Dynamics of coherence, localization and excitation transfer in disordered nanorings

Self-assembled supramolecular aggregates are excellent candidates for the design of efficient excitation transport devices. Both artificially prepared and natural photosynthetic aggregates in plants and bacteria present an important degree of disorder that is supposed to hinder excitation transport. Besides, molecular excitations couple to nuclear motion affecting excitation transport in a variety of ways. We present an exhaustive study of exciton dynamics in disordered nanorings with long-range interactions under the influence of a phonon bath and take the LH2 system of purple bacteria as a model. Nuclear motion is explicitly taken into account by employing the Davydov ansatz description of the polaron and quantum dynamics are obtained using a time-dependent variational method. We reveal an optimal exciton-phonon coupling that suppresses disorder-induced localization and facilitate excitation de-trapping. This excitation transfer enhancement, mediated by environmental phonons, is attributed to energy relaxation toward extended, low-energy excitons provided by the precise LH2 geometry with anti-parallel dipoles and long-range interactions. An analysis of localization and spectral statistics is followed by dynamical measures of coherence and localization, transfer efficiency and superradiance. Linear absorption, 2D photon-echo spectra and diffusion measures of the exciton are examined to monitor the diffusive behavior as a function of the strengths of disorder and exciton-phonon coupling.Comment: 18 pages, 13 figure

Optimized auxiliary oscillators for the simulation of general open quantum systems

A method for the systematic construction of few-body damped harmonic oscillator networks accurately reproducing the effect of general bosonic environments in open quantum systems is presented. Under the sole assumptions of a Gaussian environment and regardless of the system coupled to it, an algorithm to determine the parameters of an equivalent set of interacting damped oscillators obeying a Markovian quantum master equation is introduced. By choosing a suitable coupling to the system and minimizing an appropriate distance between the two-time correlation function of this effective bath and that of the target environment, the error induced in the reduced dynamics of the system is brought under rigorous control. The interactions among the effective modes provide remarkable flexibility in replicating non-Markovian effects on the system even with a small number of oscillators, and the resulting Lindblad equation may therefore be integrated at a very reasonable computational cost using standard methods for Markovian problems, even in strongly non-perturbative coupling regimes and at arbitrary temperatures including zero. We apply the method to an exactly solvable problem in order to demonstrate its accuracy, and present a study based on current research in the context of coherent transport in biological aggregates as a more realistic example of its use; performance and versatility are highlighted, and theoretical and numerical advantages over existing methods, as well as possible future improvements, are discussed.Comment: 23 + 9 pages, 11 + 2 figures. No changes from previous version except publication info and updated author affiliation

Quantum noise as a computational resource for materials science simulations

Quantum computing could eventually bring forth the possibility to simulate novel materials in physics and chemistry beyond the reach of classical computers. Nonetheless, current quantum hardware is inherently noisy, restricting the scope to minimal working examples that do not represent any computational advantage. Although noise is typically considered undesirable, recent works propose to exploit the intrinsic noise in NISQ-devices as an integral part of the algorithm. In this work, we aim to construct a toolbox that is tailored to the simulation of non-equilibrium dynamics in electronic networks. Given the ubiquity and generality of this formalism in materials science, possible applications range from ultrafast process in photovoltaics, cavity-enhanced catalysis in electrochemistry or the characterization of the noise present in quantum hardware

Nonlinear dynamics as a ground-state problem on quantum computers

For the solution of time-dependent nonlinear differential equations, we present variational quantum algorithms (VQAs) that encode both space and time in qubit registers. The spacetime encoding enables us to obtain the entire time evolution from a single ground-state computation. We describe a general procedure to construct efficient quantum circuits for the cost function evaluation required by VQAs. To mitigate the barren plateau problem during the optimization, we propose an adaptive multigrid strategy. The approach is illustrated for the nonlinear Burgers equation. We classically optimize quantum circuits to represent the desired ground-state solutions, run them on IBM Q System One and Quantinuum System Model H1, and demonstrate that current quantum computers are capable of accurately reproducing the exact results

