Many-body formalism for thermally excited wave-packets: A way to connect the quantum regime to the classical regime

Free classical particles have well-defined momentum and position, while free quantum particles have well-defined momentum but a position fully delocalized over the sample volume. We develop a many-body formalism based on wave-packet operators that connects these two limits, the thermal energy being distributed between the state spatial extension and its thermal excitation. The corresponding `mixed quantum-classical' states, which render the Boltzmann operator diagonal, are the physically relevant states when the temperature is finite. The formulation of many-body Hamiltonians in terms of these thermally excited wave-packets and the resulting effective scatterings is provided.Comment: 7 pages, 2 figures, 2 pages supplementary material. (v2) link to the coherent states added. Final published version. (v3) 1 Ref. adde

Composite boson signature in the interference pattern of atomic dimer condensates

We predict the existence of high frequency modes in the interference pattern of two condensates made of fermionic-atom dimers. These modes, which result from fermion exchanges between condensates, constitute a striking signature of the dimer composite nature. From the 2-coboson spatial correlation function, that we derive analytically, and the Shiva diagrams that visualize many-body effects specific to composite bosons, we identify the physical origin of these high frequency modes and determine the conditions to see them experimentally by using bound fermionic-atom pairs trapped on optical lattice sites. The dimer granularity which appears in these modes comes from Pauli blocking that prevents two dimers to be located at the same lattice site.Comment: 10+7 pp, 3 figures. v2: version accepted for publication in New J. Phy

Work Statistics, Loschmidt Echo and Information Scrambling in Chaotic Quantum Systems

Characterizing the work statistics of driven complex quantum systems is generally challenging because of the exponential growth with the system size of the number of transitions involved between different energy levels. We consider the quantum work distribution associated with the driving of chaotic quantum systems described by random matrix Hamiltonians and characterize exactly the work statistics associated with a sudden quench for arbitrary temperature and system size. Knowledge of the work statistics yields the Loschmidt echo dynamics of an entangled state between two copies of the system of interest, the thermofield double state. This echo dynamics is dictated by the spectral form factor. We discuss its relation to frame potentials and its use to assess information scrambling.Comment: 11+6pp, 5 figures. v3: version accepted for publication in Quantu

Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes

A vibronic-exciton model is applied to investigate the mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom recently proposed as the origin of the long-lived oscillations in 2D spectra of the FMO complex [Christensson et al. J. Phys. Chem. B 116 (2012) 7449]. We reduce the problem to a model BChl dimer to elucidate the role of resonance coupling, site energies, nuclear mode 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 the two coupled BChls 3 and 4 of the FMO complex, the initial amplitude of the vibronic-exciton and vibrational coherences are enhanced by up to 15 and 5 times, respectively, compared to the vibrational coherences in the isolated monomer. This maximum initial amplitude enhancement occurs when there is a resonance between the electronic energy gap and the frequency of the vibrational mode. The bandwidth of this enhancement is about 100 cm-1 for both mechanisms. The excitonic mixing of electronic and vibrational DOF leads to additional dephasing relative to the vibrational coherences. We evaluate the dephasing dynamics by solving the quantum master equation in Markovian approximation and observe a strong dependence of the life-time enhancement on the mode frequency. Long-lived vibronic-exciton coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic resonance. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that both type have a similar magnitude at long time for the present model. The ability to distinguish between vibronic-exciton and ground-state vibrational coherences in the general case of molecular aggregate is discussed.Comment: 16 pages, 6 figure

Single- and Two-Phase Flow Modeling for Coupled Neutronics/Thermal-Hydraulics Transient Analysis of Advanced Sodium-Cooled Fast Reactors

