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- ItemQuantum measurement transition and entanglement of trapped ions and optomechanical systems(2024) Araya Sossa, Kevin Jordan; Orszag Posa, Miguel; Pontificia Universidad Católica de Chile. Instituto de FísicaAlthough quantum mechanics has been able to explain a wide range of physical, chemical, and even biological events with unprecedented accuracy, fundamental problems remain. For instance, the problem of quantum measurement and quantum entanglement, which are the most perplexing problems that have persisted since the foundation of quantum mechanics. Both are crucial quantum resources with broad applications in quantum information science, quantum computing and quantum optics. For this reason, this thesis is devoted to research the quantum measurement from the weakest regime to the strongest one as well as the dynamics of entanglement of different quantum systems. In this work, we study the measurement transition for a coherent-squeezed pointer state through a transition factor Γ that involves a system-pointer coupling by using an arbitrary measured observable A. In addition, we show that the shift in the pointer’s position and momentum establishes a relationship with a new value defined as the transition value, which generalizes the weak value as well as the conditional expectation value. Furthermore, a new strategy is introduced to achieve different measurement regimes by just adjusting the r and ϕξ parameters of the coherent-squeezed pointer state, opening an interesting way to test quantum mechanics foundations. Our scheme has been theoretically applied in a trapped ion illuminated by a bichromatic laser beam, with a high potential to be implemented in future experimental setups. Besides, we propose a method to regulate the quantum entanglement in the system mentioned before as well as a dispersive-hybrid system where a qubit is directly coupled to a cavity and a mechanical resonator. Entanglement can be controlled by only tuning the squeezing parameters associated with the vibrational mode. As the squeezing amplitude becomes larger, the maximal entanglement abruptly falls to zero at specific squeezing phases. For the hybrid system, it is also possible to generate entanglement for bipartitions from the qubit-cavity-resonator system after applying this strategy. Entangled qubit-cavity states are created through squeezing, even though there is no direct interaction between them. We also analyze the effect of atomic, optical, and vibrational losses on the quantum entanglement. We finally discuss our schemes to be implemented in future experimental setups and promote further studies to generalize the concept of “monogamy of entanglement” in tripartite systems outside qubit-composite states, in particular, (2 ⊗ 2 ⊗ n)-dimensional systems.
- ItemCharacterization of extragalactic fast X-ray transients from X-ray archival searches(2023) Quirola Vasquez, Jonathan Alexander; Bauer, Franz Erik; Jonker, Peter; Pontificia Universidad Católica de Chile. Instituto de AstrofísicaExtragalactic fast X-ray transients (FXTs) are non-Galactic short flashes of X-ray photons (in the narrow range of ≈0.3–10 keV) of unclear origin that last a few minutes to hours. A variety of astronomical objects and physical mechanisms have been proposed for the origin of extragalactic FXTs, such as core-collapse SNe shock breakout (SBOs), gamma-ray bursts (GRBs), and intermediate massive black holes (IMBH)-white dwarf (WD) tidal disruption events. During the last two decades, several FXTs have been detected by Chandra, XMM-Newton, and Swift-XRT, serendipitously (for instance, Soderberg et al. 2008; Bauer et al. 2017; Xue et al. 2019; Alp & Larsson 2020; Lin et al. 2022). Previously, Yang et al. (2019) developed a method that can efficiently detect single X-ray burst light curves in a single Chandra exposure, and systematically applied it to ≈19 Ms Chandra. While this method efficiently detected all past known FXTs (2), it failed to find any new FXT candidates, setting loose bounds on their space densities. The main objective of this thesis is to identify and characterize extragalactic FXTs hidden in the Chandra archive. We apply here two modified versions of the algorithm developed by Yang et al. 2019 to X-ray sources located at |b|>10 deg (i.e., 14281 Chandra observations, totaling ≈258 Ms and 857 deg 2 ) to minimize stellar flares contamination. In Chapter 2, we consider the X-ray sources of the Chandra Source Catalog 2.0 (data available until the end of 2014; CSC2). In Chapter 3, we extend our systematic search by reprocessing the Chandra data not covered by CSC2. In both instances, we adopt additional criteria to rule out strong contamination from persistent X-ray sources (analyzing further X-ray observations taken by Chandra, XMM-Newton, Swift–XRT, Einstein, and ROSAT, and considering other astronomical catalogs such as Gaia, NED, SIMBAD, VHS, DES, Pan-STARRS), in order to identify 22 FXTs (14 and 8 FXTs identified inside CSC2 and beyond it, respectively) consistent with an extragalactic origin. We rediscover all previously reported Chandra events from the literature (Jonker et al. 2013; Glennie et al. 2015; Bauer et al. 2017; Xue et al. 2019; Lin et al. 2019, 2021, 2022). The 22 FXT candidates have peak 0.3–10 keV fluxes between F X,peak ≈6×10^−14 to 2×10^−10 erg cm−2 s−1 and T 90 durations from ≈0.3 to 40 ks. The sample is split into two groups: five "nearby" FXTs that occurred within d≲100 Mpc, and 17 "distant" FXTs at d>100 Mpc. Indeed, the latter have redshifts between ≈0.3 to 2.2. Thus, the local and distant samples have associated peak X-ray luminosities of L X,peak ≈10^39 − 10^40 and 10^44 − 10^47 erg s−1, respectively. After applying completeness corrections, we calculate the first FXT X-ray luminosity function and derive event rates for the nearby and distant samples of 34.3_{−10.8}^{+13.7} and 36.9_{−8.3}^{+9.7} deg-2 yr−1, respectively, for a limiting flux of Fpeak=10^−13 erg cm−2 s−1. We compare the volumetric density rate of FXTs with well-known transient classes such as SBOs, GRBs, and TDEs, concluding that FXTs remain broadly consistent with different transients at distinct cosmic epochs. Regarding their host properties, local hosts tend to lie just below the star-forming main sequence, with many FXTs situated in or near HII regions, implying some relation to massive stars. On the other hand, distant hosts tend to be spread all over (starburst, main sequence, and green valley regions), potentially consistent with GRB and SNe hosts. Timing and spectral properties, combined with other properties such as galactic parameters and volumetric rates, might imply that we have a mix of origins related to this novel sample of FXTs. Finally, in Chapter 4 we interpret a subset of nine FXTs with plateau or fast-rise light curves in the context of an X-ray magnetar model produced after the merger of two neutron stars. The model produces good fits to the light curves of this sub-sample, and the best-fit magnetar parameters suggest a common origin. Although the interpretation is consistent with most of the observational parameters, exploring other scenarios remains a necessary future task.
