Non-Unitarity, sterile neutrinos, and Non-Standard neutrino Interactions

The simplest Standard Model extension to explain neutrino masses involves the addition of right-handed neutrinos. At some level, this extension will impact neutrino oscillation searches. In this work we explore the differences and similarities between the case in which these neutrinos are kinematically accessible (sterile neutrinos) or not (mixing matrix non-unitarity). We clarify apparent inconsistencies in the present literature when using different parametrizations to describe these effects and recast both limits in the popular neutrino non-standard interaction (NSI) formalism. We find that, in the limit in which sterile oscillations are averaged out at the near detector, their effects at the far detector coincide with non-unitarity at leading order, even in presence of a matter potential. We also summarize the present bounds existing in both limits and compare them with the expected sensitivities of near future facilities taking the DUNE proposal as a benchmark. We conclude that non-unitarity effects are too constrained to impact present or near future neutrino oscillation facilities but that sterile neutrinos can play an important role at long baseline experiments. The role of the near detector is also discussed in detail.Comment: 19 pages, 2 figures: minor changes and references added, version published in JHE

The superadditivity effects of quantum capacity decrease with the dimension for qudit depolarizing channels

Quantum channel capacity is a fundamental quantity in order to understand how good can quantum information be transmitted or corrected when subjected to noise. However, it is generally not known how to compute such quantities, since the quantum channel coherent information is not additive for all channels, implying that it must be maximized over an unbounded number of channel uses. This leads to the phenomenon known as superadditivity, which refers to the fact that the regularized coherent information of $n$ channel uses exceeds one-shot coherent information. In this article, we study how the gain in quantum capacity of qudit depolarizing channels relates to the dimension of the systems considered. We make use of an argument based on the no-cloning bound in order to proof that the possible superadditive effects decrease as a function of the dimension for such family of channels. In addition, we prove that the capacity of the qudit depolarizing channel coincides with the coherent information when $d\rightarrow\infty$. We conclude that when high dimensional qudits experiencing depolarizing noise are considered, the coherent information of the channel is not only an achievable rate but essentially the maximum possible rate for any quantum block code.Comment: 7 pages, 2 figure

Sediment connectivity for Leitzaran river basin.

El río Leitzaran es el principal afluente del río Oria, que desemboca en el mar Cantábrico. Dentro de los ríos de Gipuzkoa se trata de un valle singular y que ha logrado mantener un grado de naturalidad alto. Para poder hacer un planeamiento territorial óptimo, así como mejorar la calidad geomorfológica y ecológica del río, conocer la conectividad de sedimentos de los sistemas fluviales es un requisito indispensable. Por ello se ha sometido a estudio la cuenca del río Leitzaran, donde se ha implementado el índice de conectividad de sedimentos desarrollado por Cavalli et al., (2013), mediante diferentes técnicas SIG. Por una parte, se ha relacionado la conectividad de sedimentos con la propia rugosidad de la topografía; por otra parte, con los cambios de usos de suelo entre los años 1956 y 2018, que han producido cambios significativos en la conectividad de la cuenca. Además, se han identificado las fuentes de sedimentos de la cuenca, ya que el conocimiento de áreas específicas que aportan sedimentos a la red de drenaje es necesario para el estudio del transporte de sedimentos. Los resultados sugieren que la conectividad general de la cuenca ha mejorado a lo largo de los años y apuntan a que las fuertes pendientes de la cuenca condicionan completamente el índice de conectividad, dejando los usos de suelo en un segundo plano.The Leitzaran River is the main tributary of the Oria River, which empties into the Cantabrian Sea. Within the rivers of Gipuzkoa it is a unique valley that has managed to maintain a high degree of naturalness. To be able to do an optimal territorial planning, as well as to improve the geomorphological and ecological quality of the river, knowing the sediment connectivity of the river systems is an indispensable requirement. For this reason, the Leitzaran river basin has been studied, where the sediment connectivity index developed by Cavalli et al. (2013), through different GIS techniques, has been implemented. On the one hand, sediment connectivity has been related to the roughness of the topography itself; on the other hand, with the changes in land use between 1956 and 2018, which have produced significant changes in the connectivity of the basin. In addition, the sources of sediments in the basin have been identified, since the knowledge of specific areas that contribute sediments to the drainage network is necessary for the study of sediment transport. The results suggest that the general connectivity of the basin has improved over the years and suggest that the steep slopes of the basin completely condition the connectivity index, leaving land uses in the background.Depto. de GeografíaFac. de Geografía e HistoriaTRUEunpu

