Construction of Multichromophoric Spectra from Monomer Data: Applications to Resonant Energy Transfer

We develop a model that establishes a quantitative link between the physical properties of molecular aggregates and their constituent building blocks. The relation is built on the coherent potential approximation, calibrated against exact results, and proven reliable for a wide range of parameters. It provides a practical method to compute spectra and transfer rates in multichromophoric systems from experimentally accessible monomer data. Applications to Förster energy transfer reveal optimal transfer rates as functions of both the system-bath coupling and intra-aggregate coherence.National Science Foundation (U.S.) (CHE-1112825

Thermal Light as a Mixture of Sets of Pulses: the Quasi-1D Example

The relationship between thermal light and coherent pulses is of fundamental and practical interest. We now know that thermal light cannot be represented as a statistical mixture of single pulses. In this paper we ask whether or not thermal light can be represented as a statistical mixture of sets of pulses. We consider thermal light in a one-dimensional wave-guide, and find a convex decomposition into products of orthonormal coherent states of localized, nonmonochromatic modes.Comment: 6 pages and 3 figures, published versio

Quantum Speed Limits across the Quantum-to-Classical Transition

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.National Institute of General Medical Sciences (U.S.) (Grant R25GM076321

Quantum Simulation of Generic Many-Body Open System Dynamics Using Classical Noise

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; non-Markovian dynamics requires colored noise. The time scale governing the fidelity decay under many-body decoherence is shown to scale as N[superscript -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.University of Massachusetts at Boston (Project P20150000029279)Templeton FoundationSwiss National Science FoundationNational Science Foundation (U.S.) (Grant CHE-1112825

Thermofield dynamics: Quantum Chaos versus Decoherence

Quantum chaos imposes universal spectral signatures that govern the thermofield dynamics of a many-body system in isolation. The fidelity between the initial and time-evolving thermofield double states exhibits as a function of time a decay, dip, ramp and plateau. Sources of decoherence give rise to a nonunitary evolution and result in information loss. Energy dephasing gradually suppresses quantum noise fluctuations and the dip associated with spectral correlations. Decoherence further delays the appearance of the dip and shortens the span of the linear ramp associated with chaotic behavior. The interplay between signatures of quantum chaos and information loss is determined by the competition among the decoherence, dip and plateau characteristic times, as demoonstrated in the stochastic Sachdev-Ye-Kitaev model.Comment: 6+8 pages, 2+3 figure

A wrapper tube for a fuel subassembly of a nuclear reactor core and method for protecting fuel against overheating in case of coolant boiling

he present invention discloses an innovative design of the sodium-cooled fast reactor subassembly wrapper tube (Figure 1 (b)) and a method for protecting the sodium-cooled fast reactor core against overheating and melting in case of the accidents accompanied by a sodium boiling. More generally, the invention is considered to be valuable in any field of technology where a heat generating fuel, heater or the like need to be cooled sufficiently to prevent the resources involved from damage caused by overheating and/or melting. The innovation of the wrapper tube design compared to the traditional wrapper tube design of a sodium-cooled fast reactor fuel subassembly (Figure 1 (a)) consists in an introduction of openings (5). The openings are made in all faces of the wrapper tubes of all fuel subassemblies at the same axial location. The goal of the openings (5) is to provide the sodium flow bypath between the neighbouring subassemblies, in the case when the sodium vapour obstructs or fully blocks the liquid sodium flow at the top of the fuel subassembly (i.e. above the axial level of the openings). Due to this bypath, the sodium flow rate at the inlet of the subassembly, where the boiling occurs, does not significantly drop (Figure 2 (b)). As a result, the sodium vapour does not propagate downwards to the core centre, where the void reactivity coefficient is positive, but rather in radial direction from one channel to another, being always kept in the region of low or even negative void reactivity coefficient. Moreover, the availability of the sodium flow rate through the subassembly, where boiling occurs, is beneficial for preventing the cladding dryout, because a part of sodium will flow through the boiling region in the form of a liquid film on the surface of the claddings

Coupled 3-D Neutronics/Thermal-Hydraulics Optimization Study For Improving The Response Of A 3600 Mw(Thermal) Sfr Core To An Unprotected Loss-Of-Flow Accident

