arXiv subjects

Jun Zhang

Publications and source records attributed to Jun Zhang.

At least 1,189 records · Page 66Linked to original sources

arXiv2015

High-efficiency and low-jitter Silicon single-photon avalanche diodes based on nanophotonic absorption enhancement

Silicon single-photon avalanche diode (SPAD) is a core device for single-photon detection in the visible and the near-infrared range, and widely used in many applications. However, due to limits of the structure design and device fabrication for current silicon SPADs, the key parameters of detection befficiency and timing jitter are often forced to compromise. Here, we propose a nanostructured silicon SPAD, which achieves high detection efficiency with excellent timing jitter simultaneously over a broad spectral range. The optical and electric simulations show significant performance enhancement compared with conventional silicon SPAD devices. This nanostructured devices can be easily fabricated and thus well suited for practical applications.

physics.ins-det↗

arXiv2015

High-Dimensional CSI Acquisition in Massive MIMO: Sparsity-Inspired Approaches

Massive MIMO has been regarded as one of the key technologies for 5G wireless networks, as it can significantly improve both the spectral efficiency and energy efficiency. The availability of high-dimensional channel side information (CSI) is critical for its promised performance gains, but the overhead of acquiring CSI may potentially deplete the available radio resources. Fortunately, it has recently been discovered that harnessing various sparsity structures in massive MIMO channels can lead to significant overhead reduction, and thus improve the system performance. This paper presents and discusses the use of sparsity-inspired CSI acquisition techniques for massive MIMO, as well as the underlying mathematical theory. Sparsity-inspired approaches for both frequency-division duplexing and time-division duplexing massive MIMO systems will be examined and compared from an overall system perspective, including the design trade-offs between the two duplexing modes, computational complexity of acquisition algorithms, and applicability of sparsity structures. Meanwhile, some future prospects for research on high-dimensional CSI acquisition to meet practical demands will be identified.

cs.IT↗

arXiv2015

Oscillating light wall above a sunspot light bridge

With the high tempo-spatial \emph{Interface Region Imaging Spectrograph} 1330 Å images, we find that many bright structures are rooted in the light bridge of NOAA 12192, forming a \emph{light wall}. The light wall is brighter than the surrounding areas, and the wall top is much brighter than the wall body. The New Vacuum Solar Telescope H$α$ and the \emph{Solar Dynamics Observatory} 171 Å and 131 Å images are also used to study the light wall properties. In 1330 Å, 171 Å, and 131 Å, the top of the wall has a higher emission, while in the H$α$ line, the wall top emission is very low. The wall body corresponds to bright areas in 1330 Å and dark areas in the other lines. The top of the light wall moves upward and downward successively, performing oscillations in height. The deprojected mean height, amplitude, oscillation velocity, and the dominant period are determined to be 3.6 Mm, 0.9 Mm, 15.4 km s$^{-1}$, and 3.9 min, respectively. We interpret the oscillations of the light wall as the leakage of \emph{p}-modes from below the photosphere. The constant brightness enhancement of the wall top implies the existence of some kind of atmospheric heating, e.g., via the persistent small-scale reconnection or the magneto-acoustic waves. In another series of 1330 Å images, we find that the wall top in the upward motion phase is significantly brighter than in the downward phase. This kind of oscillations may be powered by the energy released due to intermittent impulsive magnetic reconnection.

astro-ph.SR↗

arXiv2015

The Linear Perturbation Theory of Reionization in Position-Space: Cosmological Radiative Transfer Along the Light-Cone

