Faculty of Science
Recent Submissions
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(2024)A quantum computer is a new kind of computer which utilizes quantum phenomena in computing. This machine has the potential to solve specific tasks faster than the most powerful supercomputers and therefore has potential real-life applications across various sectors of society. One promising approach to realize a quantum computer is to store information in superconducting qubits, which are artificial two-level quantum systems made from superconducting electrical circuits. Extremely precise control of these qubits is essential but also challenging due to the excitations out of the two lowest energy states of the quantum system that constitute the computational subspace. In this thesis, we propose a new way to control a superconducting multimode qubit using the unimon qubit as an example. By coupling differently to the different modes of the multimode qubit circuit, we cancel the transition from the first excited state to the second excited state, which is typically the main transition causing a leakage out of the computational subspace. We present a theoretical description of this model by utilizing methods of circuit quantum electrodynamics to compute the energy spectrum and the transition matrix elements of the qubit. By using these results, we simulate the dynamics of the driven unimon qubit undergoing as a single-qubit gate. The result of the simulation shows that this method decreases the leakage relative to the conventional method of driving a qubit, where only one external drive is applied. However, by improving the conventional method with a more advanced pulse optimization method, the leakage becomes smaller than in the standard case of two drive fields. In addition, we find that the practical implementation of our method may be sensitive to variations in the qubit parameters. Therefore, the practical implementation of the method needs further research in the future. By cancelling one energy-level transition of the qubit, we find that other transitions in modes of similar frequency were strongly suppressed. Therefore, this method might be potentially utilized in other qubit operations than the quantum gates, such as in the qubit resetting process, where driving to higher frequency modes of the unimon is preferred.
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(2024)Despite the continuous efforts to unveil the true nature of Dark Matter (DM), it still remains as a mystery. In this thesis we propose one model that can produce the correct relic abundance of DM in the current Universe, while fitting into the existing experimentally obtained constraints. In this model we add a singlet fermion, which is a not completely sterile right-handed neutrino, and a heavy real scalar singlet into the Standard Model of Particle Physics (SM) and carry out the relic density calculations. The DM candidate here is the singlet fermion, which acts as a thermally decoupled Weakly Interacting Massive Particle. Theoretical framework is laid out in detail. Special attention is given to obtaining the definition for relic abundance from Lee-Weinberg equation in terms of yield, and the decoupling temperature. It is found, that the usual way of handling the thermally averaged cross section appearing in these definitions is not suitable in this case. In fact, the usual approximations can only be done when thermally averaged cross section is almost linear in $s$, and this is a demand that very few models can satisfy. The correct way to treat the cross section by taking the expansion in terms of the relative velocity is presented with careful attention to detail. This methodology is then applied to the extension of the SM we introduced. Only tree-level processes are being considered. Cross sections are calculated for each possible process to obtain the total cross section needed for the DM relic density calculations. We present how the different free parameters in the theory affect the relic abundance and what masses are allowed for the right-handed neutrino to obtain. It is found out that the parameters in this model are heavily constrained. Yet the model is able to fit into the constraints obtained from branching ratio and direct detection (DD) experiments, while producing the correct relic density. This is true when the mixing angle $\theta$ is of the order $1 \times 10^{-4}$, and right-handed neutrino has the mass of exactly half of the mass of the heavy scalar or higher than the mass of the heavy scalar. It is proposed that allowing lepton mixing and adding a separate mass term for fermion in the model could make the model less restricted. Investigating this would be interesting thing to do in the future. However, the proposed DM candidate remains viable and the upcoming DD experiments will relatively soon reveal if the singlet fermion is the DM particle we are seeking.
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(2024)The planetary boundary layer (PBL) is a layer of the atmosphere directly influenced by the presence of Earth's surface. In addition to its importance to the weather and climate systems, it plays significant role in controlling the air pollution levels and low-level heat conditions, thereby directly influencing the general well-being. While the modification of the boundary layer conditions by varying atmospheric forcings has been widely studied and discussed, it remains unknown what the dominant states of the PBL variation in response to this modification are. In this study, the dominant boundary layer types in both daytime and nighttime layers are examined. To understand the factors contributing to the development of these layers, weather regimes in the northern Atlantic-European region are considered. Machine learning techniques are utilized to study both the boundary layer and the large-scale flow classes, with an emphasis on unsupervised learning methods. It was found that the boundary layers in Helsinki, Finland, can be categorized into four daytime and three nighttime boundary layers, each characterized by the dominant turbulence production mechanism or the absence thereof. During the daytime, layers driven by both mechanical and buoyant turbulence are observed in summer, autumn, and spring, while individually buoyancy-driven layers occur in summer and winter, and individually mechanically-driven layers emerge in autumn, winter, and spring. Additionally, a layer characterized by overall reduced turbulence production is present throughout all seasons. During the nighttime, all three boundary layer types---individually buoyancy-driven, individually mechanically-driven, and stable layer---are observed in all seasons. Each boundary layer type exhibits season-specific variations, whereas daytime and nighttime boundary layers driven by the same mechanisms reflect the diurnal cycle of their relative intensities. The analysis revealed that the weather regimes producing cyclonic and anticyclonic flow anomalies over southern Finland collectively influence the boundary layer conditions, whereas the impact of individual weather regimes remains relatively small. Large-scale flow variation is associated with changes in the boundary layer dynamics through alterations in surface radiation budget (cloudiness) and wind conditions, thereby influencing the relative intensities of mechanical and buoyant turbulence production. However, inconsistencies in the analysis suggest that additional mechanisms, such as mesoscale phenomena, must also contribute to the development of the observed boundary layer types.