The sodium-cooled fast reactor (SFR), as a fast neutron spectrum system, is characterized by several performance advantages. In particular, the long-term operation of an SFR core in a closed fuel cycle will lead to an equilibrium state, where both reactivity and fuel mass flow stabilize. However, the SFR has one dominating neutronics drawback, namely, there is generally a positive reactivity effect when there is voiding of the sodium coolant in the core. Furthermore, this effect becomes even stronger in the equilibrium closed fuel cycle. Considering that in a hypothetical SFR unprotected loss-of-flow (ULOF) accident scenario, i.e., flow rundown without SCRAM, sodium boiling can be anticipated to occur, it is crucial to assess the corresponding impact of the positive sodium void effect. An optimization study for improving the safety characteristics of a large [3600-MW(thermal)] SFR has currently been conducted in the above context. The dynamic core response to a reference ULOF scenario is investigated with the use of a coupled three-dimensional neutronics/thermal-hydraulics PARCS/TRACE model. The starting point of the study is the reference core design considered in the framework of the Collaborative Project on the European Sodium Fast Reactor (CP-ESFR). To reduce the sodium void effect, the core has been modified by introducing an upper sodium plenum, along with a boron layer above it. Furthermore, the original core height-to-diameter ratio is reduced. In comparison to the reference ESFR core behavior, certain improvements are achieved, thanks to the static neutronics optimization carried out. However, these changes are found, in themselves, to be insufficient as regards the prevention of cladding and fuel melting during the considered ULOF event. Thermal-hydraulics optimization has thus been considered necessary, in order to (a) prevent sodium flow blockage in the fuel channel and (b) avoid boiling instabilities caused by the vaporization/condensation process in the upper sodium plenum. The corresponding measures taken are (a) the introduction of an innovative wrapper design, which features small openings in each side surface of the fuel assembly, and (b) replacement of the original upper sodium plenum by an extended fission gas plenum. Following implementation of these thermal-hydraulics-related design changes, one arrives at a final configuration of the SFR core, in which, for the selected accident scenario, a new "steady state" involving stable sodium boiling is found to be achievable, with melting of neither cladding nor fuel. Such a satisfactory behavior has been confirmed not only for the beginning-of-life state of the core but also for the equilibrium closed fuel cycle

Spectral Filtering Induced by Non-Hermitian Evolution with Balanced Gain and Loss: Enhancing Quantum Chaos

The dynamical signatures of an isolated quantum chaotic system are captured by the spectral form factor, which exhibits as a function of time a dip, a ramp, and a plateau, with the ramp being governed by the correlations in the level spacing distribution. These dynamical signatures are generally suppressed by decoherence. We consider the nonlinear non-Hermitian evolution associated with balanced gain and loss (BGL) in an energy-dephasing scenario and show that dissipation in this setting enhances manifestations of quantum chaos. Using the Sachdev-Ye-Kitaev model as a test-bed, BGL is shown to increase the span of the ramp, lowering the dip as well as the value of the plateau, providing an experimentally realizable physical mechanism for spectral filtering. The enhancement due to BGL is shown to be optimal with respect to the choice of the filter function.Comment: 6+4 pages, 2+4 figure

FAST code system: Review of recent developments and near-future plans

The FAST code system is currently being developed and used at the Paul Scherrer Institut for static and transient analysis of the main Generation 4 fast-spectrum reactor concepts: sodium-, helium-, and gas-cooled fast reactors. The code system includes the ERANOS code system for static neutronics calculations, as well as coupled TRACE/PARCS/FRED for neutron kinetics, thermal hydraulic, and fuel transient analysis. The paper presents the status of the recent developments in neutronics (new 3D procedure for equilibrium cycle simulation and new transient cross section generation procedure), in thermal hydraulics and chemistry (equations-of-state for new coolants, two-phase flow models for sodium, and new model for oxide layer buildup in heavy-metal flow), and in fuel behavior (new model for the dispersed gas-cooled fast reactor fuel). Near-future plans for the further development of FAST are outlined. 2010 by ASME

Impact of the lipid bilayer on energy transfer kinetics in the photosynthetic protein LH2

Photosynthetic purple bacteria convert solar energy to chemical energy with near unity quantum efficiency. The light-harvesting process begins with absorption of solar energy by an antenna protein called Light-Harvesting Complex 2 (LH2). Energy is subsequently transferred within LH2 and then through a network of additional light-harvesting proteins to a central location, termed the reaction center, where charge separation occurs. The energy transfer dynamics of LH2 are highly sensitive to intermolecular distances and relative organizations. As a result, minor structural perturbations can cause significant changes in these dynamics. Previous experiments have primarily been performed in two ways. One uses non-native samples where LH2 is solubilized in detergent, which can alter protein structure. The other uses complex membranes that contain multiple proteins within a large lipid area, which make it difficult to identify and distinguish perturbations caused by protein-protein interactions and lipid-protein interactions. Here, we introduce the use of the biochemical platform of model membrane discs to study the energy transfer dynamics of photosynthetic light-harvesting complexes in a near-native environment. We incorporate a single LH2 from Rhodobacter sphaeroides into membrane discs that provide a spectroscopically amenable sample in an environment more physiological than detergent but less complex than traditional membranes. This provides a simplified system to understand an individual protein and how the lipid-protein interaction affects energy transfer dynamics. We compare the energy transfer rates of detergent-solubilized LH2 with those of LH2 in membrane discs using transient absorption spectroscopy and transient absorption anisotropy. For one key energy transfer step in LH2, we observe a 30% enhancement of the rate for LH2 in membrane discs compared to that in detergent. Based on experimental results and theoretical modeling, we attribute this difference to tilting of the peripheral bacteriochlorophyll in the B800 band. These results highlight the importance of well-defined systems with near-native membrane conditions for physiologically-relevant measurements.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0018097