DSpace Collection:
https://hdl.handle.net/1889/636
2024-09-21T20:37:14ZMolecular Spins for Quantum Information: New Platforms for Error Correction, Simulation and Initialization
https://hdl.handle.net/1889/5701
Title: Molecular Spins for Quantum Information: New Platforms for Error Correction, Simulation and Initialization
Authors: Chizzini, Mario
Abstract: Quantum information processing has emerged as a forefront area of research, holding the promise to address classical intractable problems. There are several areas in which quantum computers are expected to bring a disruptive contribution and one among all is the simulation of quantum systems. In the last decades, several quantum computing architectures have been proposed and experimentally tested: among the most developed are superconducting qubits, trapped ions, and photonic systems. However, despite the significant technological advancement, the path towards the realization of a scalable and fault-tolerant quantum computer remains full of challenges. This scenario underscores the need to explore alternative possibilities and an intriguing one are Molecular Nanomagnets (MNMs).
Discovered in the early 90s, these systems exhibit classical macroscopic magnet behavior while also showing quantum properties. This combination of classical and quantum properties makes them excellent candidates as building blocks for quantum architectures. Leveraging on the spin degrees of freedom inherent in these systems, they can show at low energy a well resolved energy spectrum addressable by electromagnetic pulses. These systems have naturally long coherence times that reach in some cases up to times on the order of milliseconds allowing a coherent control and manipulation. Furthermore, MNMs can be chemical engineered allowing for a precise tuning of their properties and thus a great flexibility for different applications.
This thesis collects the results of the research conducted by my group and me over the past three years. The thematic common at all works is the use of MNMs for the quantum architectures. Within this broad field, research can be divided into two primary strands: exploring the potential of a qudit-based computation and designing molecules that are intrinsically protected against decoherence. Regarding the former, a qudit approach can simplify quantum circuits by reducing the number of objects to be controlled and thus the number of two-body gates. This approach has been investigated both for quantum simulation and for quantum error correction applications. Regarding the latter, through the modelling of decoherence and the analysis of the mechanisms that generate it, it has been possible to identify a general property that systems must have in order to be intrinsically protected against decoherence. As in precedence, this property can be useful for both quantum computation and error correction. Besides these main topics, we have also worked on the design of time-resolved EPR experiments aimed at detecting chiral induced spin selectivity (CISS). Although this phenomenon is not fully understood, it is interesting for several applications, including quantum technologies. In particular, it could be exploited for the initialization and readout of MNMs. Finally, we have worked on state of art quantum computers exploring the variational quantum eigensolver algorithms. We have worked on the design of physically motivated ansatz, i.e., ansatz tailored to respect the properties of the target system such as conserved quantities or symmetries, showing that this approach can significantly extend the capabilities of these algorithms. As a study case, we look at small spin models.2024-01-01T00:00:00ZConfronting Large Fluctuations in Numerical Stochastic Perturbation Theory
https://hdl.handle.net/1889/5700
Title: Confronting Large Fluctuations in Numerical Stochastic Perturbation Theory
Authors: Baglioni, Paolo
Abstract: Perturbation theory is universally recognized as a fundamental tool in modern theoretical physics. In the functional integral formalism, perturbation theory provides a method for studying field theories, offering both mathematical rigor and substantial physical insights. It is challenging to name an area of theoretical physics where perturbation theory does not play a fundamental role: even in theories specifically designed for a non-perturbative approach like lattice gauge theories, perturbation theory remains relevant and interesting. Lattice gauge theories offer a powerful framework for understanding non-perturbative aspects of quantum field theories. By discretizing space-time on a lattice, these theories enable detailed Monte Carlo simulations that are crucial for probing phenomena beyond the reach of perturbation theory, shedding light on subtle features such as quark confinement in QCD and many others. In the mid-1990s, a new method was developed that in a sense integrates traditional perturbation theory with Monte Carlo simulations of lattice field theories (in particular lattice gauge theories, for which traditional diagrammatic perturbation theory is cumbersome). This approach is known as Numerical Stochastic Perturbation Theory (NSPT). NSPT offers a fully automated stochastic method for calculating loop corrections in lattice field theories, using the power of Monte Carlo simulations. Its numerical implementation requires minimal changes with respect to traditional Monte Carlo simulations; (also due to this) NSPT enables the calculation of loop corrections at very high perturbative orders. The ease of implementation and advanced capability explain why NSPT has captured the attention of lattice practitioners. A not-well explored feature of NSPT is the freedom to choose any vacuum for calculating perturbation theory, in principle without encountering the complexities of the diagrammatic perturbation theory. If one had to make a natural choice, low-dimensional models are the best candidates for exploratory analysis of the feasibility of perturbative expansions on top of non-trivial vacua. This way, one immediately encounters problems: it is known that NSPT simulations exhibit large fluctuations in low-dimensional models. As the perturbative order increases, huge fluctuations show up, completely obscuring the signal at even not-so-high perturbative orders.
