1,720,963 research outputs found
Validity of the Lindblad master equations
Get PDFTheory of open quantum systems, which studies quantum systems interacting with their environments, has a wide variety of applications in physics, ranging from quantum optics and condensed matter physics to quantum informatics and quantum computation. One important feature of open quantum systems is quantum mechanical treatment for damping, which is usually described by Lindblad master equations derived using the Born–Markov approximation. However, because of their perturbative nature, such master equations can only model systems with weak system-environment coupling. For example, in superconducting quantum circuits the environmental coupling can be increased to values for which the master equations are no longer accurate. A more accurate equation of motion, the formally exact stochastic Liouville–von Neumann equation, can be obtained by using the path integral formalism of quantum mechanics.
We study a simple but important model where a two state system, qubit, is coupled to a quantum harmonic oscillator, which is further coupled to a harmonic oscillator bath. The qubit-oscillator system can be described by the Rabi Hamiltonian. We solve the dynamics of this system numerically with the stochastic Liouville–von Neumann equation and two different Lindblad master equations, the quantum optical master equation and the eigenstate master equation. The former treats the qubit and the oscillator separately and the transitions occur between the eigenstates of the oscillator, whereas the latter treats them as a single system with transitions between the eigenstates of the whole system Hamiltonian. Numerical solutions of the stochastic Liouville–von Neumann equation are unstable with long simulation times. Because of this we are only able to solve it in the case where the qubit and the oscillator are in resonance. In this case we find parameters with which all three equations produce nearly the same results. We also see that there exists cases where only one of the master equations agrees with the stochastic Liouville–von Neumann equation. Further on, we find parameters for which both master equations produce results that deviate notably from those given by the stochastic Liouville–von Neumann equation. In off-resonance, we find that the solutions of the two master equations do not agree with each other for any parameters we study.
These results suggest that the current state of the numerical simulations of open quantum systems can be improved by using the formally exact methods instead of the approximate ones for situations where the environmental coupling is of the order of the qubit-cavity coupling or stronger
Surface scratch on string theory : elementary particles as string vibrations and the existence of compact dimensions
No full textYksi fysiikan suurimpia avoimia kysymyksiä on tällä hetkellä gravitaation kvanttiteoria. Varteenotettava ehdokas täksi teoriaksi on säieteoria, jonka mukaan alkeishiukkaset syntyvät yksiulotteisten relativististen kvanttisäikeiden erilaisina värähtelytiloina. Yksi näistä hiukkastiloista on gravitoni, gravitaation välittäjähiukkanen. Koska säikeiden värähtelytiloissa esiintyy gravitonin lisäksi myös muita alkeishiukkasia, on mahdollista, että säieteoria on gravitaation kvanttiteorian lisäksi myös kaiken teoria, joka voisi siis selittää kaikki maailmankaikkeuden perusvuorovaikutukset.
Tämän työn tarkoituksena on tarkastella säieteoriaa ja selvittää, miten hiukkastilat syntyvät. Yksinkertaisuuden vuoksi tutkitaan bosonisäikeitä ja ratkaistaan säikeiden liikeyhtälöistä tilat, jotka voidaan identifioida gravitoniksi ja fotoniksi. Lisäksi havaitaan, että toimiakseen säieteoria vaatii tietyn, tarkasti rajatun, määrän ulottuvuuksia. Bosonisäieteoriasta saadaan ulottuvuuksien lukumääräksi 26. Tutkimalla supersäieteoriaa, josta saadaan bosonien lisäksi myös fermionitiloja, saadaan sen sijaan ulottuvuuksien lukumääräksi kymmenen. Ylimääräisten ulottuvuuksien olemassaolo pyritään selittämään sillä, että ne ovat käpertyneet hyvin pieniksi. Tarkastelemalla yksiulotteista ääretöntä potentiaalikuoppaa ja lisäämällä siihen tällainen käpertynyt ulottuvuus osoitetaan, että tällaisten kompaktien ulottuvuuksien olemassaolo on ainakin mahdollista. Lopuksi tarkastellaan näiden ylimääräisten ulottuvuuksien vaikutusta gravitaatioon
Bosonic many-body localization and collective phenomena in arrays of transmon devices
Get PDFAbstractSuperconducting circuits are electric devices in which information can be stored and processed on quantum level. The spectrum of these devices is often anharmonic, which means that their two lowest states can be used as a qubit. Moreover, their parameters are highly versatile and their energies are in-situ tunable, they are resistant to thermal noise, and they can be controlled and measured with high accuracy. These properties make superconducting circuits promising candidates for the basic units of a quantum computer. Currently available technology does not yet allow the construction of these machines, partly due to the errors in the qubits caused by external noise. In the meantime, these circuits could be used for simpler tasks such as quantum simulations, where the behavior of a complicated quantum system is studied experimentally using a simpler system.In this thesis we study many-body phenomena in arrays of specific superconducting circuits called transmons, which we model as anharmonic oscillators in contrast to the conventional two-level approximation. In the first part of this thesis, the transmons are embedded inside a waveguide. The electromagnetic field allows the transmons to interact with each other over long distances, which results in collective effects such as correlated decay and coherent exchange interaction. Correlated decay can be observed as superradiant and subradiant states, whose properties are well known in two-level systems. The bosonic nature of transmons distinguishes them from real two-level systems by enhancing the superradiance in this setup. We model the system with a specific master equation, from which we recover a non-Hermitian effective Hamiltonian whose eigenvalues describe the radiative properties of the system. Our model is in good agreement with experimental data, showing the inadequacy of the two-level approximation.In the latter part of this thesis we study how the