The linear perturbation theory of inhomogeneous reionization (LPTR) has been developed as an analytical tool for predicting the global ionized fraction and large-scale power spectrum of ionized density fluctuations during reionization. In the original formulation of the LPTR, the ionization balance and radiative transfer equations are linearized and solved in Fourier space. However, the LPTR's approximation to the full solution of the radiative transfer equation is not straightforward to interpret, since the latter is most intuitively conceptualized in position space. To bridge the gap between the LPTR and the language of numerical radiative transfer, we present a new, equivalent, position-space formulation of the LPTR that clarifies the approximations it makes and facilitates its interpretation. We offer a comparison between the LPTR and the excursion-set model of reionization (ESMR), and demonstrate the built-in capability of the LPTR to explore a wide range of reionization scenarios, and to go beyond the ESMR in exploring scenarios involving X-rays.

astro-ph.CO↗

arXiv2015

Rényi entropy uncertainty relation for successive projective measurements

We investigate the uncertainty principle for two successive projective measurements in terms of Rényi entropy based on a single quantum system. Our results cover a large family of the entropy (including the Shannon entropy) uncertainty relations with a lower optimal bound. We compare our relation with other formulations of the uncertainty principle in a two-spin observables measured on a pure quantum state of qubit. It is shown that the low bound of our uncertainty relation has better tightness.

quant-ph↗

arXiv2015

Filament Activation in Response to Magnetic Flux Emergence and Cancellation in Filament Channels

We make a comparative analysis for two filaments that showed quite different activation in response to the flux emergence within the filament channels. The observations from the Solar Dynamics Observatory (SDO) and Global Oscillation Network Group (GONG) are carried out to analyze the two filaments on 2013 August 17-20 and September 29. The first event showed that the main body of the filament was separated into two parts when an active region (AR) emerged with a maximum magnetic flux of about 6.4*10^21 Mx underlying the filament. The close neighborhood and common direction of the bright threads in the filament and the open AR fan loops suggest similar magnetic connectivity of these two flux systems. The equilibrium of the filament was not destroyed within 3 days after the start of the emergence of the AR. To our knowledge, similar observations have never been reported before. In the second event, the emerging flux occurred nearby a barb of the filament with a maximum magnetic flux of 4.2*10^20 Mx, about one order of magnitude less than that of the first event. The emerging flux drove the convergence of two patches of parasitic polarity in the vicinity of the barb, and resulted in cancellation between the parasitic polarity and nearby network fields. About 20 hours after the onset of the emergence, the filament was entirely erupted. Our findings imply that the location of emerging flux within the filament channel is probably crucial to filament evolution. If the flux emergence appears nearby the barbs, flux cancellation of emerging flux with the filament magnetic fields is prone to occur, which probably causes the filament eruption. The comparison of the two events shows that the emergence of an entire AR may still not be enough to disrupt the stability of a filament system and the actual eruption does occur only after the flux cancellation sets in.

astro-ph.SR↗

arXiv2015

Quasi-periodic Slipping Magnetic Reconnection During an X-class Solar Flare Observed by the Solar Dynamics Observatory and Interface Region Imaging Spectrograph

We firstly report the quasi-periodic slipping motion of flare loops during an eruptive X-class flare on 2014 September 10. The slipping motion was investigated at a specific location along one of the two ribbons and can be observed throughout the impulsive phase of the flare. The apparent slipping velocity was 20-110 km/s and the associated period was 3$-$6 min. The footpoints of flare loops appeared as small-scale bright knots observed in 1400 Å, corresponding to fine structures of the flare ribbon. These bright knots were observed to move along the southern part of the longer ribbon and also exhibited a quasi-periodic pattern. The Si IV 1402.77 Å line was redshifted by 30-50 km/s at the locations of moving knots with a ~ 40-60 km/s line width, larger than other sites of the flare ribbon. We suggest that the quasi-periodic slipping reconnection is involved in this process and the redshift at the bright knots is probably indicative of reconnection downflow. The emission line of Si IV at the northern part of the longer ribbon also exhibited obvious redshifts of about 10-70 km/s in the impulsive phase of the flare, with the redshifts at the outer edges of the ribbon larger than those in the middle. The redshift velocities at post-flare loops reached about 80-100 km/s in the transition region.

