Spectral and Steady-State Properties of Random Liouvillians

We study generic open quantum systems with Markovian dissipation, focusing on a class of stochastic Liouvillian operators of Lindblad form with independent random dissipation channels (jump operators) and a random Hamiltonian. We establish that the global spectral features, the spectral gap, and the steady-state properties follow three different regimes as a function of the dissipation strength, whose boundaries depend on the particular quantity. Within each regime, we determine the scaling exponents with the dissipation strength and system size. We find that, for two or more dissipation channels, the spectral gap increases with the system size. The spectral distribution of the steady state is Poissonian at low dissipation strength and conforms to that of a random matrix once the dissipation is sufficiently strong. Our results can help to understand the long-time dynamics and steady-state properties of generic dissipative systems.Comment: 21 pages, 11 figures. v2: minor corrections. v3: additional details, as publishe

Analytical querying with typed linear algebra: integration with MonetDB

Dissertação de mestrado integrado em Informatics EngineeringCurrent digital transformations in society heavily rely on safe, easy-to-use, high-performance data storage and analysis for smart decision taking. This triggered the need for efficient analytical querying solutions and the columnar database model is increasingly regarded as the most efficient model for data organization in large data banks. MonetDB is a pioneer in the column-wise database model and is currently at the forefront of high performance DBMS engine. A Linear Algebra Querying (LAQ) engine, using a columnar database paradigm and strongly inspired on Typed Linear Algebra (TLA), was developed in a former MSc. dissertation, with a prototype Web interface. Performance benchmarking of this engine showed it outperformed conventional referenced DBMS but it failed to beat MonetDB’s performance. This dissertation aims to improve the performance of the LAQ engine by following a different path: instead of a standalone engine, the new approach implements the engine on top of MonetDB extended with RMA (Relational Matrix Algebra) and inspired by the TLA approach. This enables the use of LAQ scripting to replace the main stream relational algebra query language approach given by SQL. Matrix operations commonly used in LAQ/TLA, such as matrix-matrix multiplication, Khatri-Rao product or Hadamard-Schur product, had to be implemented in RMA to shift from the relational algebra paradigm to TLA. A thorough analysis of the MonetDB/RMA showed the need to implement key TLA operators that are not available at the frontend. Such operators were implemented and successfully tested and validated, paving the way to future benchmarking its performance with TPC-H/OLAP queries and consequent fine tuning of the engine.Atualmente, as transformações digitais na sociedade confiam fortemente no armazenamento e na análise de dados seguros, fáceis de usar e de alto desempenho para tomadas de decisão inteligentes. Este facto desencadeou a necessidade de soluções de consultas analíticas eficientes, em que o modelo de bases de dados colunar é cada vez mais considerado o modelo mais eficiente para organização de dados em grandes bancos de dados. MonetDB é um sistema pioneiro no modelo de bases de dados colunar e atualmente está na vanguarda de DBMS’s de alto desempenho. Um motor Linear Algebra Querying (LAQ), que usa o paradigma de bases de dados colunar e fortemente inspirado em Álgebra Linear Tipada (TLA), foi desenvolvido numa antiga dissertação de mestrado em Engenharia Informática. O benchmarking do desempenho deste motor mostrou que supera DBMS tradicionais, mas não conseguiu superar o desempenho do MonetDB. Esta dissertação visa melhorar o desempenho do motor LAQ seguindo um caminho diferente: em vez de um motor autónomo, a nova abordagem implementa o motor sobre o motor do MonetDB estendido com RMA (Álgebra Relacional Matricial) e inspirado na abordagem de TLA. Isto permite o uso de scripts LAQ para substituir a abordagem da linguagem de consulta de álgebra relacional fornecida pelo SQL. Operações de matrizes comumente usadas em LAQ / TLA, como multiplicação de matrizes, produto Khatri-Rao ou produto Hadamard-Schur, tiveram de ser implementadas em RMA para mudar do paradigma da álgebra relacional para TLA. Uma análise completa do MonetDB / RMA mostrou a necessidade de implementar os principais operadores de TLA que não estão disponíveis no front-end. Esses operadores foram implementados, testados e validados com sucesso, abrindo caminho para um futuro benchmarking do seu desempenho com queries TPC-H / OLAP e consequente, ajuste do motor

