Excitonic Wave Function Reconstruction from Near-Field Spectra Using Machine Learning Techniques

A general problem in quantum mechanics is the reconstruction of eigenstate wave functions from measured data. In the case of molecular aggregates, information about excitonic eigenstates is vitally important to understand their optical and transport properties. Here we show that from spatially resolved near field spectra it is possible to reconstruct the underlying delocalized aggregate eigenfunctions. Although this high-dimensional nonlinear problem defies standard numerical or analytical approaches, we have found that it can be solved using a convolutional neural network. For both one-dimensional and two-dimensional aggregates we find that the reconstruction is robust to various types of disorder and noise

Photoinduced Intra- and Intermolecular Energy Transfer in Chlorophyll a Dimer

Applying nonadiabatic excited-state molecular dynamics, we investigate excitation energy transfer and exciton localization dynamics in a chlorophyll a (Chla) dimer system at the interface of two monomers of light-harvesting complex II trimer. After its optical excitation at the red edge of the Soret (B) band, the Chla dimer experiences an ultrafast intra- and intermolecular nonradiative relaxation process to the lowest band (Qy). The energy relaxation is found to run faster in the Chla dimer than in the Chla monomer. Once the molecular system reaches the lowest Qy band composed of two lowest excited states S1 and S2, the concluding relaxation step involves the S2 → S1 population transfer, resulting in a relatively slower relaxation rate. The strength of thermal fluctuations exceeds intraband electronic coupling between the states belonging to a certain band (B, Qx, and Qy), producing localized states on individual chromophores. Therefore, time evolution of spatial electronic localization during internal conversion reveals transient trapping on one of the Chla monomers participating in the events of intermonomeric energy exchange. In the phase space domains where electronic states are strongly coupled, these states become nearly degenerate promoting Frenkel-exciton-like delocalization and interchromophore energy transfer. As energy relaxation occurs, redistribution of the transition density on two Chla monomers leads to nearly equal distribution of the exciton among the molecules. For a single Chla, our analysis of excitonic dynamics reveals wave function amplitude transfer from nitrogen and outer carbon atoms to inner carbon atoms during nonradiative relaxation.Fil: Zheng, Fulu. Nanyang Technological University; SingapurFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados UnidosFil: Zhao, Yang. Nanyang Technological University; Singapu

Dissipative dynamics in a tunable Rabi dimer with periodic harmonic driving

Recent progress on qubit manipulation allows application of periodic driving signals on qubits. In this study, a harmonic driving field is added to a Rabi dimer to engineer photon and qubit dynamics in a circuit quantum electrodynamics device. To model environmental effects, qubits in the Rabi dimer are coupled to a phonon bath with a sub-Ohmic spectral density. A non-perturbative treatment, the Dirac-Frenkel time-dependent variational principle together with the multiple Davydov D$_2$ {\it Ansatz} is employed to explore the dynamical behavior of the tunable Rabi dimer. In the absence of the phonon bath, the amplitude damping of the photon number oscillation is greatly suppressed by the driving field, and photons can be created thanks to resonances between the periodic driving field and the photon frequency. In the presence of the phonon bath, one still can change the photon numbers in two resonators, and indirectly alter the photon imbalance in the Rabi dimer by directly varying the driving signal in one qubit. It is shown that qubit states can be manipulated directly by the harmonic driving. The environment is found to strengthen the interqubit asymmetry induced by the external driving, opening up a new venue to engineer the qubit states

Engineering Photon Delocalization in a Rabi Dimer with a Dissipative Bath

A Rabi dimer is used to model a recently reported circuit quantum electrodynamics system composed of two coupled transmission-line resonators with each coupled to one qubit. In this study, a phonon bath is adopted to mimic the multimode micromechanical resonators and is coupled to the qubits in the Rabi dimer. The dynamical behavior of the composite system is studied by the Dirac-Frenkel time-dependent variational principle combined with the multiple Davydov D$_{2}$ ans\"{a}tze. Initially all the photons are pumped into the left resonator, and the two qubits are in the down state coupled with the phonon vacuum. In the strong qubit-photon coupling regime, the photon dynamics can be engineered by tuning the qubit-bath coupling strength $\alpha$ and photon delocalization is achieved by increasing $\alpha$. In the absence of dissipation, photons are localized in the initial resonator. Nevertheless, with moderate qubit-bath coupling, photons are delocalized with quasiequilibration of the photon population in two resonators at long times. In this case, high frequency bath modes are activated by interacting with depolarized qubits. For strong dissipation, photon delocalization is achieved via frequent photon-hopping within two resonators and the qubits are suppressed in their initial down state.Comment: 11 pages, 11 figure