- ItemTransport phenomena in nontrivial topological materials(2023) Bonilla Moreno, Daniel Alejandro; Muñoz Tavera, Enrique; Pontificia Universidad Católica de Chile. Instituto de FísicaIn this Ph.D. thesis, we present our work related to electronic quantum transport in materials with nontrivial topology. The fundamental objectives of our work were as follows: Firstly, to study ballistic transport in a nano junction made of a Type I Weyl semimetal material that contains a cylindrical defect created by the application of mechanical strain. In addition to the torsion effect modeled by a pseudo-gauge field, we added an external magnetic field and the repulsive effect of the deformation produced by the mismatch of the crystal lattice. Using the appropriate Landauer ballistic formalism to describe this type of system, we calculated their transport coefficients. Secondly, to study diffusive transport using the linear response regime, of a uniform and diluted concentration of the aforementioned defects through the bulk of a Weyl semimetal slab. For this purpose, we used the standard particle scattering theory, along with Green's functions techniques and diagrammatic methods. Finally, to study the diffusive transport through a single-layer graphene sheet doped with charged impurities, and influenced by the electromagnetic coupling to a topological insulator or a semiconductor. We pursued to investigate the role played by the magneto-electric effect produced by the topological insulator in transport properties, such as electrical conductivity. Here, we also applied a combination of methods based on scattering, linear response, Green's functions, and diagrammatics. We have obtained analytical expressions for the electrical and thermal conductivities, as well as for the Seebeck coefficient. Our results demonstrate the promising nature of these novel topological materials as thermoelectrics for future applications.
- ItemLoops in Holographic Correlators(2023) Muñoz Sandoval, Iván Ignacio; Bañados, Máximo; Pontificia Universidad Católica de Chile. Instituto de FísicaIn the context of the Anti de-Sitter (AdS)/Conformal Field Theory (CFT) correspondence, we investigate the computation of holographic correlation functions for quantum fields in the bulk. Unlike the semi-classical approach, quantum computations involve Infra-Red (IR) and Ultra-Violet (UV) divergences. However, consistent with the semiclassical approximation, we find that IR infinities correspond to boundary divergences, while UV divergences correspond to the bulk. We present a systematic procedure for solving the perturbative quantum problem in the bulk. To illustrate our approach, we consider a Φ4 scalar field on a fixed AdS background and obtain the boundary correlation function in position and momentum space. In position space, we use two approximations: (i) we assume that the field is composed of the classical solution plus a quantum fluctuation, and we solve the classical part before using the holographic dictionary to obtain the quantum correction to the 2- and 4-point functions, requiring UV and IR renormalizations;(ii) using the quantum effective action, we renormalize the UV divergence from the equation of motions and then use the holographic dictionary to obtain the dual correlation function. Both formulations lead to the same conclusions and demonstrate that the bulk theory is renormalizable up to AdS7. Meanwhile, in momentum space, we use the background field method and renormalize the two-point function up to one loop, finding exact agreement with the position space computation. Finally, we provide a general set-up for obtaining the off-shell graviton bulk propagator, which is crucial for obtaining correlation functions for more realistic models.
- ItemAmorphous materials under stresses: understanding critical behavior(2022) Villarroel Cortés, Carlos Javier; Düring, Gustavo; Pontificia Universidad Católica de Chile. Facultad de FísicaGran parte de los materiales que utilizamos a diario no se comportan como un sólido elástico o un fluido newtoniano cuando se les aplica deformación. Dentro de la extensa lista de estos materiales que presentan características no lineales en cantidades macroscópicas, podemos encontrar muchos alimentos que consumimos o incluso la piel humana. Comprender estos comportamientos particulares actualmente representa un desafóo importante con aplicaciones en la industria y la medicina. Para responder algunas de las preguntas que estos materiales particulares presentan hoy en día, en esta tesis, utilizando simulaciones numéricas de alto nivel, estudiamos dos fenómenos no lineales críticos, el fenómeno de “Yielding” y el fenómeno de “Strain-Stiffening”. En particular, la transición de “Yielding” se observa en materiales donde, dependiendo de la tensión aplicada, es posible pasar de un estado mecánicamente estable a uno que fluye como un líquido. En este contexto, mediante simulaciones de partículas blandas, se realiza el cálculo de los exponentes críticos que gobiernan el régimen fluido para dos escenarios de esfuerzos aplicados, y se estudia cómo las estadísticas de avalanchas pueden caracterizar el flujo. Finalmente, para el fenómeno “Strain-Stiffening”, se propone un nuevo modelo de redes semi-flexibles capaces de replicar este comportamiento, donde un sistema blando se transforma en uno rígido mediante la aplicación de una deformación. A su vez, este modelo es capaz de explicar y predecir de buena manera los exponentes críticos que gobiernan la transición.