Quantum Coherence Effects Research Articles

Spatial and Electronic Correlations in the PE545 Light-Harvesting Complex.

The recent discovery of long-lasting quantum coherence effects in photosynthetic pigment-protein complexes has challenged our view of the role that protein motions play in light-harvesting processes. Several groups have suggested that correlated fluctuations involving the pigments site energies and couplings could be at the origin of such unexpected behavior. Here we combine molecular dynamics simulations with quantum mechanics/molecular mechanics calculations to analyze the degree of correlated fluctuations in the PE545 complex of Rhodomonas sp. strain CS24. We find that correlations between the motions of the chromophores, which are significantly assisted by the water solvent, do not translate into appreciable site energy correlations but do lead to significant cross-correlations of energies and couplings. Such behavior, not observed in a recent study on the Fenna-Mathews-Olson complex, seems to provide phycobiliproteins with an additional fundamental mechanism to control quantum coherence and light-harvesting efficiency compared with chlorophyll-containing complexes.

Quantum process in living cells

Coherent quantum effects have been confirmed for several biological processes. These processes exist in the environment of a warm wet cell where decoherence can be a serious concern. Here we propose a mechanism whereby quantum coherence may extend through the water matrix of a cell. The model is based on coherent waves of established ultrafast energy transfers in water. Computations based on the model are found to agree with several experimental results and numerical and descriptive predictions are presented. We compute wave speed, ~156 km/s, and wavelength, ~9.3 nm, and determine that these waves retain local coherence. Close agreements are found for the dipole moment of water dimers, results of microwave radiation on yeast, and the Kleiber law of metabolic rates. The theory requires that a spherical cell must have a minimum diameter of ~20 nm to accommodate a standing energy wave. The quantum properties of the modelsuggest that cellular chemistry favors reactions that support perpetuation of the energy waves.

On the Physical Basis of Self-Organization

Experiments performed with the aim to explain pattern formation in plasma devices offer, as I will show in this survey, a new insight into the mechanism by which locally matter transits spontaneously from a disordered state into an ordered one. The essential news revealed by these experiments is the identification of a population of electrons that, driven at a critical distance from thermal equilibrium, is able to act as the organizer of the emergence and the survival of a complexity starting from chaos, i.e., from electric sparks the appearance of which is controlled by deterministic chaos. Supplied at a constant rate with thermal energy extracted by electrons from plasma, the complexity survives in a dynamical state performing operations in agreement with a code directly related to electrons thermal energy distribution function. Acting as a constituent of the matter, the population of electrons intrinsically controls the emergence and the survival of the complexity. Performing operations directly related to electron’s thermal energy distribution function, the complexity evolves stepwise in more advanced self-organized dynamical states, when this function is changed by an additional injection of energy. A set of nonlinear phenomena, not explainable by classical processes is involved in the mechanism by which the complexity emerges, survives and evolves. Thus, phenomena like Bose-Einstein condensation, macroscopic quantum coherence, direct and alternate Josephson effects, electron tunneling, negative differential impedance and others, potentially explain the emergence, functionality and vitality, i.e., the dynamical state of the complexity.

Quantum coherence effects in hybrid nanoparticle molecules in the presence of ultra-short dephasing times

We study quantum coherence effects in single nanoparticle systems consisting of a semiconductor quantum dot and a metallic nanoparticle in the presence of the ultra-short dephasing times of the quantum dots. The results suggest that coherent exciton-plasmon coupling can sustain the collective molecular resonances (plasmonic meta-resonances) of these systems at about room temperature. We investigate quantum optical properties of the quantum dots under this condition, demonstrating formation of ultranarrow gain and absorption spectral lines. These results are discussed in terms of plasmonic normalization of coherent population oscillation and the collective states of the nanoparticle systems.

