2,283 research outputs found
Low-cost error mitigation by symmetry verification
We investigate the performance of error mitigation via measurement of
conserved symmetries on near-term devices. We present two protocols to measure
conserved symmetries during the bulk of an experiment, and develop a zero-cost
post-processing protocol which is equivalent to a variant of the quantum
subspace expansion. We develop methods for inserting global and local symetries
into quantum algorithms, and for adjusting natural symmetries of the problem to
boost their mitigation against different error channels. We demonstrate these
techniques on two- and four-qubit simulations of the hydrogen molecule (using a
classical density-matrix simulator), finding up to an order of magnitude
reduction of the error in obtaining the ground state dissociation curve.Comment: Published versio
A unified approach to explain contrary effects of hysteresis and smoothing in nonsmooth systems
Piecewise smooth dynamical systems make use of discontinuities to model
switching between regions of smooth evolution. This introduces an ambiguity in
prescribing dynamics at the discontinuity: should it be given by a limiting
value on one side or other of the discontinuity, or a member of some set
containing those values? One way to remove the ambiguity is to regularize the
discontinuity, the most common being either to smooth out the discontinuity, or
to introduce a hysteresis between switching in one direction or the other
across the discontinuity. Here we show that the two can in general lead to
qualitatively different dynamical outcomes. We then define a higher dimensional
model with both smoothing and hysteresis, and study the competing limits in
which hysteretic or smoothing effect dominate the behaviour, only the former of
which correspond to Filippov's standard `sliding modes'
Damage identification in composite panels using guided waves
A methodology for the identification of barely visible impact damage using guided waves on a typical aircraft composite structure is implemented. Delaminations and debondings have been introduced in two stiffened panels by means of impact loads
A unified approach to explain contrary effects of hysteresis and smoothing in nonsmooth systems
Piecewise smooth dynamical systems make use of discontinuities to model switching between regions of smooth evolution. This introduces an ambiguity in prescribing dynamics at the discontinuity: should the dynamics be given by a limiting value on one side or other of the discontinuity, or a member of some set containing those values? One way to remove the ambiguity is to regularize the discontinuity, the most common being either to smooth it out, or to introduce a hysteresis between switching in one direction or the other across it. Here we show that the two can in general lead to qualitatively different dynamical outcomes. We then define a higher dimensional model with both smoothing and hysteresis, and study the competing limits in which hysteretic or smoothing effects dominate the behaviour, only the former of which correspond to Filippovâs standard âsliding modesâ.Peer ReviewedPostprint (author's final draft
High Impact Practices: Student Engagement and Retention
Community college students face special challenges that can impede their academic progress, resulting in lower grades and persistence than students in selective four-year colleges. Kingsborough Community College in Brooklyn, New York, successfully addresses these challenges with learning communities: small cohorts of students in a blocked program of study, which includes developmental or basic English, a one-credit student skills course, and a social or behavioral science course. This research analyzes the short-term effects of the model by comparing a sample of 267 students enrolled in four learning community and four regular sections of sociology and psychology classes. The results demonstrate a high positive impact for learning communities on student success as measured by grades and course completion rates, with higher levels of engagement and lower rates of absences in learning community sections as the key causal mechanisms. That is, statistically significant correlations between mode of delivery and grades are reduced when controlling for absences, elaborating on and perhaps explaining the well-established relationship between learning communities and short-term student success
A computational framework for polyconvex large strain elasticity
This paper presents a novel computational formulation for large strain polyconvex elasticity. The formulation, based on the original ideas introduced by Schröder etal. (2011), introduces the deformation gradient (the fibre map), its adjoint (the area map) and its determinant (the volume map) as independent kinematic variables of a convex strain energy function. Compatibility relationships between these variables and the deformed geometry are enforced by means of a multi-field variational principle with additional constraints. This process allows the use of different approximation spaces for each variable. The paper extends the ideas presented in Schröder etal. (2011) by introducing conjugate stresses to these kinematic variables which can be used to define a generalised convex complementary energy function and a corresponding complementary energy principle of the Hellinger-Reissner type, where the new conjugate stresses are primary variables together with the deformed geometry. Both compressible and incompressible or nearly incompressible elastic models are considered. A key element to the developments presented in the paper is the new use of a tensor cross product, presented for the first time by de Boer (1982), page 76, which facilitates the algebra associated with the adjoint of the deformation gradient. For the numerical examples, quadratic interpolation of the displacements, piecewise linear interpolation of strain and stress fields and piecewise constant interpolation of the Jacobian and its stress conjugate are considered for compressible cases. In the case of incompressible materials two formulations are presented. First, continuous quadratic interpolation for the displacement together with piecewise constant interpolation for the pressure and second, linear continuous interpolation for both displacement and pressure stabilised via a Petrov-Galerkin technique
Temporary withdrawal of immunosuppression for life-threatening infections after liver transplantation
Vortices in simulations of solar surface convection
We report on the occurrence of small-scale vortices in simulations of the
convective solar surface. Using an eigenanalysis of the velocity gradient
tensor, we find the subset of high vorticity regions in which the plasma is
swirling. The swirling regions form an unsteady, tangled network of filaments
in the turbulent downflow lanes. Near-surface vertical vortices are underdense
and cause a local depression of the optical surface. They are potentially
observable as bright points in the dark intergranular lanes. Vortex features
typically exist for a few minutes, during which they are moved and twisted by
the motion of the ambient plasma. The bigger vortices found in the simulations
are possibly, but not necessarily, related to observations of granular-scale
spiraling pathlines in "cork animations" or feature tracking.Comment: 11 pages, 13 figures, accepted for publication in A&A, complementary
movies at http://www.mps.mpg.de/homes/moll/strudel/papermovies
Calculating energy derivatives for quantum chemistry on a quantum computer
Modeling chemical reactions and complicated molecular systems has been
proposed as the `killer application' of a future quantum computer. Accurate
calculations of derivatives of molecular eigenenergies are essential towards
this end, allowing for geometry optimization, transition state searches,
predictions of the response to an applied electric or magnetic field, and
molecular dynamics simulations. In this work, we survey methods to calculate
energy derivatives, and present two new methods: one based on quantum phase
estimation, the other on a low-order response approximation. We calculate
asymptotic error bounds and approximate computational scalings for the methods
presented. Implementing these methods, we perform the world's first geometry
optimization on an experimental quantum processor, estimating the equilibrium
bond length of the dihydrogen molecule to within 0.014 Angstrom of the full
configuration interaction value. Within the same experiment, we estimate the
polarizability of the H2 molecule, finding agreement at the equilibrium bond
length to within 0.06 a.u. (2% relative error).Comment: 19 pages, 1 page supplemental, 7 figures. v2 - tidied up and added
example to appendice
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