arXiv subjects

Hao Zhang

Publications and source records attributed to Hao Zhang.

At least 1,315 records · Page 73Linked to original sources

arXiv2013

Phase characteristics of electromagnetically induced transparency analogue in coupled resonant systems

Electromagnetically induced transparency (EIT) and EIT-like effects have been investigated in a wide variety of coupled resonant systems. Here, a classification of the phase characteristics of the EIT-like spectral responses is presented. Newly identified phase responses reveal unexplored operation regimes of EIT-like systems. Taking advantage of the new phase regimes, one can obtain group delay, dispersion and nonlinearity properties greatly enhanced by almost one order of magnitude, compared to the traditionally constructed EIT-like devices, which breaks the fundamental limitation (e.g., delay-bandwidth product) intrinsic to atomic EIT and EIT-like effects. Optical devices and electrical circuits are analyzed as examples showing the universality of our finding. We show that cavity-QED-based quantum phase gates can be greatly improved to achieve a phase shift of π. The new phase characteristics are also believed to be useful to build novel doubly resonant devices in quantum information based on cavity QED, optomechanics, and metamaterials.

physics.optics↗

arXiv2013

Spatial-Spectral Boosting Analysis for Stroke Patients' Motor Imagery EEG in Rehabilitation Training

Current studies about motor imagery based rehabilitation training systems for stroke subjects lack an appropriate analytic method, which can achieve a considerable classification accuracy, at the same time detects gradual changes of imagery patterns during rehabilitation process and disinters potential mechanisms about motor function recovery. In this study, we propose an adaptive boosting algorithm based on the cortex plasticity and spectral band shifts. This approach models the usually predetermined spatial-spectral configurations in EEG study into variable preconditions, and introduces a new heuristic of stochastic gradient boost for training base learners under these preconditions. We compare our proposed algorithm with commonly used methods on datasets collected from 2 months' clinical experiments. The simulation results demonstrate the effectiveness of the method in detecting the variations of stroke patients' EEG patterns. By chronologically reorganizing the weight parameters of the learned additive model, we verify the spatial compensatory mechanism on impaired cortex and detect the changes of accentuation bands in spectral domain, which may contribute important prior knowledge for rehabilitation practice.

stat.ML↗

arXiv2013

Generating Entangled Spin States for Quantum Metrology by Single-Photon Detection

We propose and analyze a probabilistic but heralded scheme to generate pure, entangled, non-Gaussian states of collective spin in large atomic ensembles by means of single-photon detection. One photon announces the preparation of a Dicke state, while two or more photons announce Schrödinger cat states. The method produces pure states even for finite photon detection efficiency and weak atom-photon coupling. The entanglement generation can be made quasi-deterministic by means of repeated trial and feedback, enabling metrology beyond the standard quantum limit.

physics.atom-ph↗

arXiv2013

Interpretations and Implications of the Top Quark Rapidity Asymmetries $A_{FB}^t$ and $A_{FB}^{\ell}$

Forward-backward asymmetries $A_{FB}^t$ and $A_{FB}^\ell$ are observed in the top quark $t$ rapidity distribution and in the rapidity distribution of charged leptons $\ell$ from top quark decay at the Tevatron proton-antiproton collider, and a charge asymmetry $A_C$ is seen in proton-proton collisions at the Large Hadron Collider (LHC). In this paper, we update our previous studies of the Tevatron asymmetries using the most recent data. We provide expectations for $A_C$ at the LHC based first on model independent extrapolations from the Tevatron, and second based on new physics models that can explain the Tevatron asymmetries. We examine the relationship of the two asymmetries $A_{FB}^t$ and $A_{FB}^\ell$. We show their connection through the $(V-A)$ spin correlation between the charged lepton and the top quark with different polarization states. We show that the ratio of the two asymmetries provides independent insight into new physics models that are invoked to fit the top quark asymmetry. We emphasize the value of the measurement of both asymmetries, and we conclude that a model which produces more right-handed than left-handed top quarks is favored by the present Tevatron data.

