937 research outputs found
Non-Abelian Majorana Doublets in Time-Reversal Invariant Topological Superconductor
The study of non-Abelian Majorana zero modes advances our understanding of
the fundamental physics in quantum matter, and pushes the potential
applications of such exotic states to topological quantum computation. It has
been shown that in two-dimensional (2D) and 1D chiral superconductors, the
isolated Majorana fermions obey non-Abelian statistics. However, Majorana modes
in a time-reversal invariant (TRI) topological superconductor come in
pairs due to Kramers' theorem. Therefore, braiding operations in TRI
superconductors always exchange two pairs of Majoranas. In this work, we show
interestingly that, due to the protection of time-reversal symmetry,
non-Abelian statistics can be obtained in 1D TRI topological superconductors
and may have advantages in applying to topological quantum computation.
Furthermore, we unveil an intriguing phenomenon in the Josephson effect, that
the periodicity of Josephson currents depends on the fermion parity of the
superconducting state. This effect provides direct measurements of the
topological qubit states in such 1D TRI superconductors.Comment: Manuscript: 9 pages + 6 Figs + Appendix + Supplementary material. 2nd
version for PRX. Discussions are updated according to Referee report
Majorana Flat Bands and Uni-directional Majorana Edge States in Gapless Topological Superconductors
In this work, we show that an in-plane magnetic field can drive a fully
gapped topological superconductor into a gapless phase which
supports symmetry protected Majorana edge states (MESs). Specifically, an
in-plane magnetic field can close the bulk gap and create zero energy Majorana
flat bands (MFBs) in the excitation spectrum. We show that the MFBs in the
gapless regime are protected by symmetry and are associated with MESs. The MFBs
acquire finite slopes when s-wave pairing and Rashba spin-oribit coupling terms
are added to the Hamiltonian. In this case, novel uni-directional MESs which
propagate in the same direction on opposite edges may appear. Uni-directional
MESs can also be found in pure s-wave superconductors with spin-orbit coupling.
The MFBs and the uni-directional MESs induce nearly quantized zero bias
conductance in tunneling experiments even in the presence of a gapless bulk and
disorder.Comment: 5 pages, 5 figure
Generic searches for alternative gravitational wave polarizations with networks of interferometric detectors
The detection of gravitational wave signals by Advanced LIGO and Advanced
Virgo enables us to probe the polarization content of gravitational waves. In
general relativity, only tensor modes are present, while in a variety of
alternative theories one can also have vector or scalar modes. Recently test
were performed which compared Bayesian evidences for the hypotheses that either
purely tensor, purely vector, or purely scalar polarizations were present.
Indeed, with only three detectors in a network and allowing for mixtures of
tensor polarizations and alternative polarization states, it is not possible to
identify precisely which non-standard polarizations might be in the signal and
by what amounts. However, we demonstrate that one can still infer whether, in
addition to tensor polarizations, alternative polarizations are present in the
first place, irrespective of the detailed polarization content. We develop two
methods to do this for sources with electromagnetic counterparts, both based on
the so-called null stream. Apart from being able to detect mixtures of tensor
and alternative polarizations, these have the added advantage that no waveform
models are needed, and signals from any kind of transient source with known sky
position can be used. Both formalisms allow us to combine information from
multiple sources so as to arrive at increasingly more stringent bounds. For now
we apply these on the binary neutron star signal GW170817, showing consistency
with the tensor-only hypothesis with p-values of 0.315 and 0.790 for the two
methods.Comment: 8 pages, 3 figure
Generic searches for alternative gravitational wave polarizations with networks of interferometric detectors
The detection of gravitational wave signals by Advanced LIGO and Advanced Virgo enables us to probe the polarization content of gravitational waves. In general relativity, only tensor modes are present, while in a variety of alternative theories one can also have vector or scalar modes. Recently test were performed which compared Bayesian evidences for the hypotheses that either purely tensor, purely vector, or purely scalar polarizations were present. Indeed, with only three detectors in a network and allowing for mixtures of tensor polarizations and alternative polarization states, it is not possible to identify precisely which nonstandard polarizations might be in the signal and by what amounts. However, we demonstrate that one can still infer whether, in addition to tensor polarizations, alternative polarizations are present in the first place, irrespective of the detailed polarization content. We develop two methods to do this for sources with electromagnetic counterparts, both based on the so-called null stream. Apart from being able to detect mixtures of tensor and alternative polarizations, these have the added advantage that no waveform models are needed, and signals from any kind of transient source with known sky position can be used. Both formalisms allow us to combine information from multiple sources so as to arrive at increasingly more stringent bounds. For now we apply these on the binary neutron star signal GW170817, showing consistency with the tensor-only hypothesis with p-values of 0.315 and 0.790 for the two methods
