Localized qubits in curved spacetimes

We provide a systematic and self-contained exposition of the subject of localized qubits in curved spacetimes. This research was motivated by a simple experimental question: if we move a spatially localized qubit, initially in a state |\psi_1>, along some spacetime path \Gamma from a spacetime point x_1 to another point x_2, what will the final quantum state |\psi_2> be at point x_2? This paper addresses this question for two physical realizations of the qubit: spin of a massive fermion and polarization of a photon. Our starting point is the Dirac and Maxwell equations that describe respectively the one-particle states of localized massive fermions and photons. In the WKB limit we show how one can isolate a two-dimensional quantum state which evolves unitarily along \Gamma. The quantum states for these two realizations are represented by a left-handed 2-spinor in the case of massive fermions and a four-component complex polarization vector in the case of photons. In addition we show how to obtain from this WKB approach a fully general relativistic description of gravitationally induced phases. We use this formalism to describe the gravitational shift in the COW 1975 experiment. In the non-relativistic weak field limit our result reduces to the standard formula in the original paper. We provide a concrete physical model for a Stern-Gerlach measurement of spin and obtain a unique spin operator which can be determined given the orientation and velocity of the Stern-Gerlach device and velocity of the massive fermion. Finally, we consider multipartite states and generalize the formalism to incorporate basic elements from quantum information theory such as quantum entanglement, quantum teleportation, and identical particles. The resulting formalism provides a basis for exploring precision quantum measurements of the gravitational field using techniques from quantum information theory.Comment: 53 pages, 7 figures; v2: published version with further corrections. v3: some references and equation typesetting fixe

Diamond Integrated Quantum Photonics: A Review

Integrated quantum photonics devices in diamond have tremendous potential for many quantum applications, including long-distance quantum communication, quantum information processing, and quantum sensing. These devices benefit from diamond's combination of exceptional thermal, optical, and mechanical properties. Its wide electronic bandgap makes diamond an ideal host for a variety of optical active spin qubits that are key building blocks for quantum technologies. In landmark experiments, diamond spin qubits have enabled demonstrations of remote entanglement, memory-enhanced quantum communication, and multi-qubit spin registers with fault-tolerant quantum error correction, leading to the realization of multinode quantum networks. These advancements put diamond at the forefront of solid-state material platforms for quantum information processing. Recent developments in diamond nanofabrication techniques provide a promising route to further scaling of these landmark experiments towards real-life quantum technologies. In this paper, we focus on the recent progress in creating integrated diamond quantum photonic devices, with particular emphasis on spin-photon interfaces, cavity optomechanical devices, and spin-phonon transduction. Finally, we discuss prospects and remaining challenges for the use of diamond in scalable quantum technologies.Comment: 31 pages, 8 figure