Gaussian Process Regression for Absorption Spectra Analysis of Molecular Dimers

A common task is the determination of system parameters from spectroscopy, where one compares the experimental spectrum with calculated spectra, that depend on the desired parameters. Here we discuss an approach based on a machine learning technique, where the parameters for the numerical calculations are chosen from Gaussian Process Regression (GPR). This approach does not only quickly converge to an optimal parameter set, but in addition provides information about the complete parameter space, which allows for example to identify extended parameter regions where numerical spectra are consistent with the experimental one. We consider as example dimers of organic molecules and aim at extracting in particular the interaction between the monomers, and their mutual orientation. We find that indeed the GPR gives reliable results which are in agreement with direct calculations of these parameters using quantum chemical methods

Photon-assisted Landau-Zener transitions in a periodically driven Rabi dimer coupled to a dissipative mode

We investigate multiple photon-assisted Landau-Zener (LZ) transitions in a hybrid circuit quantum electrodynamics device in which each of two interacting transmission-line resonators is coupled to a qubit, and the qubits are driven by periodic driving fields and also coupled to a common phonon mode. The quantum state of the entire composite system is modeled using the multi-$\rm D_2$ Ansatz in combination with the time-dependent Dirac-Frenkel variational principle. Applying a sinusoidal driving field to one of the qubits, this device is an ideal platform to study the photon-assisted LZ transitions by comparing the dynamics of the two qubits. A series of interfering photon-assisted LZ transitions take place if the photon frequency is much smaller than the driving amplitude. Once the two energy scales are comparable, independent LZ transitions arise and a transition pathway is revealed using an energy diagram. It is found that both adiabatic and nonadiabatic transitions are involved in the dynamics. Used to model environmental effects on the LZ transitions, the common phonon mode coupled to the qubits allows for more available states to facilitate the LZ transitions. An analytical formula is obtained to estimate the short-time phonon population and produces results in reasonable agreement with numerical calculations. Equipped with the knowledge of the photon-assisted LZ transitions in the system, we can precisely manipulate the qubit state and successfully generate the qubit dynamics with a square-wave pattern by applying driving fields to both qubits, opening up new venues to manipulate the states of qubits and photons in quantum information devices and quantum computer

Photon-assisted Landau Zener transitions in a tunable driven Rabi dimer coupled to a micromechanical resonator

Employing the multiple Davydov D$_2$ Ansatz with the time-dependent variational principle, we have investigated photon-assisted Landau-Zener (LZ) transitions and qubit manipulation in a hybrid quantum electrodynamics device. Modelled as a Rabi dimer, the device comprises of two interacting transmission-line resonators, each coupled to a qubit. The qubits, driven by independent harmonic fields, are further modulated by a micromechanical resonator mimicked by a phonon mode. The impacts of two independent driving fields on the qubit dynamics are carefully examined. The energy diagram of the system and the photon number mobilization on the resonators are analyzed to explain the behaviour of the LZ transitions and qubit dynamics while taking into account the influence of the single phonon mode. Results show that low phonon frequencies can alter the qubit dynamics, particularly in the absence of the driving fields, {and a strong phonon coupling strength can significantly perturb the qubit dynamics thanks to a high influx of phonon energy}. Notably, only the photon frequency affects the oscillation frequency of qubit polarization. This study unveils the imperative roles that photons and phonons play in the Rabi dimer model

Schr\"{o}dinger-cat states in Landau-Zener-St\"{u}ckelberg-Majorana interferometry: a multiple Davydov Ansatz approach

Employing the time-dependent variational principle combined with the multiple Davydov $\mathrm{D}_2$ Ansatz, we investigate Landau-Zener (LZ) transitions in a qubit coupled to a photon mode with various initial photon states at zero temperature. Thanks to the multiple Davydov trial states, exact photonic dynamics taking place in the course of the LZ transition is also studied efficiently. With the qubit driven by a linear external field and the photon mode initialized with Schr\"odinger-cat states, asymptotic behavior of the transition probability beyond the rotating-wave approximation is uncovered for a variety of initial states. Using a sinusoidal external driving field, we also explore the photon-assisted dynamics of Landau-Zener-St\"{u}ckelberg-Majorana interferometry. Transition pathways involving multiple energy levels are unveiled by analyzing the photon dynamics.Comment: 25 pages, 11 figure

