1,720,963 research outputs found
MODELS AND QUANTUM ALGORITHMS FOR OPEN SYSTEM DYNAMICS: THE CASE STUDY OF EXCITON TRANSPORT IN MOLECULAR NETWORKS
Get PDFAs physical systems of chemical interest are rarely isolated, molecular processes should always be intended within the framework of open system dynamics. Notable examples are charge and energy transfer in molecular networks, for which intense theoretical and experimental research has highlighted the central role of the interplay between the system Hamiltonian and decoherences due to the interaction with the environment. At intermediate system-environment interaction strengths, the cooperation between coherent and incoherent dynamics can result in a prototypical effect called environment-assisted quantum transport (ENAQT), which consists of the enhancement of transport efficiency. ENAQT is believed to play a primary part in the high efficiency of natural light-harvesting complexes. A comprehensive understanding and powerful simulation strategies for these dynamical phenomena could help us, for example, in the design of artificial devices, based on the engineering of materials and their environment, for high-performance cells for photovoltaic applications. However, the simulation of open quantum systems poses the theoretical challenge of devising an adequate equation of motion for the dynamics and a computational strategy for its solution, which becomes prohibitively difficult for classical computers when handling large quantum systems.
Thanks to the theoretical and experimental scientific advances of the last decades, we are now at the dawn of the so-called second quantum revolution that promises novel technological tools based on harnessing quantum coherence. Quantum computers, i.e., physical systems manipulated at the quantum level with high precision, are concrete examples. In recent years, quantum computers have already demonstrated they can tackle some complex problems considered intractable by classical computers: the so-called quantum advantage. The simulation of quantum systems has always been a strong motivation behind the development of quantum computers, as they are expected to provide advantages in dealing with large systems based on their huge computational space. However, despite its importance, the simulation of open system dynamics has received relatively little attention. One of the reasons is the non-trivial challenge of reproducing the evolution of open quantum systems in the framework of quantum circuits.
In this thesis, we approach the study of open system dynamics by drawing two parallel paths. On the one hand, we intend to explore in detail some salient features of quantum transport in molecular networks. To do so, we will critically analyse existing models for open system dynamics, ranging from Markovian to non-Markovian regime, from weak to strong coupling and from infinite to finite temperature. On the other hand, we consider the problem of simulating the dynamics underlying ENAQT with digital quantum computers. An algorithmic package is developed to implement the dynamics in different conditions. The algorithms are designed with two different strategies, the first one based on stochastic Hamiltonians and the second one based on a collision model. We demonstrate the potentiality of our algorithms by simulating ENAQT on a quantum computer emulator and provide a comparative analysis of the two approaches. Both algorithmic strategies can be implemented in a memory-efficient encoding with the number of required qubits scaling logarithmically with the size of the simulated system, while the number of gates scales polynomially depending on the target environmental conditions. We discuss the algorithmic quantum trajectories generated during the execution of the algorithms showing that they realize distinct unravellings of the dynamics of the open system.As physical systems of chemical interest are rarely isolated, molecular processes should always be intended within the framework of open system dynamics. Notable examples are charge and energy transfer in molecular networks, for which intense theoretical and experimental research has highlighted the central role of the interplay between the system Hamiltonian and decoherences due to the interaction with the environment. At intermediate system-environment interaction strengths, the cooperation between coherent and incoherent dynamics can result in a prototypical effect called environment-assisted quantum transport (ENAQT), which consists of the enhancement of transport efficiency. ENAQT is believed to play a primary part in the high efficiency of natural light-harvesting complexes. A comprehensive understanding and powerful simulation strategies for these dynamical phenomena could help us, for example, in the design of artificial devices, based on the engineering of materials and their environment, for high-performance cells for photovoltaic applications. However, the simulation of open quantum systems poses the theoretical challenge of devising an adequate equation of motion for the dynamics and a computational strategy for its solution, which becomes prohibitively difficult for classical computers when handling large quantum systems.
Thanks to the theoretical and experimental scientific advances of the last decades, we are now at the dawn of the so-called second quantum revolution that promises novel technological tools based on harnessing quantum coherence. Quantum computers, i.e., physical systems manipulated at the quantum level with high precision, are concrete examples. In recent years, quantum computers have already demonstrated they can tackle some complex problems considered intractable by classical computers: the so-called quantum advantage. The simulation of quantum systems has always been a strong motivation behind the development of quantum computers, as they are expected to provide advantages in dealing with large systems based on their huge computational space. However, despite its importance, the simulation of open system dynamics has received relatively little attention. One of the reasons is the non-trivial challenge of reproducing the evolution of open quantum systems in the framework of quantum circuits.