Effects of quantum coherence in metalloprotein electron transfer

Many intramolecular electron transfer (ET) reactions in biology are mediated by metal centers in proteins. This process is commonly described by a model of diffusive hopping according to the semiclassical theories of Marcus and Hopfield. However, recent studies have raised the possibility that nontrivial quantum mechanical effects play a functioning role in certain biomolecular processes. Here, we investigate the potential effects of quantum coherence in biological ET by extending the semiclassical model to allow for the possibility of quantum coherent phenomena using a quantum master equation based on the Holstein Hamiltonian. We test the model on the structurally defined chain of seven iron-sulfur clusters in nicotinamide adenine dinucleotide plus hydrogen:ubiquinone oxidoreductase (complex I), a crucial respiratory enzyme and one of the longest chains of metal centers in biology. Using experimental parameters where possible, we find that, in limited circumstances, a small quantum mechanical contribution can provide a marked increase in the ET rate above the semiclassical diffusive-hopping rate. Under typical biological conditions, our model reduces to well-known diffusive behavior.

Tunable amplification and absorption properties in double-Λ system of GaAs/AlGaAs multiple quantum wells

We study the optical amplification and absorption properties in a double-Λ four level system of GaAs/AlGaAs multiple quantum wells (MQWs) under realistic experimental conditions. The amplification and absorption responses of two weak fields can be achieved by adjusting the relative phase, the probe detuning, and the two pump Rabi frequencies appropriately. The investigation is much more practical than its atomic counterpart because of its flexible design and the wide adjustable parameters. It may provide a new possibility in technological applications for the light amplifier working on quantum coherence effects in MQWs solid-state system.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic

Space-time resolved simulation of femtosecond nonlinear light-matter interactions using a holistic quantum atomic model : Application to near-threshold harmonics

We introduce a new computational approach for femtosecond pulse propagation in the transparency region of gases that permits full resolution in three space dimensions plus time while fully incorporating quantum coherent effects such as high-harmonic generation and strong-field ionization in a holistic fashion. This is achieved by utilizing a one-dimensional model atom with a delta-function potential which allows for a closed-form solution for the nonlinear optical response due to ground-state to continuum transitions. It side-steps evaluation of the wave function, and offers more than one hundred-fold reduction in computation time in comparison to direct solution of the atomic Schrödinger equation. To illustrate the capability of our new computational approach, we apply it to the example of near-threshold harmonic generation in Xenon, and we also present a qualitative comparison between our model and results from an in-house experiment on extreme ultraviolet generation in a femtosecond enhancement cavity.

Coherent molecular resonances in quantum dot–metallic nanoparticle systems: coherent self-renormalization and structural effects

It is known that surface-plasmon resonances of metallic nanoparticles can significantly enhance the field experienced by semiconductor quantum dots. In this paper we show that, when quantum dots are in the vicinity of metallic nanoparticles and interact with coherent light sources (laser fields), coherent exciton–plasmon coupling (quantum coherence effects) can increase the amount of the plasmonic field enhancement significantly. We also study how the coherent molecular resonances generated by such a coupling process are influenced by the self-renormalization of the plasmonic fields and the structural parameters of the systems, particularly the size and shape of the metallic nanoparticle. The renormalization process happens via mutual impacts of the radiative decay rate of excitons and the coherent exciton–plasmon coupling on each other. Our results highlight the conditions where the molecular resonances become very sharp, offering optical switching processes with high extinction ratio and wide ranging device applications.

Exactly soluble model of resonant energy transfer between molecules

F\"orster's theory of resonant energy transfer (FRET) predicts the strength and range of exciton transport between separated molecules. We introduce an exactly soluble model for FRET which reproduces F\"orster's results as well as incorporating quantum coherence effects. As an application, the model is used to analyze a system composed of quantum dots and the protein bacteriorhodopsin.