Excitation energy transfer in photosynthetic systems

As one of the most significant natural processes providing food and energy for almost all life on the Earth, photosynthesis starts with sunlight absorption by specialized light harvesting complexes (LHCs) and the excitation energy is then transferred with nearly perfect efficiency to reaction centers (RCs). The extraordinary energy transfer efficiency in natural photosynthesis has been attracting increasing attention. Despite that extensive experimental and theoretical efforts have been devoted to studies of the excitation energy transfer in photosynthesis, the mystery of the extremely high energy transfer rate has not yet been well understood. An in-depth understanding of the underlying mechanisms for the energy transfer in photosynthesis can benefit the design of artificial photosynthetic devices. To this end, the current thesis conducts a systematic study of excitation energy transfer in various photosynthetic systems by employing hybrid theoretical methodologies. Electronic structures and pigment-environment interactions are key factors that determine the energy transfer in photosynthetic complexes. Compared to extensive investigations of such quantities for LHCs, relatively few studies have been proceeded to obtain these properties of RC complexes in which charge separation occurs. In order to fill in this gap, quantum chemistry calculations combined with molecular dynamics simulations are applied in this thesis to evaluate static quantities of the RC complex from purple bacteria Thermochromatium tepidum, including excitation energies, excitonic coupling, and spectral densities of the pigments. Effects of protein environments on the electronic structures are taken into account by treating atoms surrounding the pigments as point charges, producing reliable site energies of the RC pigments. In addition to helping interpret asymmetric charge separation pathways in the RCs, comprehensive electronic structures and spectral densities constructed in this thesis can be used in future explorations of dynamical processes in the RCs. Beyond static property calculations, simulations of energy transfer in different photosynthetic aggregates are then carried out in this thesis. In the preliminary stage, a relatively small complex, a chlorophyll (Chl) a dimer is modeled as the target system to investigate the competitive intra- and inter-chromophore energy transfer by employing the nonadiabatic excited-state molecular dynamics (NA-ESMD). The real time energy relaxation at an atomic level is monitored, and relaxation rates in the Chla dimer are obtained within the NA-ESMD framework. Thanks to the level splitting induced by the excitonic coupling, the overall energy relaxation in the Chla dimer is faster than that in the monomeric Chla. The NA-ESMD trajectories are also utilized to reveal energy relaxation pathways. The electronic transition density is applied to visualize the exciton redistribution on the pigments upon photoexcitation, disclosing detailed intra- and inter-molecular energy transfer mechanisms in natural photosynthetic systems. In addition to energy transfer in the Chla dimer, exciton diffusion in large-scale artificial B850 nanoarrays is simulated using the Dirac-Frenkel time-dependent variational principle combined with the Davydov trial state, aiming to study the excitation energy transfer in realistic photosynthetic systems typically composed of hundreds of pigments. An efficient program is developed and implemented on the state-of-the-art graphic processor units (GPUs). The excellent scaling properties of GPU architectures facilitate fully quantum mechanical simulations of energy transfer in huge systems. From coherent exciton-phonon dynamics in one-dimensional and two-dimensional nanoarrays, exciton delocalization is scrutinized by analyzing the coherence length and the superradiance enhancement factor. The mean square displacement is measured to characterize the exciton diffusion behavior, and a superdiffusive component is found in the exciton propagation. In summary, this thesis presents a comprehensive investigation of static as well as dynamics properties in natural and engineered photosynthetic systems by employing various theoretical methodologies, including the combined quantum chemistry calculations and classical molecular dynamics simulations, the mixed quantum-classical dynamics, and the fully quantum mechanical modelling. It is expected that findings obtained in this thesis provide advantageous insights on energy transfer processes in photosynthetic complexes and guiding principles on the design of manufactured photosynthetic devices.Doctor of Philosophy (MSE

Optimal Energy Transfer in Light-Harvesting Systems

Photosynthesis is one of the most essential biological processes in which specialized pigment-protein complexes absorb solar photons, and with a remarkably high efficiency, guide the photo-induced excitation energy toward the reaction center to subsequently trigger its conversion to chemical energy. In this work, we review the principles of optimal energy transfer in various natural and artificial light harvesting systems. We begin by presenting the guiding principles for optimizing the energy transfer efficiency in systems connected to dissipative environments, with particular attention paid to the potential role of quantum coherence in light harvesting systems. We will comment briefly on photo-protective mechanisms in natural systems that ensure optimal functionality under varying ambient conditions. For completeness, we will also present an overview of the charge separation and electron transfer pathways in reaction centers. Finally, recent theoretical and experimental progress on excitation energy transfer, charge separation, and charge transport in artificial light harvesting systems is delineated, with organic solar cells taken as prime examples