hep-ph↗

arXiv2013

Quasi-Equiangular Frame (QEF) : A New Flexible Configuration of Frame

Frame theory is a powerful tool in the domain of signal processing and communication. Among its numerous configurations, the ones which have drawn much attention recently are Equiangular Tight Frame (ETF) and Grassmannian Frame. These frames both have some kind of optimality in coherence, thus bring robustness or optimal performance in applications such as digital fingerprint, erasure channels, and Compressive Sensing. However, too strict constraint on existence and construction of ETF and Grassmannian Frame became the main obstacle for widespread use. In this paper, we propose a new configuration of frame: Quasi-Equiangular Frame, as a compromise but more convenient and flexible approximation of ETF and Grassmannian Frame. We will give formal definition of Quasi-Equiangular Frame and analyze its relationship with ETF and Grassmannian frame. Furthermore, for popularity of ETF and Grassmannian frame in Compressive Sensing, we utilize the technique of random matrices to obtain asymptotical concentration estimation of the Restricted Isometry Constant (RIC) of Quasi-Equiangular Frame with respect to its key parameter.

cs.IT↗

arXiv2013

Tight is better: Performance Improvement of the Compressive Classifier Using Equi-Norm Tight Frames

Detecting or classifying already known sparse signals contaminated by Gaussian noise from compressive measurements is different from reconstructing sparse signals, as its objective is to minimize the error probability which describes performance of the detectors or classifiers. This paper is concerned about the performance improvement of a commonly used Compressive Classifier. We prove that when the arbitrary sensing matrices used to get the Compressive Measurements are transformed into Equi-Norm Tight Frames, i.e. the matrices that are row-orthogonal, The Compressive Classifier achieves better performance. Although there are other proofs that among all Equi-Norm Tight Frames the Equiangular tight Frames (ETFs) bring best worst-case performance, the existence and construction of ETFs on some dimensions is still an open problem. As the construction of Equi-Norm Tight Frames from any arbitrary matrices is very easy and practical compared with ETF matrices, the result of this paper can also provide a practical method to design an improved sensing matrix for Compressive Classification. We can conclude that: Tight is Better!

cs.IT↗

arXiv2013

Assemblathon 2: evaluating de novo methods of genome assembly in three vertebrate species

Background - The process of generating raw genome sequence data continues to become cheaper, faster, and more accurate. However, assembly of such data into high-quality, finished genome sequences remains challenging. Many genome assembly tools are available, but they differ greatly in terms of their performance (speed, scalability, hardware requirements, acceptance of newer read technologies) and in their final output (composition of assembled sequence). More importantly, it remains largely unclear how to best assess the quality of assembled genome sequences. The Assemblathon competitions are intended to assess current state-of-the-art methods in genome assembly. Results - In Assemblathon 2, we provided a variety of sequence data to be assembled for three vertebrate species (a bird, a fish, and snake). This resulted in a total of 43 submitted assemblies from 21 participating teams. We evaluated these assemblies using a combination of optical map data, Fosmid sequences, and several statistical methods. From over 100 different metrics, we chose ten key measures by which to assess the overall quality of the assemblies. Conclusions - Many current genome assemblers produced useful assemblies, containing a significant representation of their genes, regulatory sequences, and overall genome structure. However, the high degree of variability between the entries suggests that there is still much room for improvement in the field of genome assembly and that approaches which work well in assembling the genome of one species may not necessarily work well for another.

q-bio.GN↗

arXiv2013

Measuring Top-Quark Polarization in Top-Pair + Missing Energy Events

The polarization of a top-quark can be sensitive to new physics beyond the standard model. We propose a novel method to measure top-quark polarization, based on the charged lepton energy fraction in top-quark decay, and illustrate the method with a detailed simulation of top-quark pairs produced in supersymmetric top squark pair production. We show that the lepton energy ratio distribution that we define is very sensitive to the top-quark polarization but insensitive to the precise measurement of the top-quark energy.

hep-ph↗

arXiv2013

Quasibound States and Evidence for a Spin 1 Kondo Effect in Asymmetric Quantum Point Contacts