Conductance fluctuations in chaotic systems with tunnel barriers

Quantum effects are expected to disappear in the short-wavelength, semiclassical limit. As a matter of fact, recent investigations of transport through quantum chaotic systems have demonstrated the exponential suppression of the weak localization corrections to the conductance and of the Fano factor for shot noise when the Ehrenfest time ${\ensuremath{\tau}}_{E}$ exceeds the electronic dwell time ${\ensuremath{\tau}}_{D}$. On the other hand, conductance fluctuations, an effect of quantum coherence, retain their universal value in the limit ${\ensuremath{\tau}}_{E}/{\ensuremath{\tau}}_{D}\ensuremath{\rightarrow}\ensuremath{\infty}$, when the system is ideally coupled to external leads. Motivated by this intriguing result we investigate conductance fluctuations through quantum chaotic cavities coupled to external leads via (tunnel) barriers of arbitrary transparency $\ensuremath{\Gamma}$. Using the trajectory-based semiclassical theory of transport, we find that the linear ${\ensuremath{\tau}}_{E}$ dependence of the conductance variance shows a nonmonotonous, sinusoidal behavior as a function of $\ensuremath{\Gamma}$. Most notably, we find an increase of the conductance fluctuations with ${\ensuremath{\tau}}_{E}$, above their universal value, for $\ensuremath{\Gamma}\ensuremath{\lesssim}0.5$. These results, confirmed by numerical simulations, show that, contrary to common wisdom, effects of quantum coherence may increase in the semiclassical limit, under special circumstances.

Fluctuation relations for current components in mesoscopic electric circuits

We present a new class of fluctuation relations, to which we will refer as Fluctuation Relations for Current Components (FRCCs). FRCCs can be used to estimate system parameters when complete information about nonequilibrium many-body electron interactions is unavailable. We show that FRCCs are often robust in the sense that they do not depend on some basic types of electron interactions and some quantum coherence effects.

Semiclassical study of quantum coherence and isotope effects in ultrafast electron transfer reactions coupled to a proton and a phonon bath

The linearized semiclassical initial value representation is employed to describe ultrafast electron transfer processes coupled to a phonon bath and weakly coupled to a proton mode. The goal of our theoretical investigation is to understand the influence of the proton on the electronic dynamics in various bath relaxation regimes. More specifically, we study the impact of the proton on coherences and analyze if the coupling to the proton is revealed in the form of an isotope effect. This will be important in distinguishing reactions in which the proton does not undergo significant rearrangement from those in which the electron transfer is accompanied by proton transfer. Unlike other methodologies widely employed to describe nonadiabatic electron transfer, this approach treats the electronic and nuclear degrees of freedom consistently. However, due to the linearized approximation, quantum interference effects are not captured accurately. Our study shows that at small phonon bath reorganization energies, coherent oscillations and isotope effect are observed in both slow and fast bath regimes. The coherences are more substantially damped by deuterium in comparison to the proton. Further, in contrast to the dynamics of the spin-boson model, the coherences are not long-lived. At large bath reorganization energies, the decay is incoherent in the slow and fast bath regimes. In this case, the extent of the isotope effect depends on the relative relaxation timescales of the proton mode and the phonon bath. The isotope effect is magnified for baths that relax on picosecond timescales in contrast to baths that relax in femtoseconds.

Mid-infrared pump-related electric-field domains in GaAs/(Al,Ga)As quantum-cascade structures for terahertz lasing without population inversion

We investigate the effect of mid-infrared (MIR) pumping on the transport properties of GaAs/(Al,Ga)As terahertz (THz) quantum lasers (TQLs), which rely on quantum coherence effects of intersubband transitions. Aiming at THz lasing at elevated temperatures, we extend the concept of THz gain with and without population inversion of a single, MIR-pumped, electrically driven THz stage proposed by Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)] to an entire TQL. However, experiments using a CO2 as well as a free-electron laser and numerical simulations show that this resonant MIR pumping causes a negative differential conductivity (NDC) in addition to the NDC caused by sequential tunneling. Lasing of these TQLs is prevented by the formation of electric-field domains below the resonance field strength for gain of each single THz stage.

Quantum coherent effects in layered nanostructures

Magnetotransport in structures based on NbSe3, Bi2Sr2CaCu2O8 + x, and graphite has been examined. The stimulation of the charge density wave gap by a high magnetic field in NbSe3 has been detected and studied using interlayer tunneling spectroscopy. It has been shown that this stimulation is of orbital character and the effect of the Zeeman splitting of the ground state of the charge density wave on this stimulation has been analyzed. The second harmonic of the gap singularity of the charge density wave in tunneling spectra, which is associated with the cascade relaxation of quasiparticles with the excess energy 2Δ. The effects caused by the interaction between Josephson vortices and Abrikosov vortex filaments have been studied on Bi2Sr2CaCu2O8 + x mesas. Studies of the magnetoresistance of thin graphite with nanoholes reveal a significant contribution to the magnetotransport from Dirac fermions and provide experimental evidence of the existence of edge states around nanoholes.

