Solmujen luokittelumenetelmien arviointi tieteellisten julkaisujen verkossa

Large graphs often have labels only for a subset of nodes. Node classification is a semi-supervised learning task where unlabeled nodes are assigned labels utilizing the known information of the graph. In this thesis, three node classification methods are evaluated based on two metrics: computational speed and node classification accuracy. The three methods that are evaluated are label propagation, harmonic functions with Gaussian fields, and Graph Convolutional Neural Network (GCNN). Each method is tested on five citation networks of different sizes extracted from a large scientific publication graph, MAG240M-LSC. For each graph, the task is to predict the subject areas of scientific publications, e.g., cs.LG (Machine Learning). The motivation of the experiments is to give insight on whether the methods would be suitable for automatic labeling of scientific publications. The results show that label propagation and harmonic functions with Gaussian fields reach mediocre accuracy in the node classification task, while GCNN had a low accuracy. Label propagation was computationally slow compared to the other methods, whereas harmonic functions were exceptionally fast. Training of the GCNN took a long time compared to harmonic functions, but computational speed was acceptable. However, none of the methods reached a high enough classification accuracy to be utilized in automatic labeling of scientific publications

Quantum jump model for a system with a finite-size environment

© 2016 American Physical Society. Measuring the thermodynamic properties of open quantum systems poses a major challenge. A calorimetric detection has been proposed as a feasible experimental scheme to measure work and fluctuation relations in open quantum systems. However, the detection requires a finite size for the environment, which influences the system dynamics. This process cannot be modeled with the standard stochastic approaches. We develop a quantum jump model suitable for systems coupled to a finite-size environment. We use the method to study the common fluctuation relations and prove that they are satisfied

Fluctuations of work in nearly adiabatically driven open quantum systems

We extend the quantum jump method to nearly adiabatically driven open quantum systems in a way that allows for an accurate account of the external driving in the system-environment interaction. Using this framework, we construct the corresponding trajectory-dependent work performed on the system and derive the integral fluctuation theorem and the Jarzynski equality for nearly adiabatic driving. We show that such identities hold as long as the stochastic dynamics and work variable are consistently defined. We numerically study the emerging work statistics for a two-level quantum system and find that the conventional diabatic approximation is unable to capture some prominent features arising from driving, such as the continuity of the probability density of work. Our results reveal the necessity of using accurate expressions for the drive-dressed heat exchange in future experiments probing jump time distributions. © 2015 American Physical Society

Work and heat for two-level systems in dissipative environments: Strong driving and non-Markovian dynamics

Work, moments of work and heat flux are studied for the generic case of a strongly driven twolevel system immersed in a bosonic heat bath in domains of parameter space where perturbative treatments fail. This includes particularly the interplay between non-Markovian dynamics and moderate to strong external driving. Exact data are compared with predictions from weak coupling approaches. Further, the role of system-bath correlations in the initial thermal state and their impact on the heat flux are addressed. The relevance of these results for current experimental activities on solid state devices is discussed.Comment: 9 pages, 9 figures, this version: added one new author, changed figures 2 and 4, minor changes in the tex

Quantifying non-Markovianity due to driving and a finite-size environment in an open quantum system

© 2017 American Physical Society.We study non-Markovian effects present in a driven qubit coupled to a finite environment using a recently proposed model developed in the context of calorimetric measurements of open quantum systems. To quantify the degree of non-Markovianity we use the Breuer-Laine-Piilo (BLP) measure [H.-P. Breuer, Phys. Rev. Lett. 103, 210401 (2009)PRLTAO0031-900710.1103/PhysRevLett.103.210401]. We show that information backflow only occurs in the case of driving, in which case we investigate the dependence of memory effects on the environment size, driving amplitude, and coupling to the environment. We show that the degree of non-Markovianity strongly depends on the ratio between the driving amplitude and the coupling strength. We also show that the degree of non-Markovianity does not decrease monotonically as a function of the environment size

Työn statistiikka avoimissa kvanttimekaanisissa systeemeissä

Rapid progress in the fabrication and manipulation of micro and nanoscale devices has made it necessary to extend the concepts of thermodynamics to small systems and finally to quantum systems. In such systems the extensive thermodynamic quantities, such as entropy, heat, and work, are not described by their average values alone but by distributions. Remarkably, it has been shown that the stochastic thermodynamic variables often obey fluctuation relations which usually appear in the form of relations between exponential averages. While the two-measurement protocol of thermodynamic variables, especially work, is now well studied in closed quantum systems, there have been conceptual and experimental problems in open quantum systems. In this thesis, we theoretically study work statistics and fluctuation relations in open quantum systems. To calculate the work statistics, we use the quantum jump method and direct master equation calculations. We show that work definitions equivalent in closed systems can lead to mutually different results in the case of open quantum systems due to a different order of approximations. Moreover, we show that the fluctuation relations can be extended to nearly adiabatically driven systems by using the adiabatic renormalization procedure. Last, we focus on how a finite size of the environment affects the dynamics and work statistics of open quantum systems. This is important from the experimental point of view because a finite-size environment allows the detection of heat exchange between the system and the environment. We derive a master equation that takes into account these effects and develop a stochastic model that unravels the master equation. We explicitly show that the finite size of the environment influences the dynamics and the work statistics. We also show that the common fluctuation relations are still valid.Nopea kehitys pienten komponenttien valmistamisessa on tehnyt tarpeelliseksi laajentaa termodynamiikan konseptit pieniin systeemeihin ja lopulta kvanttimekaanisiin systeemeihin. Näiden systeemien pienen koon takia termodynaamisia suureita, kuten työtä ja lämpöä, ei voida kuvata pelkällä keskiarvolla, vaan koko jakauma täytyy ottaa huomioon. Tässä väitöskirjassa tutkitaan työn statistiikkaa avoimissa kvanttisysteemeissä. Väitöskirjassa näytetään, että työn määritelmät, jotka ovat yhtenevät suljetuille systeemeille, voivat johtaa eri tuloksiin avoimissa kvanttisysteemeissä. Työssä laajennetaan myös työn fluktuaatiorelaatiot lähes adiabaattisesti ajasta riippuviin systeemeihin. Väitöskirjassa kehitetään myös malli, joka ottaa huomioon ympäristön äärellisen koon. Tämä malli mahdollistaa työn mittaamiseen ehdotettujen kokeiden mallintamisen, joissa ympäristön koon täytyy olla pieni lämmönvaihdon mittaamiseksi. Mallille kehitetään myös stokastinen vastine, joka mahdollistaa stokastisen termodynaamikan käytön työn ja lämmön analysoimiseen. Väitöskirjassa kehitetyt menetelmät mahdollistavat termodynamiikan tutkimisen systeemeissä, joita ei tavanomaisilla menetelmillä pystytä tutkimaan

Calorimetric measurement of work for a driven harmonic oscillator

A calorimetric measurement has recently been proposed as a promising technique to measure thermodynamic quantities in a dissipative superconducting qubit. These measurements rely on the fact that the system is projected into energy eigenstates whenever energy is exchanged with the environment. This requirement imposes a restriction on the class of systems that can be measured in this way. Here we extend the calorimetric protocol to the measurement of work in a driven quantum harmonic oscillator. We employ a scheme based on a two-level approximation that makes use of an experimentally accessible quantity and show how it relates to the work obtained through the standard two-measurement protocol. We find that the average work is well approximated in the underdamped regime for short driving times and, in the overdamped regime, for any driving time. However, this approximation fails for the variance and higher moments of work at finite temperatures. Furthermore, we show how to relate the work statistics obtained through this scheme to the work statistics given by the two-measurement protocol.Peer reviewe