Energy efficiency of mmWave massive MIMO precoding with low-resolution DACs

With the congestion of the sub-6 GHz spectrum, the interest in massive multiple-input multiple-output (MIMO) systems operating on millimeter wave spectrum grows. In order to reduce the power consumption of such massive MIMO systems, hybrid analog/digital transceivers and application of low-resolution digital-to-analog/analog-to-digital converters have been recently proposed. In this work, we investigate the energy efficiency of quantized hybrid transmitters equipped with a fully/partially-connected phase-shifting network composed of active/passive phase-shifters and compare it to that of quantized digital precoders. We introduce a quantized single-user MIMO system model based on an additive quantization noise approximation considering realistic power consumption and loss models to evaluate the spectral and energy efficiencies of the transmit precoding methods. Simulation results show that partially-connected hybrid precoders can be more energy-efficient compared to digital precoders, while fully-connected hybrid precoders exhibit poor energy efficiency in general. Also, the topology of phase-shifting components offers an energy-spectral efficiency trade-off: active phase-shifters provide higher data rates, while passive phase-shifters maintain better energy efficiency.Comment: Published in IEEE Journal of Selected Topics in Signal Processin

CNN-LSTM-based models to predict the heart rate using PPG signal from wearables during physical exercise

Atrial fibrillation, or AFib is the most common form of arrhythmia, in fact, 3\% of people over the age of 20 suffer from this condition and more shockingly, it is found that patients with arrhythmias are 5 times more likely to have a stroke [1]. These events of irregularity in the heart beat occur briefly and can be very sporadic which leads their detection to be rather cumbersome, with the standard diagnostic procedure being a long term continuous ECG. This leads to multiple problems, first of all, the ECG is commonly performed as the person is laying down in a hospital bed, which immediately distances the test environment from the real world scenario of living with AFib or another kind of arrhythmia, especially since arrhythmias are more likely to manifest during the practice of physical exercise. From this need arises the alternative of using a PPG (Photoplethysmography) signal, which is an optical method of measuring the blood volume in surfaces such as the finger tip, wrist or ear lobe[2] and can be present in many portable devices like fitness bands and smartwatches, therefore enabling it to be used during the practice of physical exercise [3]. This alternative heart rate monitor is substantially less invasive and more mobile but it is also much more susceptible to motion artifacts. However the motion artifacts that create this noise can be quantified through the pairing of an accelerometer to this device, which provides us with data regarding the acceleration of the devices over the 3 axis. Data like this is available and we will be using the dataset from the IEEE Signal Processing Cup 2015, with which, a plethora of different approaches to remove the noise becomes available, from more traditional filtering methods to the more modern Artificial Intelligence approaches, like the neural networks and support vector machines that have been used in the related work. We believe this multimodal approach will provide us with substantially better results than traditional methods that used the signal itself as the only input of the model

The effect of firm and country characteristics on mandatory disclosure compliance

Our study investigates the role of firm and country characteristics in determining the level of compliance with mandatory disclosure requirements. We also examine whether the role of firm characteristics hold across different country environments. Our empirical study relies on European Union listed firms included on the STOXX Europe 600 Index and on their level of compliance with IFRS 3, Business Combinations disclosure requirements. Our results demonstrate that both firm and country characteristics develop a significant task in explaining the level of compliance with mandatory disclosure requirements. They confirm that firms located in a common-law country have the strongest, and firms located in a French-civil-law country the weakest, level of compliance with IFRS 3 disclosure requirements, with firms located in a Scandinavian- and German-civil-law country placed in the middle. Our findings also suggest that return on assets is the main determinant of the level of compliance with IFRS 3 disclosure requirements in the group of common-law plus Scandinavian- and Germancivil- law countries, while leverage is the main determinant in the group of French-civil-law countries.O nosso estudo analisa a influência das características das empresas e dos países sobre o nível de cumprimento dos requisitos de divulgação obrigatórios. Também analisamos se o impacto das características das empresas sobre o nível de cumprimento dos requisitos de divulgação obrigatórios varia de acordo com os diferentes países. O nosso estudo empírico baseia-se em empresas cotadas na União Europeia, incluídas no Índice STOXX 600 no final de 2009, e no seu nível de cumprimento dos requisitos de divulgação exigidos pela Norma Internacional e de Relato Financeiro 3, Concentrações de Negócios. Os resultados obtidos indicam que tanto as características das empresas como dos países influenciam de modo significativo o nível de cumprimento dos requisitos de divulgação obrigatórios. Eles demonstram que as empresas localizadas em países com sistemas de common-law têm superiores níveis de cumprimento dos requisitos de divulgação, que as empresas localizadas em países com sistemas de civil-law Francês têm piores níveis de cumprimento e que as empresas localizadas em países com sistemas de civil-law Escandinavo ou Alemão se encontram entre as anteriores em termos de cumprimento dos requisitos de divulgação exigidos pela Norma Internacional e de Relato Financeiro 3. Os nossos resultados também sugerem que a Rendibilidade dos Activos é a principal determinante nas empresas localizadas em países com sistemas de common-law e de civil-law Escandinavo ou Alemão, e que o rácio de Alavanca Financeira é o principal determinante nas empresas localizadas em países com sistemas de civil-law Francês