Nuclear power is nowadays in the front rank as regards helping to meet the growing worldwide energy demand while avoiding an excessive increase in greenhouse gas emissions. However, the operating nuclear power plants are mainly thermal-neutron reactors and, as such, can not be maintained on the basis of the currently identified uranium resources beyond one century at the present consumption rate. Sustainability of nuclear power thus involves closure of the fuel cycle through breeding. With a uranium-based fuel, breeding can only be achieved using a fast-neutron reactor. Sodium-cooled fast reactor (SFR) technology benefits from 400 reactor-years of accumulated experience and is thus a prime candidate for the implementation of so-called Generation-IV nuclear energy systems. In this context, the safety demonstration of SFRs remains a major R&D related issue. The current doctoral research aims at the development of a computational tool for the in-depth understanding of SFR core behavior during accidental transients, particularly those including boiling of the coolant. An accurate modeling of the core physics during such transients requires the coupling between 3D neutron kinetics and thermal-hydraulics in the core, to account for the strong interactions between the two-phase coolant flow and power variations caused by the sodium void effect. Models for the representation of sodium two-phase flow are not present in most of the thermal-hydraulics codes used currently, and these have specifically been focused upon. The particular contributions of the present research are: (1) implementation of sodium two-phase flow models into the thermal-hydraulics code TRACE, which forms part of the FAST code system at PSI and, as such, can easily be coupled to the spatial neutron kinetics code PARCS; (2) validation of the TRACE sodium single- and two-phase flow modeling using out-of-pile sodium boiling experiments; (3) validation of the coupled TRACE/PARCS code system on the basis of experimental reactor data; and (4) application of the developed new tool for the core behavior analysis of an advanced SFR during a transient with boiling onset. The extension of the TRACE code, previously limited to the simulation of single-phase sodium flow, has been carried out through the implementation of equations-of-state and closure relations specific to sodium. A review has first been performed of the models available in the open literature for the representation of the interfacial and wall-to-fluid transfer mechanisms. The different correlations have then been implemented as options in the extended TRACE code. From the validation study carried out, it has been possible to recommend a set of models which provide satisfactory results, while considering annular flow as the dominant regime up to dryout and a smooth breakdown of the liquid film after dryout onset. The validation of the extended TRACE code has been achieved through the successful simulation of out-of-pile experiments. A review of available sodium boiling test data has first been carried out, and complementary tests have then been selected to assess the quality of the different physical models. These tests, performed in the 1980s, include the study of the pressure drop and cooling limits under quasi steady-state conditions, as well as the simulation of a loss-of-flow transient. Sensitivity analyses, using the specifically implemented correlations, have enabled one to identify the most pertinent physical parameters and to define, for each, the most appropriate model. Usage of the set of models thus selected has demonstrated the capacity of the extended TRACE code to predict, with satisfactory accuracy, the main thermal-hydraulics characteristics such as the single- and two-phase pressure drop and heat transfer, as also the characteristic quantities describing the sodium two-phase flow, e.g. boiling inception, void fraction evolution and expansion of the boiling region, pressure evolution, as well as coolant and clad temperatures. The natural convection test conducted in 2009 in the Phenix reactor has been used to validate TRACE single-phase sodium flow modeling. This represents the first international benchmark exercise conducted on the basis of actual SFR experimental data. Data from the Phenix test have additionally been used as basis for the validation of the FAST code system as a whole. Analyses based on a point-kinetics TRACE model and on coupled TRACE/PARCS 3D-kinetics modeling have enabled an in-depth understanding of the transient behavior of a sodium-cooled fast reactor core, as well as the identification of potential improvements in the FAST code system. The experimental power evolution could be satisfactorily reproduced within the measurement uncertainties with both models, and the detailed analysis of the core neutronics has enabled one to define the most important reactivity feedbacks taking place during the considered transient. In the final part of the thesis, the developed tool has been applied to the simulation of a hypothetical, unprotected loss-of-flow event for one of the European SFR (ESFR) core concepts. This study has demonstrated the new calculational tool's capability to adequately simulate the core response through the modeling of single- and two-phase sodium flow, coupled to 3D neutron kinetics. Thereby, the space-dependent reactivity feedbacks, such as the void and Doppler effects, have been shown to be modeled accurately. The resulting analysis has shown the ability of the TRACE/PARCS modeling to predict the expansion of the boiling region and calculate the resulting feedbacks, as well as to predict the interactions between parallel boiling channels. This first-of-a-kind study has provided detailed results for the thermal-hydraulics and neutronics parameters during the pre-severe phase of the simulated accident, thus allowing a comprehensive understanding of the core behavior during such transients. In brief, the present research has led to the development of a key calculational tool for SFR safety analysis. A first application of the new tool has demonstrated its potential for usage in SFR design optimization aimed at enhanced safety

Thermal light cannot be represented as a statistical mixture of single pulses

We ask whether or not thermal light can be represented as a mixture of single broadband coherent pulses. We find that it cannot. Such a mixture is simply not rich enough to mimic thermal light; indeed, it cannot even reproduce the first-order correlation function. We show that it is possible to construct a modified mixture of single coherent pulses that does yield the correct first-order correlation function at equal space points. However, as we then demonstrate, such a mixture cannot reproduce the second-order correlation function.Comment: 5 pages, 2 figures. Published versio

Extreme Decoherence and Quantum Chaos

We study the ultimate limits to the decoherence rate associated with dephasing processes. Fluctuating chaotic quantum systems are shown to exhibit extreme decoherence, with a rate that scales exponentially with the particle number, thus exceeding the polynomial dependence of systems with fluctuating $k$-body interactions. Our findings suggest the use of quantum chaotic systems as a natural test-bed for spontaneous wave function collapse models. We further discuss the implications on the decoherence of AdS/CFT black holes resulting from the unitarity loss associated with energy dephasing.Comment: 6+10 pp, 2+3 figures; published versio

Quantum work statistics, Loschmidt echo and information scrambling

A universal relation is established between the quantum work probability distribution of an isolated driven quantum system and the Loschmidt echo dynamics of a two-mode squeezed state. When the initial density matrix is canonical, the Loschmidt echo of the purified double thermofield state provides a direct measure of information scrambling and can be related to the analytic continuation of the partition function. Information scrambling is then described by the quantum work statistics associated with the time-reversal operation on a single copy, associated with the sudden negation of the system Hamiltonian.Comment: 6 pages, 1 figure, published versio

Light Adaptation in Phycobilisome antennas: Influence on the Rod Length and Structural Arrangement

Phycobilisomes, the light-harvesting antennas of cyanobacteria, can adapt to a wide range of environments thanks to a composition and function response to stress conditions. We study how structural changes influence excitation transfer in these super-complexes. Specifically, we show the influence of the rod length on the photon absorption and subsequent excitation transport to the core. Despite the fact that the efficiency of individual disks on the rod decreases with increasing rod length, we find an optimal length for which the average rod efficiency is maximal. Combining this study with experimental structural measurements, we propose models for the arrangement of the phycobiliproteins inside the thylakoid membranes, evaluate the importance of rod length, and predict the corresponding transport properties for different cyanobacterial species. This analysis, which links the functional and structural properties of full phycobilisome complexes, thus provides further rationals to help resolving their exact structure.Comment: 7 pages, 7 figures, 2 pages supplementary materia