In this thesis, we approach the study of open system dynamics by drawing two parallel paths. On the one hand, we intend to explore in detail some salient features of quantum transport in molecular networks. To do so, we will critically analyse existing models for open system dynamics, ranging from Markovian to non-Markovian regime, from weak to strong coupling and from infinite to finite temperature. On the other hand, we consider the problem of simulating the dynamics underlying ENAQT with digital quantum computers. An algorithmic package is developed to implement the dynamics in different conditions. The algorithms are designed with two different strategies, the first one based on stochastic Hamiltonians and the second one based on a collision model. We demonstrate the potentiality of our algorithms by simulating ENAQT on a quantum computer emulator and provide a comparative analysis of the two approaches. Both algorithmic strategies can be implemented in a memory-efficient encoding with the number of required qubits scaling logarithmically with the size of the simulated system, while the number of gates scales polynomially depending on the target environmental conditions. We discuss the algorithmic quantum trajectories generated during the execution of the algorithms showing that they realize distinct unravellings of the dynamics of the open system
Simulating action-2D electronic spectroscopy of quantum dots: insights on the exciton and biexciton interplay from detection-mode and time-gating
Get PDFA Quantum Algorithm from Response Theory: Digital Quantum Simulation of Two-Dimensional Electronic Spectroscopy
No full textMultidimensional optical spectroscopies are powerful techniques to investigate energy transfer pathways in natural and artificial systems. Because of the high information content of the spectra, numerical simulations of the optical response are of primary importance to assist the interpretation of spectral features. However, the increasing complexity of the investigated systems and their quantum dynamics call for the development of novel simulation strategies. In this work, we consider using digital quantum computers. By combining quantum dynamical simulation and nonlinear response theory, we present a quantum algorithm for computing the optical response of molecular systems. The quantum advantage stems from the efficient quantum simulation of the dynamics governed by the molecular Hamiltonian, and it is demonstrated by explicitly considering exciton-vibrational coupling. The protocol is tested on a near-term quantum device, providing the digital quantum simulation of the linear and nonlinear response of simple molecular models
Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy
Get PDFAction-detection has expanded the scope and applicability of 2D electronic spectroscopy, while posing new challenges for the unambiguous interpretation of spectral features. In this context, identifying the origin of cross-peaks at early waiting times is not trivial, and incoherent mixing is often invoked as an unwanted contribution masking the nonlinear signal. In this work, we elaborate on the relation between the nonlinear response and the incoherent mixing contribution by analyzing the action signal in terms of one- and two-particle observables. Considering a weakly interacting molecular dimer, we show how cross-peaks at early waiting times, reflecting exciton-exciton annihilation dynamics, can be equivalently interpreted as arising from incoherent mixing. This equivalence, on the one hand, highlights the information content of spectral features related to incoherent mixing and, on the other hand, provides an efficient numerical scheme to simulate the action response of weakly interacting systems
From stochastic Hamiltonian to quantum simulation: exploring memory effects in exciton dynamics
Get PDFThe unraveling of open quantum system dynamics in terms of stochastic quantum trajectories offers a picture of open system dynamics that consistently considers memory effects stemming from the finite correlation time of environment fluctuations. These fluctuations significantly influence the coherence and energy transport properties of excitonic systems. When their correlation time is comparable to the timescale of the Hamiltonian evolution, it leads to the departure of open system dynamics from the Markovian limit. In this work, we leverage the unraveling of exciton dynamics through stochastic Hamiltonian propagators to design quantum circuits that simulate exciton transport, capturing finite memory effects. In addition to enabling the synthesis of parametrizable quantum circuits, stochastic unitary propagators provide a transparent framework for investigating non-Markovian effects on exciton transport. Our analysis reveals a nuanced relationship between environment correlation time and transport efficiency, identifying a regime of 'memory-assisted' quantum transport where time-correlated fluctuations allow the system to reach higher efficiency. However, this property is not universal and can only be realized in conjunction with specific features of the system Hamiltonian
Strategies to simulate dephasing-assisted quantum transport on digital quantum computers
No full textSimulating charge and energy transfer in extended molecular networks requires an effective model to include the environment because it significantly affects the quantum dynamics. A prototypical effect known as environment-assisted quantum transport (ENAQT) consists in the enhancement of the transfer efficiency by the interaction with an environment. A simple description of this phenomenon is obtained by a quantum master equation describing a quantum walk over the molecular network in the presence of inter-site decoherence. We consider the problem of simulating the dynamics underlying ENAQT in a digital quantum computer. Two different quantum algorithms are introduced, the first one based on stochastic Hamiltonians and the second one based on a collision scheme. We test both algorithms by simulating ENAQT in a small molecular network on a quantum computer emulator and provide a comparative analysis of the two approaches. Both algorithms can be implemented in a memory efficient encoding with the number of required qubits scaling logarithmically with the size of the simulated system while the number of gates increases quadratically. We discuss the algorithmic quantum trajectories generated by the two simulation strategies showing that they realize distinct unravellings of the site-dephasing master equation. In our approach, the non-unitary dynamics of the open system is obtained through effective representations of the environment, paving the way to digital quantum simulations of quantum transport influenced by structured environments
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