In this thesis, we discuss NSPT simulations for a class of highly interesting low-dimensional models, the two-dimensional O(N) Non-Linear Sigma Models (NLSMs). O(N) non-linear sigma models can be regarded as a valuable theoretical laboratory in quantum field theory, as they display in a relatively simple framework interesting features like asymptotic freedom. From a more phenomenology oriented point of view, NLSM proved to be effective in modeling different features in different contexts. As we will see, in this work our interest for O(N) models is motivated by the possibility to tune N. On general grounds we expect that huge
fluctuations in simulations of low-dimensional models are somehow connected to the limited number of degrees of freedom. From this perspective, O(N) NLSMs are an ideal laboratory: in fact we can modify the number of degrees of freedom by tuning the parameter N. Our numerical results show that in the large N limit NSPT simulations are not affected by the large fluctuations issue at high orders, in contrast to what occurs in the small N regime. Our conclusions are supported by extensive numerical studies of the properties of NSPT distributions as function of the perturbative order n and the parameter N. While a fundamental comprehension is admittedly lacking, we will consider different indicators for assessing if (and to what extent) large enough N computations are to be regarded as safe at a given perturbative order n. In particular, the study of relative errors has been particularly fruitful: in this context, the onset of fluctuations has been probed through violations of very generally expected scaling behaviors. Our numerical simulations strongly suggest that indeed for each perturbative order n, an NSPT computation in O(N) can always be found safe with respect to fluctuations if we take a large enough N. As a result, the larger the value of N, the more perturbative corrections we could compute, significantly extending the previously known results from lattice diagrammatic perturbation theory.
Once for large enough N high perturbative orders can be safely computed, we expect we can explore the asymptotic behavior of perturbative expansions. In the context of lattice gauge theories, NSPT has proven to be effective in probing infrared renormalons. In the final part of the thesis, we discuss O(N) renormalon effects in the large N limit. We will perform computations on a pretty small lattice size, but we will provide new insights on the role of finite-volume effects. In particular, by explicitly taking into account the infrared cutoff, we obtained an analytic (first-principles) estimate of finite-volume effects, assessing how they modify the factorial scaling of coefficients. Once we have such a modeling, we can compare analytical predictions and numerical results, finding agreement in the asymptotic perturbative region. We stress that this will be a parameter-free comparison (there is no space for any parameter to adjsut). Large N NSPT simulations for O(N) models can also be regarded as a preliminary step towards going back to perturbative expansions around non-trivial vacua. Quite interestingly, such computations in the (quite close) CP(N-1) models are connected to resurgence scenarios.2024-01-01T00:00:00ZLooking for non-standard cosmologies: the critical role of numerical tools
https://hdl.handle.net/1889/5646
Title: Looking for non-standard cosmologies: the critical role of numerical tools
Authors: Piga, Lorenzo
Abstract: In this thesis we study the realm of non-standard cosmologies, with a particular emphasis on the role of numerical tools in probing beyond the standard Lambda Cold Dark Matter (LCDM) model. Despite the success of the LCDM model, questions persist regarding the true nature of dark matter (DM) and dark energy (DE). We investigate several candidates and mechanisms, including primordial black holes (PBHs) as a form of cold dark matter and the normal branch of Dvali-Gabadadze-Porrati (DGP) gravity as a dynamic DE mode.
A significant portion of this thesis is dedicated to studying PBHs, analyzing the constraints placed by cosmic microwave background (CMB) observations on matter accretion into PBHs, and considering the resulting emissions and their effect on the universe's thermal history. The work explores the complex interplay of accretion processes, outflows, and non-thermal emissions, which introduces considerable theoretical uncertainty in constraining PBH parameters.
In addressing the mystery of dark energy, we investigate the nDGP model using galaxy distribution data from the Baryon Oscillation Spectroscopic Survey (BOSS). Through the effective field theory of large-scale structure (EFTofLSS) and perturbative bias expansion, we establish limits on model parameters, showcasing the relationship between these parameters and their degeneracies.
Furthermore, the thesis contributes a novel adaptive interpolation tool, leveraging hydrodynamic simulations and the Optimized Kriging technique, to analyze the Lyman-alpha forest data. This allows for enhanced constraints on cosmological models, addressing tensions such as those in the amplitude of the linear power spectrum.
The prospective data from the Euclid satellite is anticipated to test the methodologies presented herein, offering a window into the fundamental properties of DM and DE.
This thesis underlines the potential of combining astrophysics, astroparticle physics, and cosmology, using numerical simulations and machine learning tools to test a plethora of non-standard cosmologies. The developed techniques stand as a testament to the evolving landscape of cosmological research, advocating for a synergistic approach to unravel the universe's most enigmaticÂ constituents.2024-01-01T00:00:00ZA correlative and multimodal microscopy approach to study the interaction of photosensitizers with biological samples
https://hdl.handle.net/1889/5645
Title: A correlative and multimodal microscopy approach to study the interaction of photosensitizers with biological samples
Authors: Mariangeli, Matteo
Abstract: The thesis presented a comprehensive study on the interaction and the effects of the photosensitizer Hypericin with a model of enveloped virus, i.e. SARS-CoV-2, and lipid bilayers resembling the viral envelope. Multiple biophysical techniques were employed, either optical or scanning probe methods, also used in a correlative way.; La tesi ha presentato uno studio approfondito sull'interazione e gli effetti del fotosensibilizzatore Ipericina con un modello di virus a envelope, ovvero SARS-CoV-2, e membrane modello lipidiche che assomigliano all'involucro virale. Sono state impiegate molteplici tecniche biofisiche, sia ottiche che di scansione di sonda, utilizzate anche in modo correlativo.2024-01-01T00:00:00Z