interplay between many-body interactions and local disorder affects the behavior of transmon arrays. With weak disorder the system obeys the laws of statistical physics, resulting in thermalization of the system. With sufficiently strong disorder the system is instead in the many-body localized phase, characterized by the absence of transport of particles and logarithmic spreading of entanglement. The transition point is probed numerically and the phase diagram for the Bose–Hubbard Hamiltonian is constructed.Original papersOriginal papers are not included in the electronic version of the dissertation.Orell, T., Michailidis, A. A., Serbyn, M., & Silveri, M. (2019). Probing the many-body localization phase transition with superconducting circuits. Physical Review B, 100(13), 134504. https://doi.org/10.1103/PhysRevB.100.134504Self-archived versionZanner, M., Orell, T., Schneider, C. M. F., Albert, R., Oleschko, S., Juan, M. L., Silveri, M., & Kirchmair, G. (2022). Coherent control of a multi-qubit dark state in waveguide quantum electrodynamics. Nature Physics. https://doi.org/10.1038/s41567-022-01527-wSelf-archived versionOrell, T., Zanner, M., Sharafiev, A., Juan, M. L., Albert, R., Oleschko, S., Kirchmair, G., & Silveri, M. (2021). Collective bosonic effects in an array of transmon devices. Manuscript submitted for publication. https://doi.org/10.48550/arXiv.2112.08134Self-archived versionOsajulkaisutOsajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.Orell, T., Michailidis, A. A., Serbyn, M., & Silveri, M. (2019). Probing the many-body localization phase transition with superconducting circuits. Physical Review B, 100(13), 134504. https://doi.org/10.1103/PhysRevB.100.134504Rinnakkaistallennettu versioZanner, M., Orell, T., Schneider, C. M. F., Albert, R., Oleschko, S., Juan, M. L., Silveri, M., & Kirchmair, G. (2022). Coherent control of a multi-qubit dark state in waveguide quantum electrodynamics. Nature Physics. https://doi.org/10.1038/s41567-022-01527-wRinnakkaistallennettu versioOrell, T., Zanner, M., Sharafiev, A., Juan, M. L., Albert, R., Oleschko, S., Kirchmair, G., & Silveri, M. (2021). Collective bosonic effects in an array of transmon devices. Manuscript submitted for publication. https://doi.org/10.48550/arXiv.2112.08134Rinnakkaistallennettu versioAcademic Dissertation to be presented with the assent of the Doctoral Training
Committee of Technology and Natural Sciences of the University of Oulu, for
public discussion in Auditorium L5, on May 13th, 2022, at 3 p.m.Abstract
Superconducting circuits are electric devices in which information can be stored and processed on quantum level. The spectrum of these devices is often anharmonic, which means that their two lowest states can be used as a qubit. Moreover, their parameters are highly versatile and their energies are in-situ tunable, they are resistant to thermal noise, and they can be controlled and measured with high accuracy. These properties make superconducting circuits promising candidates for the basic units of a quantum computer. Currently available technology does not yet allow the construction of these machines, partly due to the errors in the qubits caused by external noise. In the meantime, these circuits could be used for simpler tasks such as quantum simulations, where the behavior of a complicated quantum system is studied experimentally using a simpler system.
In this thesis we study many-body phenomena in arrays of specific superconducting circuits called transmons, which we model as anharmonic oscillators in contrast to the conventional two-level approximation. In the first part of this thesis, the transmons are embedded inside a waveguide. The electromagnetic field allows the transmons to interact with each other over long distances, which results in collective effects such as correlated decay and coherent exchange interaction. Correlated decay can be observed as superradiant and subradiant states, whose properties are well known in two-level systems. The bosonic nature of transmons distinguishes them from real two-level systems by enhancing the superradiance in this setup. We model the system with a specific master equation, from which we recover a non-Hermitian effective Hamiltonian whose eigenvalues describe the radiative properties of the system. Our model is in good agreement with experimental data, showing the inadequacy of the two-level approximation.
In the latter part of this thesis we study how the interplay between many-body interactions and local disorder affects the behavior of transmon arrays. With weak disorder the system obeys the laws of statistical physics, resulting in thermalization of the system. With sufficiently strong disorder the system is instead in the many-body localized phase, characterized by the absence of transport of particles and logarithmic spreading of entanglement. The transition point is probed numerically and the phase diagram for the Bose–Hubbard Hamiltonian is constructed
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
Single qubit dephasing in the many-body localization phase transition
Get PDFWe are interested in a model for quantum computer that consists of superconducting qubits, transmons. These transmons are known to experience decoherence which causes unwanted dynamics in their quantum state. In this thesis, one form of decoherence experienced by the transmon, dephasing, is studied. The goal of this research is to differentiate the thermalization phase from the many-body localization phase in a transmon chain by studying dephasing of a single transmon. This phase transition is measured by simulating an array of transmons using the Bose–Hubbard model and calculating the dephasing rate R2 that a single transmon experiences. The many-body localization is shown to have significantly reduced dephasing rates compared to the rates observed at the thermalization phase in three different cases with varying ratio between the interaction strength and the hopping rate U/J = 2, 3.5 and 5. From these results we see that the phase transition is seen when the disorder strength is in the range of W/J = 10−15 on all three cases. An interesting future outlook could be to study the deeper features of the phase transition by examining the the effect of spin-echo pulses on the transmon dephasing
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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