Performance enhancement of surface codes via recursive MWPM decoding

The minimum weight perfect matching (MWPM) decoder is the standard decoding strategy for quantum surface codes. However, it suffers a harsh decrease in performance when subjected to biased or non-identical quantum noise. In this work, we modify the conventional MWPM decoder so that it considers the biases, the non-uniformities and the relationship between $X$, $Y$ and $Z$ errors of the constituent qubits of a given surface code. Our modified approach, which we refer to as the recursive MWPM decoder, obtains an $18\%$ improvement in the probability threshold $p_{th}$ under depolarizing noise. We also obtain significant performance improvements when considering biased noise and independent non-identically distributed (i.ni.d.) error models derived from measurements performed on state-of-the-art quantum processors. In fact, when subjected to i.ni.d. noise, the recursive MWPM decoder yields a performance improvement of $105.5\%$ over the conventional MWPM strategy and, in some cases, it even surpasses the performance obtained over the well-known depolarizing channel

Decoding algorithms for surface codes

Quantum technologies have the potential to solve computationally hard problems that are intractable via classical means. Unfortunately, the unstable nature of quantum information makes it prone to errors. For this reason, quantum error correction is an invaluable tool to make quantum information reliable and enable the ultimate goal of fault-tolerant quantum computing. Surface codes currently stand as the most promising candidates to build error corrected qubits given their two-dimensional architecture, a requirement of only local operations, and high tolerance to quantum noise. Decoding algorithms are an integral component of any error correction scheme, as they are tasked with producing accurate estimates of the errors that affect quantum information, so that it can subsequently be corrected. A critical aspect of decoding algorithms is their speed, since the quantum state will suffer additional errors with the passage of time. This poses a connundrum-like tradeoff, where decoding performance is improved at the expense of complexity and viceversa. In this review, a thorough discussion of state-of-the-art surface code decoding algorithms is provided. The core operation of these methods is described along with existing variants that show promise for improved results. In addition, both the decoding performance, in terms of error correction capability, and decoding complexity, are compared. A review of the existing software tools regarding surface code decoding is also provided.Comment: 54 pages, 31 figure

Collective and non-collective molecular dynamics in a ferroelectric nematic liquid crystal studied by broadband dielectric spectroscopy

A great deal of effort has been recently devoted to the study of dielectric relaxation processes in ferroelectric nematic liquid crystals, yet their interpretation remains unclear. In this work, we present the results of broadband dielectric spectroscopy experiments of a prototypical ferroelectric nematogen in the frequency range 10 Hz-110 MHz at different electrode separations and under the application of DC bias fields. The results evidence a complex behavior in all phases due to the magnitude of polar correlations in these systems. The observed modes have been assigned to different relaxation mechanisms based on existing theoretical frameworks.Comment: The following article has been submitted to The Journal of Chemical Physics. After it is published, it will be found at https://pubs.aip.org/aip/jc

Multi-qubit time-varying quantum channels for NISQ-era superconducting quantum processors

Recent experimental studies have shown that the relaxation time ($T_1$) and the dephasing time ($T_2$) of superconducting qubits fluctuate considerably over time. To appropriately consider this time-varying nature of the $T_1$ and $T_2$ parameters, a new class of quantum channels, known as Time-Varying Quantum Channels (TVQCs), has been proposed. In previous works, realizations of multi-qubit TVQCs have been assumed to be equal for all the qubits of an error correction block, implying that the random variables that describe the fluctuations of $T_1$ and $T_2$ are block-to-block uncorrelated, but qubit-wise perfectly correlated for the same block. Physically, the fluctuations of these decoherence parameters are explained by the incoherent coupling of the qubits with unstable near-resonant two-level-systems (TLS), which indicates that such variations may be local to each of the qubits of the system. In this article, we perform a correlation analysis of the fluctuations of the relaxation times of multi-qubit quantum processors ibmq\_quito, ibmq\_belem, ibmq\_lima, ibmq\_santiago and ibmq\_bogota. Our results show that it is reasonable to assume that the fluctuations of the relaxation and dephasing times of superconducting qubits are local to each of the qubits of the system. Based on these results, we discuss the multi-qubit TVQCs when the fluctuations of the decoherence parameters for an error correction block are qubit-wise uncorrelated (as well as from block-to-block), a scenario we have named the Fast Time-Varying Quantum Channel (FTVQC). Furthermore, we lower bound the quantum capacity of general FTVQCs based on a quantity we refer to as the ergodic quantum capacity. Finally, we use numerical simulations to study the performance of quantum error correction codes (QECC) when they operate over FTVQCs.Comment: 21 page

The Role of Models in Self-adaptive and Self-healing Systems

Self-healing and self-adaptive systems dynamically react on changes in the environment. They enable software systems to adjust to new conditions and work optimally even in unstable environments. However, such systems have to cope with an ever increasing complexity and size of software systems. In order to handle such systems, models are an efficient means for analysis, control, and documentation. Furthermore, hierarchically structured models can make self-healing and self-adaptation manageable. In this report, we discuss several questions that address the role of models in self-healing and self-adaptive systems. We outline today\u27s challenges and present different viewpoints on the application and benefit of models