Ultrafast energy transfer is used to transmit electronic excitation among the many molecules in photosynthetic antenna complexes. Recent experiments and theories have highlighted the role of coherent transfer in femtosecond studies of these proteins, suggesting the need for accurate dynamical models to capture the subtle characteristics of energy transfer mechanisms. Here we discuss how to think about coherence in light harvesting and electronic energy transfer. We review the various fundamental concepts of coherence, spanning from classical phenomena to the quantum superposition, and define coherence in electronic energy transfer. We describe the current status of experimental studies on light-harvesting complexes. Insights into the microscopic process are presented to highlight how and why this is a challenging problem to elucidate. We present an overview of the applicable dynamical theories to model energy transfer in the intermediate coupling regime.

A vibronic-exciton model is applied to investigate the recently proposed mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom. We study a dimer system to elucidate the role of resonance coupling, site energies, vibrational frequency and energy disorder in the enhancement of vibronic-exciton and ground-state vibrational coherences, and to identify regimes where this enhancement is significant. For a heterodimer representing two coupled bachteriochloropylls of the FMO complex, long-lived vibronic coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic energy difference. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that, due to the dephasing of the former, both type of coherences have a similar magnitude at longer population time.

Quantum speed limits set an upper bound to the rate at which a quantum system can evolve. Adopting a phase-space approach we explore quantum speed limits across the quantum to classical transition and identify equivalent bounds in the classical world. As a result, and contrary to common belief, we show that speed limits exist for both quantum and classical systems. As in the quantum domain, classical speed limits are set by a given norm of the generator of time evolution.The multi-faceted nature of time makes its treatment challenging in the quantum world [1,2]. Nonetheless, the understanding of time-energy uncertainty relations is somewhat privileged [3,4]. To a great extent, this is due to their reformulation in terms of quantum speed limits (QSL) concerning the ability to distinguish two quantum states connected via time evolution. While QSL provide fundamental constraints to the pace at which quantum systems can change, a plethora of applications have been found that well extend beyond the realm of quantum dynamics. Indeed, QSL provide limits to the computational capability of physical devices [5], the performance of quantum thermal machines in finite-time thermodynamics [6,7], parameter estimation in quantum metrology [8,9], quantum control [10][11][12][13][14], the decay of unstable quantum systems [15][16][17][18] and information scrambling [19], among other examples [3,4,20].Specifically, QSL are derived as upper bounds to the rate of change of the fidelity F(τ) = | ψ 0 |ψ τ | 2 ∈ [0, 1] between an initial quantum state |ψ 0 and the corresponding time-evolving state |ψ τ =Û(τ, 0)|ψ 0 , whereÛ(τ, 0) is the time-evolution operator. More generally quantum states need not be pure, and given two density matrices ρ 0 and ρ τ =Û(τ, 0)ρ 0Û (τ, 0) † the fidelity readsThe fidelity is useful to define a metric between quantum states in Hilbert space, known as the Bures angle, [24,25] This gives a geometric interpretation of speed limit as the minimum time required to sweep out the angle L (ρ 0 , ρ τ ) under a given dynamics [26]. For unitary processes, two seminal results are known. The Mandelstam-Tamm bound estimates the speed of evolution in terms of the energy dispersion of the initial state [15, 16, 21-23, 25, 27]. Its original derivation relies on the Heisenberg uncertainty relation. The second seminal result is named after Margolus and Levitin, and provides an upper bound to the speed of evolution in term of the difference between the mean energy and the ground state energy [28,29]. Its original derivation relies on the study of the survival amplitude ψ 0 |ψ τ . These bounds can be extended to driven and open quantum systems [30][31][32][33][34][35]. In addition, the two bounds can be unified [29] so that the time of evolution τ required to sweep an angle L (ρ 0 , ρ τ ) is lower bounded bywhere E 0 is the ground state of the system, E is its mean energy, and ∆E denotes the energy dispersion. Note however that there is an infinite family of bounds in terms of higher order moments of the energy of the system [3...

Friction in quantum thermodynamics results from fast driving schemes that generate nonadiabatic excitations.

We introduce a scheme for the quantum simulation of many-body decoherence based on the unitary evolution of a stochastic Hamiltonian. Modulating the strength of the interactions with stochastic processes, we show that the noise-averaged density matrix simulates an effectively open dynamics governed by k-body Lindblad operators. Markovian dynamics can be accessed with white-noise fluctuations; nonMarkovian dynamics requires colored noise. The time scale governing the fidelity decay under many-body decoherence is shown to scale as N −2k with the system size N. Our proposal can be readily implemented in a variety of quantum platforms including optical lattices, superconducting circuits, and trapped ions. DOI: 10.1103/PhysRevLett.118.140403 Understanding the nonequilibrium dynamics of a quantum system embedded in an environment is a longstanding problem at the core of the foundations of physics. Environmentally induced decoherence paves the way to the emergence of classical reality from a quantum substrate. The decoherence program and its extensions such as quantum Darwinism are focused on it [1]. The open quantum dynamics of a system is as well of relevance to quantum technologies.While it is often desirable to beat decoherence and dissipation by suppressing system-environment interactions [2,3], new paradigms have emerged that fully embrace this coupling. To date, a variety of approaches have been put forward to simulate the reduced dynamics of an open quantum system [4][5][6], including the engineering of quantum jump operators via digital quantum simulation [7,8], or encoding the role of the environment in an auxiliary qubit [4,9]. Important instances also include dissipative state preparation and quantum computation [10][11][12][13][14][15]. Recent efforts focus on the possibility of engineering the environment to which the system is coupled [14,16,17], which provides new avenues for quantum simulation of exotic phases of quantum matter [4][5][6]. Engineering of artificial baths is also motivated by the need to compute thermal averages in a variety of fields ranging from statistical mechanics [18,19] to machine learning [20]. Further applications include the characterization and quantification of quantum non-Markovian behavior [21] and its experimental detection [22]. As an alternative, one can resort to a unitary quantum circuit [23], e.g., in combination with measurement of multitime correlation functions [24], for which efficient quantum algorithms have been developed [25].In this Letter, we introduce a versatile scheme for the quantum simulation of the open dynamics of a many-body system embedded in an environment to which it couples via many-body interactions. The open-system dynamics is simulated in another, more controllable experimental platform, by adding appropriate classical noise processes. Our scheme exploits current technologies for digital and analog quantum simulation of unitary dynamics, and can be readily implemented in various experimental platforms such as trapped ions, superconducting ci...