Optimized high-throughput microRNA expression profiling provides novel biomarker assessment of clinical prostate and breast cancer biopsies
BACKGROUND: Recent studies indicate that microRNAs (miRNAs) are mechanistically involved in the development of various human malignancies, suggesting that they represent a promising new class of cancer biomarkers. However, previously reported methods for measuring miRNA expression consume large amounts of tissue, prohibiting high-throughput miRNA profiling from typically small clinical samples such as excision or core needle biopsies of breast or prostate cancer. Here we describe a novel combination of linear amplification and labeling of miRNA for highly sensitive expression microarray profiling requiring only picogram quantities of purified microRNA. RESULTS: Comparison of microarray and qRT-PCR measured miRNA levels from two different prostate cancer cell lines showed concordance between the two platforms (Pearson correlation R(2 )= 0.81); and extension of the amplification, labeling and microarray platform was successfully demonstrated using clinical core and excision biopsy samples from breast and prostate cancer patients. Unsupervised clustering analysis of the prostate biopsy microarrays separated advanced and metastatic prostate cancers from pooled normal prostatic samples and from a non-malignant precursor lesion. Unsupervised clustering of the breast cancer microarrays significantly distinguished ErbB2-positive/ER-negative, ErbB2-positive/ER-positive, and ErbB2-negative/ER-positive breast cancer phenotypes (Fisher exact test, p = 0.03); as well, supervised analysis of these microarray profiles identified distinct miRNA subsets distinguishing ErbB2-positive from ErbB2-negative and ER-positive from ER-negative breast cancers, independent of other clinically important parameters (patient age; tumor size, node status and proliferation index). CONCLUSION: In sum, these findings demonstrate that optimized high-throughput microRNA expression profiling offers novel biomarker identification from typically small clinical samples such as breast and prostate cancer biopsies
Using machine learning to infer reasoning provenance from user interaction log data: based on the data/frame theory of sensemaking
The reconstruction of analysts’ reasoning processes (reasoning provenance) during complex sensemaking tasks can support reflection and decision making. One potential approach to such reconstruction is to automatically infer reasoning from low-level user interaction logs. We explore a novel method for doing this using machine learning. Two user studies were conducted in which participants performed similar intelligence analysis tasks. In one study, participants used a standard web browser and word processor; in the other, they used a system called INVISQUE (Interactive Visual Search and Query Environment). Interaction logs were manually coded for cognitive actions based on captured think-aloud protocol and posttask interviews based on Klein, Phillips, Rall, and Pelusos’s data/frame model of sensemaking as a conceptual framework. This analysis was then used to train an interaction frame mapper, which employed multiple machine learning models to learn relationships between the interaction logs and the codings. Our results show that, for one study at least, classification accuracy was significantly better than chance and compared reasonably to a reported manual provenance reconstruction method. We discuss our results in terms of variations in feature sets from the two studies and what this means for the development of the method for provenance capture and the evaluation of sensemaking systems
Limits on precursor and afterglow radio emission from a fast radio burst in a star-forming galaxy
We present a new fast radio burst at 920 MHz discovered during commensal
observations conducted with the Australian Square Kilometre Array Pathfinder
(ASKAP) as part of the Commensal Real-time ASKAP Fast Transients (CRAFT)
survey. FRB 191001 was detected at a dispersion measure (DM) of 506.92(4) pc
cm and its measured fluence of 143(15) Jy ms is the highest of the
bursts localized to host galaxies by ASKAP to date. The sub-arcsecond
localisation of the FRB provided by ASKAP reveals that the burst originated in
the outskirts of a highly star-forming spiral in a galaxy pair at redshift
. Radio observations show no evidence for a compact persistent
radio source associated with the FRB 191001 above a flux density of Jy.
However, we detect diffuse synchrotron radio emission from the disk of the host
galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected
as an image-plane transient in a single 10-s snapshot with a flux density of
19.3 mJy in the low-time-resolution visibilities obtained simultaneously with
CRAFT data. The commensal observation facilitated a search for repeating and
slowly varying radio emissions 8 hrs before and 1 hr after the burst. We found
no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report
our upper limits and briefly review FRB progenitor theories in the literature
which predict radio afterglows. Our data are still only weakly constraining of
any afterglows at the redshift of the FRB. Future commensal observations of
more nearby and bright FRBs will potentially provide stronger constraints.Comment: 12 pages, 6 figures, Accepted for publication in ApJ Letter
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