9,945 research outputs found
The classical nature of nuclear spin noise near clock transitions of Bi donors in silicon
Whether a quantum bath can be approximated as classical noise is a
fundamental issue in central spin decoherence and also of practical importance
in designing noise-resilient quantum control. Spin qubits based on bismuth
donors in silicon have tunable interactions with nuclear spin baths and are
first-order insensitive to magnetic noise at so-called clock-transitions (CTs).
This system is therefore ideal for studying the quantum/classical nature of
nuclear spin baths since the qubit-bath interaction strength determines the
back-action on the baths and hence the adequacy of a classical noise model. We
develop a Gaussian noise model with noise correlations determined by quantum
calculations and compare the classical noise approximation to the full quantum
bath theory. We experimentally test our model through dynamical decoupling
sequence of up to 128 pulses, finding good agreement with simulations and
measuring electron spin coherence times approaching one second - notably using
natural silicon. Our theoretical and experimental study demonstrates that the
noise from a nuclear spin bath is analogous to classical Gaussian noise if the
back-action of the qubit on the bath is small compared to the internal bath
dynamics, as is the case close to CTs. However, far from the CTs, the
back-action of the central spin on the bath is such that the quantum model is
required to accurately model spin decoherence.Comment: 5 pages, 3 figure
An hourglass model for the flare of HST-1 in M87
To explain the multi-wavelength light curves (from radio to X-ray) of HST-1
in the M87 jet, we propose an hourglass model that is a modified two-zone
system of Tavecchio & Ghisellini (hereafter TG08): a slow hourglass-shaped or
Laval nozzle-shaped layer connected by two revolving exponential surfaces
surrounding a fast spine, through which plasma blobs flow. Based on the
conservation of magnetic flux, the magnetic field changes along the axis of the
hourglass. We adopt the result of TG08---the high-energy emission from GeV to
TeV can be produced through inverse Compton by the two-zone system, and the
photons from radio to X-ray are mainly radiated by the fast inner zone system.
Here, we only discuss the light curves of the fast inner blob from radio to
X-ray. When a compressible blob travels down the axis of the first bulb in the
hourglass, because of magnetic flux conservation, its cross section experiences
an adiabatic compression process, which results in particle acceleration and
the brightening of HST-1. When the blob moves into the second bulb of the
hourglass, because of magnetic flux conservation, the dimming of the knot
occurs along with an adiabatic expansion of its cross section. A similar broken
exponential function could fit the TeV peaks in M87, which may imply a
correlation between the TeV flares of M87 and the light curves from radio to
X-ray in HST-1. The Very Large Array (VLA) 22 GHz radio light curve of HST-1
verifies our prediction based on the model fit to the main peak of the VLA 15
GHz radio light curve.Comment: 14 pages, 2 figures, accepted for publication in A
The challenge of obtaining ground truth for GPS processing
The increasing use of GPS as a substitute or complement to conventional travel surveys has brought with it an increasing need for a reliable source of ground truth, i.e., information on the actual travel in which each respondent engaged, including the mode and purpose of such travel. Over the past decade or so, the main source that has been used for providing ground truth is the prompted recall survey. As the survey task has become less of a challenge, it has become evident that the prompted recall survey is subject to many of the same shortcomings as most selfadministered surveys, with reporting error, misunderstanding of what constitutes a trip, etc. In this paper, we review some of the common problems encountered in prompted recall surveys, especially those undertaken at some period of time after the GPS survey was undertaken, and using self-report methods. Following this, the paper describes a recent experiment in using life-logging cameras to record a person’s travel, together with GPS, thereby providing a new source of ground truth data that is not subject to processing issues of the GPS data, nor to self-report or other issues with prompted recall data
L2G Auto-encoder: Understanding Point Clouds by Local-to-Global Reconstruction with Hierarchical Self-Attention
Auto-encoder is an important architecture to understand point clouds in an
encoding and decoding procedure of self reconstruction. Current auto-encoder
mainly focuses on the learning of global structure by global shape
reconstruction, while ignoring the learning of local structures. To resolve
this issue, we propose Local-to-Global auto-encoder (L2G-AE) to simultaneously
learn the local and global structure of point clouds by local to global
reconstruction. Specifically, L2G-AE employs an encoder to encode the geometry
information of multiple scales in a local region at the same time. In addition,
we introduce a novel hierarchical self-attention mechanism to highlight the
important points, scales and regions at different levels in the information
aggregation of the encoder. Simultaneously, L2G-AE employs a recurrent neural
network (RNN) as decoder to reconstruct a sequence of scales in a local region,
based on which the global point cloud is incrementally reconstructed. Our
outperforming results in shape classification, retrieval and upsampling show
that L2G-AE can understand point clouds better than state-of-the-art methods
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