Linear conductance below $2e^2/h$ shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even odd effect, observable in the longer channels, is consistent with the formation of quasi-localized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin one Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple-peak provides the first evidence suggestive of a spin singlet triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons.

cond-mat.mes-hall↗

arXiv2013

Exotic disordered phases in the quantum $J_1-J_2$ model on the honeycomb lattice

We study the ground-state phase diagram of the frustrated quantum $J_1-J_2$ Heisenberg antiferromagnet on the honeycomb lattice using a mean field approach in terms of the Schwinger boson representation of the spin operators. We present results for the ground-state energy, local magnetization, energy gap and spin-spin correlations. The system shows magnetic long range order for $0\leq J_{2}/J_{1}\lesssim 0.2075$ (Néel) and $0.398\lesssim J_{2}/J_{1}\leq 0.5$ (spiral). In the intermediate region, we find two magnetically disordered phases: a gapped spin liquid phase which shows short-range Néel correlations $(0.2075 \lesssim J_{2}/J_{1} \lesssim 0.3732)$, and a lattice nematic phase $(0.3732 \lesssim J_{2}/J_{1}\lesssim 0.398)$, which is magnetically disordered but breaks lattice rotational symmetry. The errors in the values of the phase boundaries which are implicit in the number of significant figures quoted, correspond purely to the error in the extrapolation of our finite-size results to the thermodynamic limit.

cond-mat.str-el↗

arXiv2012

Collective state measurement of mesoscopic ensembles with single-atom resolution

For mesoscopic ensembles containing 100 or more atoms we measure the total atom number and the number of atoms in a specific hyperfine state with single-atom resolution. The measurement detects the atom-induced shift of the resonance frequency of an optical cavity containing the ensemble. This work extends the range of cavity-based detection with single-atom resolution by more than an order of magnitude in atom number, and provides the readout capability necessary for Heisenberg-limited interferometry with atomic ensembles.

physics.atom-ph↗

arXiv2012

Measuring Top Quark Polarization in Top Pair plus Missing Energy Events

The polarization of a top quark can be sensitive to new physics beyond the standard model. Since the charged lepton from top quark decay is maximally correlated with the top quark spin, it is common to measure the polarization from the distribution in the angle between the charged lepton and the top quark directions. We propose a novel method based on the charged lepton energy fraction and illustrate the method with a detailed simulation of top quark pairs produced in supersymmetric top squark pair production. We show that the lepton energy ratio distribution that we define is very sensitive to the top quark polarization but insensitive to the precise measurement of the top quark energy.

hep-ph↗

arXiv2012

Search for the signal of monotop production at the early LHC

We investigate the potential of the early LHC to discover the signal of monotops, which can be decay products of some resonances in models such as R-parity violating SUSY or SU(5), etc. We show how to constrain the parameter space of the models by the present data of $Z$ boson hadronic decay branching ratio, $K^0-\bar{K^0}$ mixing and dijet productions at the LHC. Then, we study the various cuts imposed on the events, reconstructed from the hadronic final states, to suppress backgrounds and increase the significance in detail. And we find that in the hadronic mode the information from the missing transverse energy and reconstructed resonance mass distributions can be used to specify the masses of the resonance and the missing particle. Finally, we study the sensitivities to the parameters at the LHC with $\sqrt{s}$=7 TeV and an integrated luminosity of $1 {\rm fb}^{-1}$ in detail. Our results show that the early LHC may detect this signal at 5$σ$ level for some regions of the parameter space allowed by the current data.

hep-ph↗

arXiv2012

Associated Higgs plus vector boson test of a fermiophobic Higgs boson

Production in association with an electroweak vector boson V is a distinctive mode of production for a Higgs boson H without tree-level couplings to fermions, known as a fermiophobic Higgs boson. We focus on HV associated production with H decay into a pair of photons, and V into a pair of jets, with the goal of distinguishing a fermiophobic Higgs boson from the standard model Higgs boson. Performing a simulation of the signal and pertinent QCD backgrounds, and using the same event selection cuts employed by the LHC ATLAS Collaboration, we argue that existing LHC data at 7 TeV with 4.9 fb^{-1} of integrated luminosity may distinguish a fermiophobic Higgs boson from a standard model Higgs boson near 125 GeV at about 1.9 standard deviation signal significance (1.9 sigma) per experiment. At 8 TeV we show that associated production could yield 2.8 sigma significance per experiment with 10 fb^{-1} of data.