Mesoscopic transport and interferometry with wave packets of ultracold atoms: Effects of quantum coherence and interactions

We propose a way to simulate mesoscopic transport processes with counterpropagating wave packets of ultracold atoms in quasi-one-dimensional (1D) waveguides and show quantitative agreement with analytical results. The method allows the study of a broad range of transport processes at the level of individual modes, not possible in electronic systems. Typically suppressed effects of quantum coherence become manifest, along with the effects of tunable interactions, which can be used to develop a simpler type of sensitive atom interferometer.

Renormalized perturbation theories of Anderson localization: Self‐consistent two‐particle vertices

AbstractWe study effects of quantum coherence due to scatterings of electrons on impurities on two‐particle vertex functions. We use the diagrammatic perturbation expansion in the impurity potential and analyze possible ways how to introduce renormalizations on the two‐particle level. We discuss the differences between the low‐dimensional self‐consistent theory of Vollhardt and Wölfle and the high‐dimensional one of Janiš and Kolorenč. Finally we demonstrate, using the latter approach, the emergence of the localized phase in the Falicov‐Kimball model.

Quantum coherent effect of spins in magnetic nanodots

We study a quantum coherent effect of spins in magnetic nanodots. We quantize the Heisenberg Hamiltonian in a coherent representation so as to deal with the time evolution of spin components. We observe that the number of quantum states in a nanodot is dependent on the nanodot size. For a nanodot of small size, it has very rare quantum states with a discrete energy structures. The distribution patterns of spins in these quantum states rotate about a central axis normal to the nanodot plane, and the spin reversal occurs with time. These quantum states can be achieved by a resonant excitation of a time-varying magnetic field. As a suggestion, the reversal of coherent spins may serve as a mechanism of binary information storage.

Three-coupled-quantum-well nanostructures as a source of far-infrared entangled light

A scheme is proposed to achieve the two-mode entanglement in an asymmetric semiconductor three-coupled-quantum-well (TCQW) system based on the intersubband transitions (ISBTs). In the present scheme, the TCQW structure is trapped into a doubly resonant cavity, and the required quantum coherence effects is induced by the corresponding ISBTs, which is the key of realising entanglement. By numerically simulating the dynamics of the system, we show that the entangled cavity modes with the far-infrared wavelengh can be realised in this TCQW system. The present research provides an efficient approach to achieve far-infrared entangled light in the semiconductor nanostructures, which may have significant impact on the progress of solid-state quantum information theory.

A model calculation of coherence effects in the elastic backscattering of very low energy electrons (1–20 eV) from amorphous ice

Purpose: Backscattering of very low energy electrons in thin layers of amorphous ice is known to provide experimental data for the elastic and inelastic cross sections and indicates values to be expected in liquid water. The extraction of cross sections was based on a transport analysis consistent with Monte Carlo simulation of electron trajectories. However, at electron energies below 20 eV, quantum coherence effects may be important and trajectory-based methods may be in significant error. This possibility is here investigated by calculating quantum multiple elastic scattering of electrons in a simple model of a very small, thin foil of amorphous ice.Method: The average quantum multiple elastic scattering of electrons is calculated for a large number of simulated foils, using a point-scatterer model for the water molecule and taking inelastic absorption into account. The calculation is compared with a corresponding trajectory simulation.Results: The difference between average quantum scattering and trajectory simulation at energies below about 20 eV is large, in particular in the forward scattering direction, and is found to be almost entirely due to coherence effects associated with the short-range order in the amorphous ice. For electrons backscattered at the experimental detection angle (45° relative to the surface normal) the difference is however small except at electron energies below about 10 eV.Conclusion: Although coherence effects are in general found to be strong, the mean free path values derived by trajectory-based analysis may actually be in fair agreement with the result of an analysis based on quantum scattering, at least for electron energies larger than about 10 eV.