astro-ph.SR↗

arXiv2015

Direct Evidence of Interaction-Induced Dirac Cones in Monolayer Silicene/Ag(111) System

Silicene, analogous to graphene, is a one-atom-thick two-dimensional crystal of silicon which is expected to share many of the remarkable properties of graphene. The buckled honeycomb structure of silicene, along with its enhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spin-split states in silicene are tunable with external fields. Although the low-energy Dirac cone states lie at the heart of all novel quantum phenomena in a pristine sheet of silicene, the question of whether or not these key states can survive when silicene is grown or supported on a substrate remains hotly debated. Here we report our direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved photoemission measurements on silicene(3x3)/Ag(111), we reveal the presence of six pairs of Dirac cones on the edges of the first Brillouin zone of Ag(111), other than expected six Dirac cones at the K points of the primary silicene(1x1) Brillouin zone. Our result shows clearly that the unusual Dirac cone structure originates not from the pristine silicene alone but from the combined effect of silicene(3x3) and the Ag(111) substrate. This study identifies the first case of a new type of Dirac Fermion generated through the interaction of two different constituents. Our observation of Dirac cones in silicene/Ag(111) opens a new materials platform for investigating unusual quantum phenomena and novel applications based on two-dimensional silicon systems.

cond-mat.mtrl-sci↗

arXiv2015

Relationship between EUV microflares and small-scale magnetic fields in the quiet Sun

Microflares are small dynamic signatures observed in X-ray and extreme-ultraviolet channels. Because of their impulsive emission enhancements and wide distribution, they are thought to be closely related to coronal heating. By using the high resolution 171 Å images from the Atmospheric Imaging Assembly and the lines-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we trace 10794 microflares in a quiet region near the disk center with a field of view of 960 arcsec $\times$ 1068 arcsec during 24 hr. The microflares have an occurrence rate of 4.4 $\times$ 10$^{3}$ hr$^{-1}$ extrapolated over the whole Sun. Their average brightness, size, and lifetime are 1.7 I$_{0}$(of the quiet Sun), 9.6 Mm$^{2}$, and 3.6 min, respectively. There exists a mutual positive correlation between the microflares' brightness, area and lifetime. In general, the microflares distribute uniformly across the solar disk, but form network patterns locally, which are similar to and matched with the magnetic network structures. Typical cases show that the microflares prefer to occur in magnetic cancellation regions of network boundaries. We roughly calculate the upper limit of energy flux supplied by the microflares and find that the result is still a factor of $\sim$15 below the coronal heating requirement.

astro-ph.SR↗

arXiv2015

Accurate Shear Measurement with Faint Sources

For cosmic shear to become an accurate cosmological probe, systematic errors in the shear measurement method must be unambiguously identified and corrected for. Previous work of this series has demonstrated that cosmic shears can be measured accurately in Fourier space in the presence of background noise and finite pixel size, without assumptions on the morphologies of galaxy and PSF. The remaining major source of error is source Poisson noise, due to the finiteness of source photon number. This problem is particularly important for faint galaxies in space-based weak lensing measurements, and for ground-based images of short exposure times. In this work, we propose a simple and rigorous way of removing the shear bias from the source Poisson noise. Our noise treatment can be generalized for images made of multiple exposures through MultiDrizzle. This is demonstrated with the SDSS and COSMOS/ACS data. With a large ensemble of mock galaxy images of unrestricted morphologies, we show that our shear measurement method can achieve sub-percent level accuracy even for images of signal-to-noise ratio less than 5 in general, making it the most promising technique for cosmic shear measurement in the ongoing and upcoming large scale galaxy surveys.

