6,895 research outputs found
Enabling Adiabatic Passages Between Disjoint Regions in Parameter Space through Topological Transitions
We explore topological transitions in parameter space in order to enable
adiabatic passages between regions adiabatically disconnected within a given
parameter manifold. To this end, we study the Hamiltonian of two coupled qubits
interacting with external magnetic fields, and make use of the analogy between
the Berry curvature and magnetic fields in parameter space, with spectrum
degeneracies associated to magnetic charges. Symmetry-breaking terms induce
sharp topological transitions on these charge distributions, and we show how
one can exploit this effect to bypass crossing degeneracies. We also
investigate the curl of the Berry curvature, an interesting but as of yet not
fully explored object, which together with its divergence uniquely defines this
field. Finally, we suggest a simple method for measuring the Berry curvature,
thereby showing how one can experimentally verify our results.Comment: 17 pages, 11 figure
Geodesic Paths for Quantum Many-Body Systems
We propose a method to obtain optimal protocols for adiabatic ground-state
preparation near the adiabatic limit, extending earlier ideas from [D. A. Sivak
and G. E. Crooks, Phys. Rev. Lett. 108, 190602 (2012)] to quantum
non-dissipative systems. The space of controllable parameters of isolated
quantum many-body systems is endowed with a Riemannian quantum metric
structure, which can be exploited when such systems are driven adiabatically.
Here, we use this metric structure to construct optimal protocols in order to
accomplish the task of adiabatic ground-state preparation in a fixed amount of
time. Such optimal protocols are shown to be geodesics on the parameter
manifold, maximizing the local fidelity. Physically, such protocols minimize
the average energy fluctuations along the path. Our findings are illustrated on
the Landau-Zener model and the anisotropic XY spin chain. In both cases we show
that geodesic protocols drastically improve the final fidelity. Moreover, this
happens even if one crosses a critical point, where the adiabatic perturbation
theory fails.Comment: 5 pages, 2 figures + 4 pages supplemen
Geodesic paths for quantum many-body systems
We propose a method to obtain optimal protocols for adiabatic ground-state preparation near the adiabatic limit, extending earlier ideas from [D. A. Sivak and G. E. Crooks, Phys. Rev. Lett. 108, 190602 (2012)] to quantum non-dissipative systems. The space of controllable parameters of isolated quantum many-body systems is endowed with a Riemannian quantum metric structure, which can be exploited when such systems are driven adiabatically. Here, we use this metric structure to construct optimal protocols in order to accomplish the task of adiabatic ground-state preparation in a fixed amount of time. Such optimal protocols are shown to be geodesics on the parameter manifold, maximizing the local fidelity. Physically, such protocols minimize the average energy fluctuations along the path. Our findings are illustrated on the Landau-Zener model and the anisotropic XY spin chain. In both cases we show that geodesic protocols drastically improve the final fidelity. Moreover, this happens even if one crosses a critical point, where the adiabatic perturbation theory fails.http://meetings.aps.org/link/BAPS.2016.MAR.F50.9First author draf
Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMeP[Pd(dmit)]
The organic charge-transfer salt EtMeP[Pd(dmit)] is a
quasi-two-dimensional Mott insulator with localized spins = 1/2 residing on
a distorted triangular lattice. Here we report measurements of the uniaxial
thermal expansion coefficients along the in-plane = - and
-axis as well as along the out-of-plane -axis for temperatures 1.4\,K
T 200\,K. Particular attention is paid to the lattice effects
around the phase transition at = 25\,K into a low-temperature
valence-bond-solid phase and the paramagnetic regime above where effects of
short-range antiferromagnetic correlations can be expected. The salient results
of our study include (i) the observation of strongly anisotropic lattice
distortions accompanying the formation of the valence-bond-solid, and (ii) a
distinct maximum in the thermal expansion coefficients in the paramagnetic
regime around 40\,K. Our results demonstrate that upon cooling through
the in-plane -axis, along which the valence bonds form, contracts
while the second in-plane -axis elongates by the same relative amount.
Surprisingly, the dominant effect is observed for the out-of-plane -axis
which shrinks significantly upon cooling through . The pronounced
anomaly in around 40\,K is attributed to short-range magnetic
correlations. It is argued that the position of this maximum, relative to that
in the magnetic susceptibility around 70\,K, speaks in favor of a more
anisotropic triangular-lattice scenario for this compound than previously
thought.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
Field-induced length changes in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
Measurements of the coefficient of thermal expansion on the spin-liquid
candidate -(BEDT-TTF)Cu(CN) have revealed distinct and
strongly anisotropic lattice effects around 6 K - a possible spin-liquid
instability. In order to study the effects of a magnetic field on the
low-temperature spin-liquid state, dilatometric measurements have been
conducted both as a function of temperature at \emph{B} = const. and as a
function of field at \emph{T} = const. While the 6 K anomaly is found to be
insensitive to magnetic fields \emph{B} 10 T, the maximum field applied,
surprisingly strong \emph{B}-induced effects are observed for magnetic fields
applied along the in-plane \emph{b}-axis. Above a threshold field of 0.5 T <
\emph{B} 1 T, a jump-like anomaly is observed in the \emph{b}-axis
lattice parameter. This anomaly, which is located at 8.7 K at \emph{B} = 1 T,
grows in size and shifts to lower temperatures with increasing the magnetic
field. Although the anomaly bears resemblance to a first-order phase
transition, the lack of hysteresis suggests otherwise.Comment: 3 pages, 3 figures, proceedings of ISCOM 2011, physica status solidi
(c)(in press
Magnetic Field-Induced Lattice Effects in a Quasi-2D Organic Conductor Close to the Mott Metal-Insulator Transition
We present ultra-high-resolution dilatometric studies in magnetic fields on a
quasi-two-dimensional organic conductor
-(D8-BEDT-TTF)Cu[N(CN)]Br, which is located close to the
Mott metal-insulator (MI) transition. The obtained thermal expansion
coefficient, , reveals two remarkable features: (i) the Mott MI
transition temperature = (13.6 0.6)\,K is insensitive to fields
up to 10\,T, the highest applied field; (ii) for fields along the interlayer
\emph{b}-axis, a magnetic-field-induced (FI) phase transition at =
(9.5 0.5)\,K is observed above a threshold field 1 T,
indicative of a spin reorientation with strong magneto-elastic coupling.Comment: 5 pages, 4 figure
LExecutor: Learning-Guided Execution
Executing code is essential for various program analysis tasks, e.g., to
detect bugs that manifest through exceptions or to obtain execution traces for
further dynamic analysis. However, executing an arbitrary piece of code is
often difficult in practice, e.g., because of missing variable definitions,
missing user inputs, and missing third-party dependencies. This paper presents
LExecutor, a learning-guided approach for executing arbitrary code snippets in
an underconstrained way. The key idea is to let a neural model predict missing
values that otherwise would cause the program to get stuck, and to inject these
values into the execution. For example, LExecutor injects likely values for
otherwise undefined variables and likely return values of calls to otherwise
missing functions. We evaluate the approach on Python code from popular
open-source projects and on code snippets extracted from Stack Overflow. The
neural model predicts realistic values with an accuracy between 79.5% and
98.2%, allowing LExecutor to closely mimic real executions. As a result, the
approach successfully executes significantly more code than any available
technique, such as simply executing the code as-is. For example, executing the
open-source code snippets as-is covers only 4.1% of all lines, because the code
crashes early on, whereas LExecutor achieves a coverage of 51.6%.Comment: Accepted in research track of the ACM Joint European Software
Engineering Conference and Symposium on the Foundations of Software
Engineering (ESEC/FSE) 202
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