Variational quantum eigensolver for nonlinear dynamics

The simulation of quantum systems constitutes today one of the most fruitful applications of quantum computing in the era of Noisy Intermediate-Scale Quantum (NISQ) computers. Nonetheless, other dynamical systems that are not necessarily governed by the laws of quantum mechanics remain a fundamental challenge. Several approaches have emerged regarding the integration of arbitrary Partial Differential Equations (PDEs) on quantum computers [1]. A method based on the Feynmann-Kitaev formalism of quantum dynamics, where the full evolution of the system can be retrieved after a single optimization routine of an appropriate cost function has been recently put forth [2]. This spacetime formulation alleviates the accumulation of errors, but its application is restricted to quantum systems only. In this work [3], we introduce an extension of the Feynman–Kitaev formalism that is tailored to the integration of arbitrary PDEs with non-linearities and provide proof-of-principle calculations that demonstrate that fundamental processes such as diffusion and turbulence can be well-reproduced on the IBM Q System One and Quantinuum’s H1 quantum computers. We find numerical evidence of a favorable scaling of the variational approach with the number of qubits and present several optimization strategies that avoid barren plateaus, providing a powerful toolbox for the scalable integration of large dynamical systems on NISQ hardware. [1] Lubasch, M., Joo, J., Moinier, P., Kiffner, M., & Jaksch, D., Variational quantum algorithms for nonlinear problems. Physical Review A, 101, 010301(R) (2020). [2] S. Barison, F. Vicentini, I. Cirac, and G. Carleo, “Variational dynamics as a ground-state problem on a quantum computer,” arXiv preprint arXiv:2204.03454, 2022. [3] A. Pool, A. Somoza, M. Lubasch, B. Horstmann, Proceedings - 2022 IEEE International Conference on Quantum Computing and Engineering, QCE 202

Nonlinear dynamics as a ground-state solution on quantum computers

For the solution of time-dependent nonlinear differential equations, we present variational quantum algorithms (VQAs) that encode both space and time in qubit registers. The spacetime encoding enables us to obtain the entire time evolution from a single ground-state computation. We describe a general procedure to construct efficient quantum circuits for the cost function evaluation required by VQAs. To mitigate the barren plateau problem during the optimization, we propose an adaptive multigrid strategy. The approach is illustrated for the nonlinear Burgers equation. We classically optimize quantum circuits to represent the desired ground-state solutions, run them on IBM Q System One and Quantinuum System Model H1, and demonstrate that current quantum computers are capable of accurately reproducing the exact results

Driving Force and Nonequilibrium Vibronic Dynamics in Charge Separation of Strongly Bound Electron-Hole Pairs

Electron-hole pairs in organic photovoltaics dissociate efficiently despite their Coulomb-binding energy exceeding thermal energy at room temperature. The electronic states involved in charge separation couple to structured vibrational environments containing multiple underdamped modes. The non-perturbative simulations of such large, spatially extended electronic-vibrational (vibronic) systems remains an outstanding challenge. Current methods bypass this difficulty by considering effective one-dimensional Coulomb potentials or unstructured environments. Here we extend and apply a recently developed method for the non-perturbative simulation of open quantum systems to the dynamics of charge separation in one, two and three-dimensional donor-acceptor networks. This allows us to identify the precise conditions in which underdamped vibrational motion induces efficient long-range charge separation. Our analysis provides a comprehensive picture of ultrafast charge separation by showing how different mechanisms driven either by electronic or vibronic couplings are well differentiated for a wide range of driving forces and how entropic effects become apparent in large vibronic systems. These results allow us to quantify the relative importance of electronic and vibronic contributions in organic photovoltaics and provide a toolbox for the design of efficient charge separation pathways in artificial nanostructures