astro-ph.CO↗

arXiv2015

Topological Defects from the Multiverse

Many theories of the early universe predict the existence of a multiverse where bubbles continuously nucleate giving rise to observers in their interior. In this paper, we point out that topological defects of several dimensionalities will also be produced in de Sitter like regions of the multiverse. In particular, defects could be spontaneously nucleated in our parent vacuum. We study the evolution of these defects as they collide with and propagate inside of our bubble. We estimate the present distribution of defects in the observable part of the universe. The expected number of such nearby defects turns out to be quite small, even for the highest nucleation rate. We also study collisions of strings and domain walls with our bubble in our past light cone. We obtain simulated full-sky maps of the loci of such collisions, and find their angular size distribution. Similarly to what happens in the case of bubble collisions, the prospect of detecting any collisions of our bubble with ambient defects is greatly enhanced in the case where the cosmological constant of our parent vacuum is much higher than the vacuum energy density during inflation in our bubble.

hep-th↗

arXiv2015

Anomalous High-Energy Waterfall-Like Electronic Structure in 5d Transition Metal Oxide Sr2IrO4 with a Strong Spin-Orbit Coupling

The layered 5d transition metal oxides like Sr2IrO4 have attracted significant interest recently due to a number of exotic and new phenomena induced by the interplay between the spin-orbit coupling, bandwidth W and on-site Coulomb correlation U. In contrast to a metallic behavior expected from the Mott-Hubbard model due to more spatially extended 5d orbitals and moderate U, an insulating ground state has been observed in Sr2IrO4. Such an insulating behavior can be understood by an effective J_eff=1/2 Mott insulator model by incorporating both electron correlation and strong spin-orbital coupling, although its validity remains under debate at present. In particular, Sr2IrO4 exhibits a number of similarities to the high temperature cuprate superconductors in the crystal structure, electronic structure, magnetic structure, and even possible high temperature superconductivity. Here we report a new observation of the anomalous high energy electronic structure in Sr2IrO4. By taking high-resolution angle-resolved photoemission measurements on Sr2IrO4 over a wide energy range, we have revealed that the high energy electronic structures show unusual nearly-vertical bands that extend over a large energy range. Such anomalous high energy behaviors resemble the high energy waterfall features observed in the cuprate superconductors, adding one more important similarity between these two systems. While strong electron correlation plays an important role in producing high energy waterfall features in the cuprate superconductors, the revelation of the high energy anomalies in Sr2IrO4 points to a novel route in generating exotic electronic excitations from the strong spin-orbit coupling and a moderate electron correlation.

cond-mat.str-el↗

arXiv2015

On the minimum distance of elliptic curve codes

Computing the minimum distance of a linear code is one of the fundamental problems in algorithmic coding theory. Vardy [14] showed that it is an \np-hard problem for general linear codes. In practice, one often uses codes with additional mathematical structure, such as AG codes. For AG codes of genus $0$ (generalized Reed-Solomon codes), the minimum distance has a simple explicit formula. An interesting result of Cheng [3] says that the minimum distance problem is already \np-hard (under \rp-reduction) for general elliptic curve codes (ECAG codes, or AG codes of genus $1$). In this paper, we show that the minimum distance of ECAG codes also has a simple explicit formula if the evaluation set is suitably large (at least $2/3$ of the group order). Our method is purely combinatorial and based on a new sieving technique from the first two authors [8]. This method also proves a significantly stronger version of the MDS (maximum distance separable) conjecture for ECAG codes.

cs.IT↗

arXiv2014

Long-range micro-pulse aerosol lidar at 1.5 um with an up-conversion single-photon detector

A micro-pulse lidar at eye-safe wavelength is constructed based on an up-conversion single-photon detector. The ultralow noise detector enables using integration technique to improve the signal-to-noise ratio of the atmospheric backscattering even at daytime. With the pulse energy of 110uJ, the pulse repetition rate of 15 kHz, the optical antenna diameter of 100 mm and integration time of 5 minutes, a horizontal detection range of 7 km is realized. In the demonstration experiment, atmospheric visibility over 24 hours is monitored continuously, with results in accordance with the weather forecasts.