hep-ph↗

arXiv2012

Fast Algorithms for Sparse Recovery with Perturbed Dictionary

In this paper, we account for approaches of sparse recovery from large underdetermined linear models with perturbation present in both the measurements and the dictionary matrix. Existing methods have high computation and low efficiency. The total least-squares (TLS) criterion has well-documented merits in solving linear regression problems while FOCal Underdetermined System Solver (FOCUSS) has low-computation complexity in sparse recovery. Based on TLS and FOCUSS methods, the present paper develops more fast and robust algorithms, TLS-FOCUSS and SD-FOCUSS. TLS-FOCUSS algorithm is not only near-optimum but also fast in solving TLS optimization problems under sparsity constraints, and thus fit for large scale computation. In order to reduce the complexity of algorithm further, another suboptimal algorithm named D-FOCUSS is devised. SD-FOCUSS can be applied in MMV (multiple-measurement-vectors) TLS model, which fills the gap of solving linear regression problems under sparsity constraints. The convergence of TLS-FOCUSS algorithm and SD-FOCUSS algorithm is established with mathematical proof. The simulations illustrate the advantage of TLS-FOCUSS and SD-FOCUSS in accuracy and stability, compared with other algorithms.

cs.IT↗

arXiv2012

The Recursive Form of Error Bounds for RFS State and Observation with Pd<1

In the target tracking and its engineering applications, recursive state estimation of the target is of fundamental importance. This paper presents a recursive performance bound for dynamic estimation and filtering problem, in the framework of the finite set statistics for the first time. The number of tracking algorithms with set-valued observations and state of targets is increased sharply recently. Nevertheless, the bound for these algorithms has not been fully discussed. Treating the measurement as set, this bound can be applied when the probability of detection is less than unity. Moreover, the state is treated as set, which is singleton or empty with certain probability and accounts for the appearance and the disappearance of the targets. When the existence of the target state is certain, our bound is as same as the most accurate results of the bound with probability of detection is less than unity in the framework of random vector statistics. When the uncertainty is taken into account, both linear and non-linear applications are presented to confirm the theory and reveal this bound is more general than previous bounds in the framework of random vector statistics.In fact, the collection of such measurements could be treated as a random finite set (RFS).

stat.AP↗

arXiv2012

Top quark forward-backward asymmetry and W^prime bosons

The top quark forward-backward asymmetry measured at the Fermilab Tevatron collider deviates from the standard model prediction. A W^prime boson model is described, where the coupling W^prime-t-d is fixed by the tt_bar forward-backward asymmetry and total cross section at the Tevatron. We show that such a W^prime boson would be produced in association with a top quark at the CERN Large Hadron Collider (LHC), thus inducing additional tt_bar+j events. We use measurements of tt_bar+n-jet production from the LHC to constrain the allowed W^prime-t-d couplings as a function of W^prime boson mass. We find that this W^prime model is constrained at the 95% C.L. using 0.7 fb^{-1} of data from the LHC, and could be fully excluded with 5 fb^{-1} of data.

hep-ph↗

arXiv2012

Codes Can Reduce Queueing Delay in Data Centers

In this paper, we quantify how much codes can reduce the data retrieval latency in storage systems. By combining a simple linear code with a novel request scheduling algorithm, which we call Blocking-one Scheduling (BoS), we show analytically that it is possible to reduce data retrieval delay by up to 17% over currently popular replication-based strategies. Although in this work we focus on a simplified setting where the storage system stores a single content, the methodology developed can be applied to more general settings with multiple contents. The results also offer insightful guidance to the design of storage systems in data centers and content distribution networks.

math.OC↗