153 research outputs found
Delocalised oxygen as the origin of two-level defects in Josephson junctions
One of the key problems facing superconducting qubits and other Josephson
junction devices is the decohering effects of bi-stable material defects.
Although a variety of phenomenological models exist, the true microscopic
origin of these defects remains elusive. For the first time we show that these
defects may arise from delocalisation of the atomic position of the oxygen in
the oxide forming the Josephson junction barrier. Using a microscopic model, we
compute experimentally observable parameters for phase qubits. Such defects are
charge neutral but have non-zero response to both applied electric field and
strain. This may explain the observed long coherence time of two-level defects
in the presence of charge noise, while still coupling to the junction electric
field and substrate phonons.Comment: 5 pages, 4 figures. This version streamlines presentation and focuses
on the 2D model. Also fixed embarrassing typo (pF -> fF
Naturally-meaningful and efficient descriptors: machine learning of material properties based on robust one-shot ab initio descriptors
Establishing a data-driven pipeline for the discovery of novel materials
requires the engineering of material features that can be feasibly calculated
and can be applied to predict a material's target properties. Here we propose a
new class of descriptors for describing crystal structures, which we term
Robust One-Shot Ab initio (ROSA) descriptors. ROSA is computationally cheap and
is shown to accurately predict a range of material properties. These simple and
intuitive class of descriptors are generated from the energetics of a material
at a low level of theory using an incomplete ab initio calculation. We
demonstrate how the incorporation of ROSA descriptors in ML-based property
prediction leads to accurate predictions over a wide range of crystals,
amorphized crystals, metal-organic frameworks and molecules. We believe that
the low computational cost and ease of use of these descriptors will
significantly improve ML-based predictions.Comment: 13 pages, accepted in Journal of Cheminformatic
Optimal Experimental Design for Partially Observable Pure Birth Processes
We develop an efficient algorithm to find optimal observation times by
maximizing the Fisher information for the birth rate of a partially observable
pure birth process involving observations. Partially observable implies
that at each of the observation time points for counting the number of
individuals present in the pure birth process, each individual is observed
independently with a fixed probability , modeling detection difficulties or
constraints on resources. We apply concepts and techniques from generating
functions, using a combination of symbolic and numeric computation, to
establish a recursion for evaluating and optimizing the Fisher information. Our
numerical results reveal the efficacy of this new method. An implementation of
the algorithm is available publicly
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