics simulations that observe no correlation between the energies of individual sites. This contradiction suggests that a new physical model for long-lived coherence may be necessary. The data presented here details experimental results that must be reproduced for a physical model of quantum coherence in photosynthetic energy transfer.     

Crystalline self-assembly into monolayers of folded oligomers at the air-water interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 A. induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 A separation between alkyl chains, yields the conventional herringbone arrangement.     

Unified treatment of coherent and incoherent electronic energy transfer dynamics using classical electrodynamics

Recent experiments on resonance energy transfer (RET) in photosynthetic systems have found evidence of quantum coherence between the donor and the acceptor. Under these conditions, Fo?rster's theory of RET is no longer applicable and no theory of coherent RET advanced to date rivals the intuitive simplicity of Fo?rster's theory. Here, we develop a framework for understanding RET that is based on classical electrodynamics but still captures the essence of the quantum coherence between the molecules. Our theory requires only a knowledge of the complex polarizabilities of the two molecules participating in the transfer as well as the distance between them. We compare our results to quantum mechanical calculations and show that the results agree quantitatively.