physics.optics↗

arXiv2014

Magnetic reconnection between small-scale loops observed with the New Vacuum Solar Telescope

Using the high tempo-spatial resolution H$α$ images observed with the New Vacuum Solar Telescope, we report the solid observational evidence of magnetic reconnection between two sets of small-scale anti-parallel loops with an X-shaped topology. The reconnection process contains two steps: a slow step with the duration of more than several tens of minutes, and a rapid step lasting for only about three minutes. During the slow reconnection, two sets of anti-parallel loops reconnect gradually, and new loops are formed and stacked together. During the rapid reconnection, the anti-parallel loops approach each other quickly, and then the rapid reconnection takes place, resulting in the disappearance of former loops. In the meantime, new loops are formed and separate. The region between the approaching loops is brightened, and the thickness and length of this region are determined to be about 420 km and 1.4 Mm, respectively. During the rapid reconnection process, obvious brightenings at the reconnection site and apparent material ejections outward along reconnected loops are observed. These observed signatures are consistent with predictions by reconnection models. We suggest that the successive slow reconnection changes the conditions around the reconnection site and triggers instabilities, thus leading to the rapid approach of the anti-parallel loops and resulting in the rapid reconnection.

astro-ph.SR↗

arXiv2014

Non-classicalities via perturbing local unitary operations

We study the nonclassical correlations in a two-qubit state by the perturbing local unitary operation method. We find that the definitions of various non-classicalities including quantum discord (QD), measurement-induced nonlocality (MIN) and so on usually do not have a unique definition when expressed as the perturbation of local unitary operations, so a given non-classicality can lead to different definitions of its dual non-classicality. In addition, it is shown that QD and MIN are not the corresponding dual expressions in a simple set of unitary operations, even though they are in their original definitions. In addition, we also consider the non-classicalities in general $2\otimes d$ dimensional systems.

quant-ph↗

arXiv2014

The roles of quantum correlations in quantum cloning

In this paper, we study the entanglement and quantum discord of the output modes in the unified $1\rightarrow 2$ state-dependent cloning and probabilistic quantum cloning. The tripartite entanglement among the output modes and the quantum cloning machine is also considered. We find that the roles of the quantum correlations including the bipartite and tripartite entanglement and quantum discord strongly depend on the quantum cloning machines as well as the cloned state. In particular, it is found that this quantum cloning scheme can be realizable even without any quantum correlation.

quant-ph↗

arXiv2014

Direct Observations of Tether-cutting Reconnection During a Major Solar Event From 2014 February 24 to 25

Using the multi-wavelength data from Atmospheric Imaging Assembly on board the Solar Dynamic Observatory, we investigated two successive solar flares, a C5.1 confined flare and an X4.9 ejective flare with a halo coronal mass ejection,in NOAA AR 11990 from 2014 Feb 24 to 25. Before the confined are onset, EUV brightening beneath the filament was detected. As the are began, a twisted helical flux rope (FR) wrapping around the filament moved upward and then stopped, and in the meantime an obvious X-ray source below it was observed. Prior to the ejective X4.9 flare, some pre-existing loop structures in the active region interacted with each other, which produced a brightening region beneath the filament. Meanwhile, a small flaring loop appeared below the interaction region and some new helical lines connecting the far ends of the loop structures was gradually formed and continually added into the former twisted FR. Then, due to the resulting imbalance between the magnetic pressure and tension, the new FR together with the filament erupted outward. Our observations coincide well with tether-cutting model, suggesting that the two flares probably have the same triggering mechanism, i.e., tether-cutting reconnection. To our knowledge, this is the first direct observation of tether-cutting reconnection occurring between the pre-existing loops in active region. In the ejective flare case, the erupting filament exhibited an omega-like kinked structure and underwent an exponential rise after a slow-rise phase, indicating the kink instability might be also responsible for the eruption initiation.

astro-ph.SR↗