Why phonon scattering in glasses is universally small at low temperatures

We present a novel view of the standard model of tunneling two level systems (TLS) to explain the puzzling universal value of a quantity, $C\sim 3\times 10^{-4}$, that characterizes phonon scattering in glasses below 1 K as reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound velocity. Physical considerations lead to a broad distribution of phonon-TLS couplings that (1) exponentially renormalize tunneling matrix elements, and (2) reduce the TLS density of states through TLS-TLS interactions. We find good agreement between theory and experiment for a variety of individual glasses.Comment: Resubmission: several typos were corrected and the notation clarified in some places. No major change

Interacting Two-Level Systems as a Source of 1/f Charge Noise in Quantum Dot Qubits

Charge noise in semiconducting quantum dots has been observed to have a 1/f spectrum. We propose a model in which a pair of quantum dots are coupled to a 2D bath of fluctuating two level systems (TLS) that have electric dipole moments and that interact with each other, i.e., with the other fluctuators. These interactions are primarily via the elastic strain field. We use a 2D nearest-neighbor Ising spin glass to represent these elastic interactions and to simulate the dynamics of the bath of electric dipole fluctuators in the presence of a ground plane representing metal gates above the oxide layer containing the fluctuators. The interactions between the TLS cause the energy splitting of individual fluctuators to change with time. We calculate the resulting fluctuations in the electric potential at the two quantum dots that lie below the oxide layer. We find that 1/f electric potential noise spectra at the quantum dots and cross correlation in the noise between the two quantum dots are in qualitative agreement with experiment. Our simulations find that the cross correlations decrease exponentially with increasing quantum dot separation.Comment: 14 pages, 11 figure

Effects of Temperature Fluctuations on Charge Noise in Quantum Dot Qubits

Silicon quantum dot qubits show great promise but suffer from charge noise with a 1/f^\alpha spectrum, where f is frequency and \alpha \lesssim 1. It has recently been proposed that 1/f^\alpha noise spectra can emerge from a few thermally activated two-level fluctuators in the presence of sub-bath temperature fluctuations associated with a two-dimensional electron gas (2DEG)~\cite{Ahn2021}. We investigate this proposal by doing Monte Carlo simulations of a single Ising spin in a bath with a fluctuating temperature. We find that to obtain noise with a $1/f^\alpha$ spectrum with $alpha \lesssim 1 down to low frequencies, the duration of temperature fluctuations must be comparable to the inverse of the lowest frequency at which the noise is measured. This result is consistent with an analytic calculation in which the fluctuator is a two-state system with dynamics governed by time-dependent switching rates. In this case we find that the noise spectrum follows a Lorentzian at frequencies lower than the inverse of the average duration of the lowest switching rate. We then estimate relaxation times of thermal fluctuations by considering thermal diffusion in an electron gas in a confined geometry. We conclude that temperature fluctuations in a 2DEG sub-bath would require an unphysically long duration to be consistent with experimental measurements of 1/f-like charge noise in quantum dots at frequencies extending well below 1 Hz.Comment: 8 pages, 5 figure

Novel stiffeners exploiting internal pressurisation to enhance buckling behaviour under bending loads

The paper proposes a novel type of stiffener designed to bear bending loads by exploiting internal pressure effects. The stiffener is made of two adjacent thin-walled pipes (r/t≥50) jointed with a connecting strip. Such a structure is shown to have higher performance against buckling failure compared to a single pipe and its geometry allows for good exploitation of internal pressurisation. The study is conducted by using the FEA software ANSYS and the analysis technique is the linear perturbation buckling analysis. Internal pressure ranges from 0 to 1.4 MPa. The buckling mechanisms are observed for a set of models with different values of length, wall thickness and geometric variation of the cross-section. It is shown that two different buckling modes can take place. However, for a given geometry, the level of pressure can alter the behaviour and lead to one mode rather than the other one. Potential of the presented structure is maximised by the use of high performance materials and a possible aerospace engineering application is presented

6 billions, mono-bit operations per second : the cellular automata approach to the simulation of natural phenomenon, using a systolic GAPP array. Application to the diffusion

The « cellular automata » approch (see the references [8] to [17]) to the simulation of natural phenomenon leads up to computation problems essentially due ta the required computation power and to the data transfer bottleneck between processing elements and memories . A real-time image processing system for low and medium level processing that we developped at LSIT, Strasbourg, enabled us to solve these problems. A systolic SIMD array (GAPP of NCR) gaved us enough processing power to handle the iterative cellular automata method . In this paper, we will first describe our image processing system (MASYVE) and give the simulated diffusion model . We will then describe the management of data transfers . At last we will present the achieved performances, the limitations of the system and the perspectives .L'approche «automate cellulaire » des phénomènes physiques pose des problèmes de simulation difficiles à résoudre par des calculateurs classiques. Les temps de calcul pour des processus itératifs, l'accès pas toujours simple à de très gros ordinateurs, font que plusieurs équipes travaillent à l'étude de machines programmables dédiées à ce genre d'applications . Cette approche est étudiée dans les références [8] à [17] . Le LSIT de Strasbourg a développé une machine multi-processeurs de traitement d'images bas et moyen niveau orientée temps réel vidéo ([1] à [7]) . La puissance de calcul apportée par une matrice SIMD de processeurs GAPP (« Geometrical Arithmetic Parallel Processors » de NCR) intégrée dans ce système nous permet de l'utiliser facilement et de façon très efficace pour les simulations de type « automates cellulaires » en visualisant toutes les itérations de calcul . Après avoir présenté succinctement la machine, nous présenterons le modèle simulé . Nous donnerons quelques détails sur la gestion des entrées-sorties pour cet exemple, puis indiquerons les performances atteintes . Nous finirons en soulignant les limitations et les perspectives envisagées