6,632 research outputs found
Quantum Belief Propagation
We present an accurate numerical algorithm, called quantum belief propagation
(QBP), for simulation of one-dimensional quantum systems at non-zero
temperature. The algorithm exploits the fact that quantum effects are
short-range in these systems at non-zero temperature, decaying on a length
scale inversely proportional to the temperature. We compare to exact results on
a spin-1/2 Heisenberg chain. Even a very modest calculation, requiring
diagonalizing only 10-by-10 matrices, reproduces the peak susceptibility with a
relative error of less than , while more elaborate calculations
further reduce the error.Comment: 4 pages, 1 figure; revised time estimates due to improved
implementation. Typographical corrections to Eq. 7 made; thanks to David
Poulin for pointing out the mistak
Diffusion Processes on Power-Law Small-World Networks
We consider diffusion processes on power-law small-world networks in
different dimensions. In one dimension, we find a rich phase diagram, with
different transient and recurrent phases, including a critical line with
continuously varying exponents. The results were obtained using self-consistent
perturbation theory and can also be understood in terms of a scaling theory,
which provides a general framework for understanding processes on small-world
networks with different distributions of long-range links.Comment: 4 pages, 3 figures, added references, modified Fig. 2 with added data
(PRL, in press
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Right Temporoparietal Junction Involvement in Autonomic Responses to the Suffering of Others: A Preliminary Transcranial Magnetic Stimulation Study.
Functional neuroimaging studies have emphasized distinct networks for social cognition and affective aspects of empathy. However, studies have not considered whether substrates of social cognition, such as the right temporoparietal junction (TPJ), play a role in affective responses to complex empathy-related stimuli. Here, we used repetitive transcranial magnetic stimulation (TMS) to test whether the right TPJ contributes to psychophysiological responses to another person's emotional suffering. We used a theory of mind functional localizer and image-guided TMS to target the sub-region of the right TPJ implicated in social cognition, and measured autonomic and subjective responses to an empathy induction video. We found evidence that TMS applied at 1 Hz over the right TPJ increased withdrawal of parasympathetic nervous system activity during the empathy induction (n = 32), but did not affect sympathetic nervous system activity (n = 27). Participants who received TMS over the right TPJ also reported feeling more irritation and annoyance, and were less likely to report feeling compassion over and above empathic sadness, than participants who received TMS over the vertex (N = 34). This study provides preliminary evidence for the role of right TPJ functioning in empathy-related psychophysiological and affective responding, potentially blurring the distinction between neural regions specific to social cognition vs. affective aspects of empathy
Finite Size Scaling of Mutual Information: A Scalable Simulation
We develop a quantum Monte Carlo procedure to compute the Renyi mutual
information of an interacting quantum many-body system at non-zero temperature.
Performing simulations on a spin-1/2 XXZ model, we observe that for a subregion
of fixed size embedded in a system of size L, the mutual information converges
at large L to a limiting function which displays non-monotonic temperature
behavior corresponding to the onset of correlations. For a region of size L/2
embedded in a system of size L, the mutual information divided by L converges
to a limiting function of temperature, with apparently nontrivial corrections
near critical points.Comment: 4 pages, 4 figure
Simulator study of vortex encounters by a twin-engine, commercial, jet transport airplane
A simulator study of vortex encounters was conducted for a twin-engine, commercial, jet transport airplane encountering the vortex flow field of a heavy, four-engine, commercial, jet transport airplane in the final-approach configuration. The encounters were conducted with fixed controls and with a pilot using a state-of-the-art, manual-control system. Piloted encounters with the base-line vortex flow field out of ground effect (unattenuated) resulted in initial bank-angle excursions greater than 40 deg, coupled with initial sideslip-angle excursions greater than 10 deg. The severity of these initial upsets was significantly reduced when the vortex center was moved laterally or vertically away from the flight path of the encountering airplane. Smaller reductions occurred when the flow field was attenuated by the flight spoilers on the generating airplane. The largest reduction in the severity of the initial upsets, however, was from aging in ground effect. The severity of the initial upsets of the following airplane was relatively unaffected by the approach speed. Increasing the lift coefficient of the generating airplane resulted in an increase in the severity of the initial upsets
Observations Outside the Light-Cone: Algorithms for Non-Equilibrium and Thermal States
We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent
and thermal quantities in quantum systems. For time-dependent systems, we
modify a previous mapping to quantum circuits to significantly reduce the
computer resources required. This modification is based on a principle of
"observing" the system outside the light-cone. We apply this method to study
spin relaxation in systems started out of equilibrium with initial conditions
that give rise to very rapid entanglement growth. We also show that it is
possible to approximate time evolution under a local Hamiltonian by a quantum
circuit whose light-cone naturally matches the Lieb-Robinson velocity.
Asymptotically, these modified methods allow a doubling of the system size that
one can obtain compared to direct simulation. We then consider a different
problem of thermal properties of disordered spin chains and use quantum belief
propagation to average over different configurations. We test this algorithm on
one dimensional systems with mixed ferromagnetic and anti-ferromagnetic bonds,
where we can compare to quantum Monte Carlo, and then we apply it to the study
of disordered, frustrated spin systems.Comment: 19 pages, 12 figure
Preliminary results of simulated vortex encounters by a twin-engine, commercial aircraft during final landing approach
Piloted simulations of encounters with vortices of various ages and degrees of attenuation were performed with the Visual Motion Simulator. In the simulations, a twin engine, commercial transport on final approach, encountered the modeled vortices of a four engine, wide body, commercial transport. The data show the effect of vortex age and attenuation on the severity of the initial upset, as well as the effect of the vortex encounters on the landing capability
The literature of low g propellant behavior
Annotated bibliography on low-g liquid propellant behavio
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