Thermodynamics and synchronization in open quantum systems

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Atómica Molecular y Nuclear, leída el 11-07-2017Los efectos disipativos tienen profundas consecuencias en el comportamiento y las propiedades de los sistemas cuánticos [72]. La inevitable interacción con el entorno circundante, con el cual los sistemas intercambian continuamente información, energía, momento angular o materia, es la última responsable de los fenómenos de decoherencia y de la emergencia del comportamiento clásico [490, 614]. Sin embargo, existe un amplio régimen intermedio en el cual efectos disipativos y cuánticos coexisten, dando lugar a una amplia gama de ricos y sorprendentes fenómenos que apenas están empezando a ser comprendidos. Además, las innovadoras técnicas desarrolladas recientemente para controlar y manipular sistemas cuánticos en el laboratorio han hecho esta fenomenología accesible experimentalmente y potencialmente aplicable [244, 586]. En esta tesis exploraremos desde un punto de vista teórico algunas de las conexiones entre efectos disipativos y cuánticos en lo concerniente a dos principales aspectos: el comportamiento termodinámico de los sistemas cuánticos y la relación entre las correlaciones dinámicas y cuánticas compartidas por éstos. Las correlaciones cuánticas son una de las características más sorprendentes de la naturaleza y han atraído un notable interés desde la misma formulación de la teoría cuántica. La comprensión de los mecanismos subyacentes que generan, preservan, o destruyen estas correlaciones resulta de gran importancia a la hora de explorar la frontera cuántico-clásica [597], mientras que es esencial en el diseño de esquemas en los que la decoherencia pueda ser evitada en aplicaciones prácticas [35, 143, 562]. Por otra parte, otro tipo importante de correlaciones dinámicas de caracter más tradicionalmente clásico son los fenómenos de sincronización, que han sido estudiados en un amplio rango de sistemas físicos, químicos y biológicos [433]. La sincronización puede aparecer como un comportamiento espontáneo y cooperativo de diferentes unidades que oscilan y que, cuando se acoplan, adaptan sus ritmos a una frecuencia común. Este fenómeno de sincronización mutua ha sido considerado con profusión desde un punto de vista clásico [249, 263, 606], mientras que los rasgos genuinamente cuánticos de la sincronización estan empezando ahora a ser investigados...Dissipation effects have profound consequences in the behavior and properties of quantum systems. The unavoidable interaction with the surrounding environment, with whom systems continuously exchange information, energy, angular momentum or matter, is ultimately responsible of decoherence phenomena and the emergence of classical behavior. However, there exist a wide intermediate regime in which the interplay between dissipative and quantum effects gives rise to a plethora of rich and striking phenomena that has only started to be understood. In addition, the recent breakthrough techniques in controlling and manipulating quantum systems in the laboratory has made this phenomenology accessible in experiments and potentially applicable. In this thesis we aim to explore from a theoretical point of view some of the connections between dissipative and quantum effects. We focus on three main topics: the relation between dynamical and quantum correlations, the thermodynamical properties of fluctuations, and the performance of quantum thermal machines. First, we study the emergence of transient and asymptotic spontaneous synchronization induced by dissipation in harmonic quantum systems and its connections with quantum correlations. Our results show that synchronization may be used a witnesses for the slow decay or even the preservation of quantum discord in many situations of interest. Furthermore, we develop methods for engineering it in complex harmonic networks or selected clusters, where noiseless subsystems can be obtained by tuning one or few system parameters. Second, we explore the quantum versions of work and entropy production fluctuation relations. We derive a general fluctuation theorem valid for a broad class of open systems dynamics and discuss the meaning of the quantity fulfilling it. Importantly, our theorem overcomes the prototypical assumption of ideal thermal reservoirs. We also study the possibility of split entropy production in arbitrary quantum processes into adiabatic and non-adiabatic contributions, each of them fulfilling an independent fluctuation theorem. Contrary to the classical case, quantum effects may break the split, and we discuss the necessary conditions to enforce it. Our findings are illustrated in three relevant examples for quantum thermodynamics. Finally, we focus on the role quantum effects in the performance of quantum thermal machines. We analyze the case of an optimized quantum Otto cycle powered by a squeezed thermal reservoir. Our previous results allow us to characterize the many striking nonequilibrium features that arise, including work extraction from a single reservoir or multi-task regimes combining both refrigeration of a cold reservoir and work extraction at the same time. On the other hand, we also explore the role of the Hilbert space dimension in the performance of autonomous quantum thermal machines. Our results point that adding extra levels constitutes a thermodynamical resource. For the case of autonomous fridges, we further obtain a statement on the third law of thermodynamics in terms of their number of levels: reaching zero temperature requires an infinite Hilbert space dimension. The research results presented in this thesis are complemented with a broad introduction to the field of open quantum systems and quantum thermodynamics. There, we review the state of the art on these topics and the reader will find the main methods and tools used along the thesis.Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEunpu

Virtual humans: thirty years of research, what next?

In this paper, we present research results and future challenges in creating realistic and believable Virtual Humans. To realize these modeling goals, real-time realistic representation is essential, but we also need interactive and perceptive Virtual Humans to populate the Virtual Worlds. Three levels of modeling should be considered to create these believable Virtual Humans: 1) realistic appearance modeling, 2) realistic, smooth and flexible motion modeling, and 3) realistic high-level behaviors modeling. At first, the issues of creating virtual humans with better skeleton and realistic deformable bodies are illustrated. To give a level of believable behavior, challenges are laid on generating on the fly flexible motion and complex behaviours of Virtual Humans inside their environments using a realistic perception of the environment. Interactivity and group behaviours are also important parameters to create believable Virtual Humans which have challenges in creating believable relationship between real and virtual humans based on emotion and personality, and simulating realistic and believable behaviors of groups and crowds. Finally, issues in generating realistic virtual clothed and haired people are presente

Proceedings of AUTOMATA 2011 : 17th International Workshop on Cellular Automata and Discrete Complex Systems

International audienceThe proceedings contain full (reviewed) papers and short (non reviewed) papers that were presented at the workshop