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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
Toista tasoa korkeammalle transmonhiloissa: perustilafaasit ja dynamiikka
No full textAbstractSuperconducting qubits are one of the most promising platforms for the construc-tion of quantum computers. The state-of-the-art qubits and associated control gates are unfortunately still too prone to errors for general purpose quantum computing to be feasible. However, the circuits used to build the qubits can already be utilised in analogue quantum simulation. That is, they can be used as a building block of relatively simple and easy to control quantum mechanical device, used to emulate some aspects of more complicated systems. Here we focus specifically on the transmon, which is currently the most common qubit design. The few lowest energy levels of transmons can be described as an anharmonic oscillator. While the anharmonicity allows the two lowest levels to be used as a qubit, that does not mean that the higher levels should necessarily be neglected. Beyond the two-level approximation, the behaviour of transmon arrays can be well approximated with the Bose–Hubbard model with attractive interactions. Here we discuss analytical and numerical studies of the model, focusing on the higher excited levels of transmons.We begin by constructing ground state phase diagrams of the attractive model for constant numbers of bosons, including the effect of disorder, an unavoidable feature of manufactured devices. In the phase diagrams we find three distinct phases: the localised phase at strong disorder, the superfluid phase at high hopping frequencies, and the W phase when both disorder and hopping are dominated by the attractive interactions. Next, we study the dynamics of the model, utilising the wide-gapped band structure of the Bose–Hubbard spectrum. We describe the dynamics within each band with a compact and unified framework based on high-order degenerate perturbation theory. The unitary dynamics effectively occur within a single band, resulting in various forms of collective behaviour, such as bosons on a single site moving as a single quasiparticle and effective longer-range interactions between multiple quasiparticles. Finally, to account for the imperfect isolation of the transmons from their environment, we include dissipation and dephasing into our model of the dynamics. We provide analytical descriptions on how the environment affects the dynamics within the bands, and causes transitions between them.Original papersOriginal papers are not included in the electronic version of the dissertation.Mansikkamäki, O., Laine, S., & Silveri, M. (2021). Phases of the disordered Bose-Hubbard model with attractive interactions. Physical Review B, 103(22), L220202. https://doi.org/10.1103/PhysRevB.103.L220202Self-archived versionMansikkamäki, O., Laine, S., Piltonen, A., & Silveri, M. (2022). Beyond hard-core bosons in transmon arrays. PRX Quantum, 3(4), 040314. https://doi.org/10.1103/PRXQuantum.3.040314Self-archived versionBusel, O., Laine, S., Mansikkamäki, O., & Silveri, M. (2023). Dissipation and dephasing of interacting photons in transmon arrays. Manuscript submitted for publication.OsajulkaisutOsajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.Mansikkamäki, O., Laine, S., & Silveri, M. (2021). Phases of the disordered Bose-Hubbard model with attractive interactions. Physical Review B, 103(22), L220202. https://doi.org/10.1103/PhysRevB.103.L220202Rinnakkaistallennettu versioMansikkamäki, O., Laine, S., Piltonen, A., & Silveri, M. (2022). Beyond hard-core bosons in transmon arrays. PRX Quantum, 3(4), 040314. https://doi.org/10.1103/PRXQuantum.3.040314Rinnakkaistallennettu versioBusel, O., Laine, S., Mansikkamäki, O., & Silveri, M. (2023). Dissipation and dephasing of interacting photons in transmon arrays. Manuscript submitted for publication.Academic 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 L6, on April 21st, 2023, at 12 o’clock noonAbstract
Superconducting qubits are one of the most promising platforms for the construc-tion of quantum computers. The state-of-the-art qubits and associated control gates are unfortunately still too prone to errors for general purpose quantum computing to be feasible. However, the circuits used to build the qubits can already be utilised in analogue quantum simulation. That is, they can be used as a building block of relatively simple and easy to control quantum mechanical device, used to emulate some aspects of more complicated systems. Here we focus specifically on the transmon, which is currently the most common qubit design. The few lowest energy levels of transmons can be described as an anharmonic oscillator. While the anharmonicity allows the two lowest levels to be used as a qubit, that does not mean that the higher levels should necessarily be neglected. Beyond the two-level approximation, the behaviour of transmon arrays can be well approximated with the Bose–Hubbard model with attractive interactions. Here we discuss analytical and numerical studies of the model, focusing on the higher excited levels of transmons.
We begin by constructing ground state phase diagrams of the attractive model for constant numbers of bosons, including the effect of disorder, an unavoidable feature of manufactured devices. In the phase diagrams we find three distinct phases: the localised phase at strong disorder, the superfluid phase at high hopping frequencies, and the W phase when both disorder and hopping are dominated by the attractive interactions. Next, we study the dynamics of the model, utilising the wide-gapped band structure of the Bose–Hubbard spectrum. We describe the dynamics within each band with a compact and unified framework based on high-order degenerate perturbation theory. The unitary dynamics effectively occur within a single band, resulting in various forms of collective behaviour, such as bosons on a single site moving as a single quasiparticle and effective longer-range interactions between multiple quasiparticles. Finally, to account for the imperfect isolation of the transmons from their environment, we include dissipation and dephasing into our model of the dynamics. We provide analytical descriptions on how the environment affects the dynamics within the bands, and causes transitions between them
Noi, bambine ad Auschwitz
No full textLa storia delle sorelle Bucci; una testimonianza biografica di due testimoni, sopravvissute alla Shoah
Naming of grammatical classes in frontotemporal dementias: Linguistic and non linguistic factors contribute to noun-verb dissociation
No full textWe studied noun and verb naming in three main variants of frontotemporal dementia: the frontal variant(Fv-FTD), primary progressive aphasia (PPA) and semantic dementia (SD). We further distinguished PPA in nonfluent and fluent forms and restricted diagnosis of SD to subjects with progressive semantic breakdown leading to agnosia for words and objects. Fv-FTD and nonfluent-PPA named objects better than actions, SD showed an inverse dissociation and no specific pattern emerged in fluent-PPA. In this last group, in spite of the broad definition of fluent aphasia, quite heterogeneous patterns of language disorders and word class dissociation emerged when single-subject analyses were performed. In fv-FTD correlations between executive tasks and action naming were stronger than between executive tasks and object naming. We conclude that both linguistic and non linguistic factors, in particular an executive deficit, contribute to grammatical class dissociation. We also suggest that the fluent vs. nonfluent distinction does not reflect the complexity of primary aphasia. © 2007 - IOS Press and the authors. All rights reserved
Semantic memory in object use
No full textWe studied five patients with semantic memory disorders, four with semantic dementia and one with herpes simplex virus encephalitis, to investigate the involvement of semantic conceptual knowledge in object use. Comparisons between patients who had semantic deficits of different severity, as well as the follow-up, showed that the ability to use objects was largely preserved when the deficit was mild but progressively decayed as the deficit became more severe. Naming was generally more impaired than object use. Production tasks (pantomime execution and actual object use) and comprehension tasks (pantomime recognition and action recognition) as well as functional knowledge about objects were impaired when the semantic deficit was severe. Semantic and unrelated errors were produced during object use, but actions were always fluent and patients performed normally on a novel tools task in which the semantic demand was minimal. Patients with severe semantic deficits scored borderline on ideational apraxia tasks. Our data indicate that functional semantic knowledge is crucial for using objects in a conventional way and suggest that non-semantic factors, mainly non-declarative components of memory, might compensate to some extent for semantic disorders and guarantee some residual ability to use very common objects independently of semantic knowledge. © 2009 Elsevier Ltd. All rights reserved
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