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(2024)Ultra-low frequency (ULF) waves in the Pc4-Pc5, 2 – 25 mHz range have been observed to accelerate trapped 1 – 10 MeV electrons in the Earth’s radiation belts. This acceleration can lead to particle losses and injections that occur on timescales comparable to the particle drift periods. Current models rely on diffusion equations written in terms of Fokker-Planck equations and are not suitable for describing fast temporal variations in the distribution function. This thesis is a study of fast transport of equatorially trapped electrons in the radiation belts. We look at solutions for the time evolution of the linear part of the perturbed distribution function using both analytical and numerical methods. Based on this work we build a simple model of fast transport in the radiation belts using a spectral PDE framework called Dedalus. The resulting program is a computationally inexpensive, simple approach to modelling drift-periodic signatures on fast timescales. In this study we investigate the behavior of the distribution function in three systems: a simple system without a wave term, and systems with a single non-resonant and resonant ULF wave. The wave solutions are evaluated with magnetic field perturbations of different magnitudes. The Earth’s magnetic field is modelled with the Mead field. The numerical solution of the perturbed differential equation is studied for relativistic equatorially trapped electrons. Phase-mixing is found to happen regardless of field fluctuations or resonance. The non-resonant wave solution shows time-delayed, spatially localized structures in the equatorial plane forming in the presence of large magnetic field fluctuations. These transients are also seen in the analytical solution and provide a new theoretical explanation for the ubiquitous observation of drift echoes in the inner and outer radiation belts of the Earth (Li et al., 2024).
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(2024)This thesis studies gas-rich galaxy mergers at redshifts of z ∼ 1-2 using numerical simulations, with a particular focus on the effect of feedback from active galactic nuclei (AGNs). In total, 16 galaxy mergers at redshifts z = 1 and z = 2 were modeled using the simulation codes KETJU and GADGET-3. The simulations were performed on the supercomputer Mahti located at the Finnish IT Centre for Science (CSC). AGN feedback can be described as the radiative and mechanical energy released through accretion, which act to heat and disperse the remaining gaseous material surrounding the central supermassive black hole (SMBH). The feedback mechanisms include, for example, photoionization heating due to high-energy photons and winds and jets driven by the AGN. Numerically, AGN feedback was implemented using two models in this thesis: thermal and kinetic AGN feedback, in which the gas particles are either heated or ‘kicked’, respectively. In addition to AGN feedback, the simulations included metal-dependent gas cooling, stochastic star formation, and stellar feedback. The simulated progenitor galaxies were gas-rich spirals consistent with observed galaxies at redshifts z = 1 and z = 2. The virial masses of the progenitors were set to correspond to typical massive galaxies at their redshifts using the Press-Schechter mass function, while the initial masses for the central SMBHs were set using observed MBH-M⋆ and MBH-σ⋆ relations. The gas fractions and metal abundances of the progenitors were calibrated using observational data at their respective redshifts. The KETJU and GADGET-3 simulations produced very similar results for the overall evolution of a given merger configuration. Consistent with earlier studies, the kinetic feedback was observed to be significantly more effective at removing gas from the galaxies than the thermal feedback. The combined effect of AGN and stellar feedback was observed to strongly suppress star formation, with the star formation of one merger being almost completely shut down. The thermal and kinetic feedback models caused noticeable differences in the orbital evolution of the SMBH binaries. Merger timescales were significantly longer for the SMBHs in the KETJU simulations with kinetic feedback. In general, the merger timescales increased with decreasing initial eccentricity for the SMBH binary. The merger remnants were compared to observed MBH-σ⋆, R1/2-M⋆, fgas-M⋆, and mass-metallicity relations. Overall, the remnants were reasonably consistent with the observed relations. Hence, we can conclude that AGN feedback plays a crucial role in galaxy evolution and that both the thermal and kinetic feedback models are able to produce realistic high-redshift galaxies.