arXiv subjects

Y. Cai

Publications and source records attributed to Y. Cai.

At least 55 records · Page 3Linked to original sources

arXiv2016

Is "genuine multipartite entanglement" really genuine?

The existence of non-local quantum correlations is certainly the most important specific property of the quantum world. However, it is a challenging task to distinguish correlations of classical origin from genuine quantum correlations, especially when the system involves more than two parties, for which different partitions must be simultaneously considered. In the case of mixed states, intermediate levels of correlations must be introduced, coined by the name inseparability. In this work, we revisit in more detail such a concept in the context of continuous-variable quantum optics. We consider a six-partite quantum state that we have effectively generated by the parametric downconversion of a femtosecond frequency comb, the full 12 x 12 covariance matrix of which has been experimentally determined. We show that, though this state does not exhibit "genuine entanglement", it is undoubtedly multipartite-entangled. The consideration of not only the entanglement of individual mode-decompositions but also of combinations of those solves the puzzle and exemplifies the importance of studying different categories of multipartite entanglement.

quant-ph↗

arXiv2015

Adaptive Reduced-Rank Minimum Symbol-Error-Rate Receive Processing for Large-Scale Multiple-Antenna Systems

In this work, we propose a novel adaptive reduced-rank receive processing strategy based on joint preprocessing, decimation and filtering (JPDF) for large-scale multiple-antenna systems. In this scheme, a reduced-rank framework is employed for linear receive processing and multiuser interference suppression based on the minimization of the symbol-error-rate (SER) cost function. We present a structure with multiple processing branches that performs a dimensionality reduction, where each branch contains a group of jointly optimized preprocessing and decimation units, followed by a linear receive filter. We then develop stochastic gradient (SG) algorithms to compute the parameters of the preprocessing and receive filters, along with a low-complexity decimation technique for both binary phase shift keying (BPSK) and $M$-ary quadrature amplitude modulation (QAM) symbols. In addition, an automatic parameter selection scheme is proposed to further improve the convergence performance of the proposed reduced-rank algorithms. Simulation results are presented for time-varying wireless environments and show that the proposed JPDF minimum-SER receive processing strategy and algorithms achieve a superior performance than existing methods with a reduced computational complexity.

cs.IT↗

arXiv2015

The First Data Release (DR1) of the LAMOST general survey

The Large sky Area Multi-Object Spectroscopic Telescope (LAMOST) General Survey is a spectroscopic survey that will eventually cover approximately half of the celestial sphere and collect 10 million spectra of stars, galaxies and QSOs. Objects both in the pilot survey and the first year general survey are included in the LAMOST First Data Release (DR1). The pilot survey started in October 2011 and ended in June 2012, and the data have been released to the public as the LAMOST Pilot Data Release in August 2012. The general survey started in September 2012, and completed its first year of operation in June 2013. The LAMOST DR1 includes a total of 1202 plates containing 2,955,336 spectra, of which 1,790,879 spectra have observed signal-to-noise S/N >10. All data with S/N>2 are formally released as LAMOST DR1 under the LAMOST data policy. This data release contains a total of 2,204,696 spectra, of which 1,944,329 are stellar spectra, 12,082 are galaxy spectra and 5,017 are quasars. The DR1 includes not only spectra, but also three stellar catalogues with measured parameters: AFGK-type stars with high quality spectra (1,061,918 entries), A-type stars (100,073 entries), and M stars (121,522 entries). This paper introduces the survey design, the observational and instrumental limitations, data reduction and analysis, and some caveats. Description of the FITS structure of spectral files and parameter catalogues is also provided.

astro-ph.GA↗

arXiv2015

MEIC Design Summary

This document summarizes the design of Jefferson Lab's electron-ion collider, MEIC, as of January 20, 2015, and describes the facility whose cost was estimated for the United States Department of Energy Nuclear Sciences Advisory Committee EIC cost review of January 26-28, 2015. In particular, each of the main technical systems within the collider is presented to the level of the best current information.

physics.acc-ph↗

arXiv2015

Full multipartite entanglement of frequency comb Gaussian states

An analysis is conducted of the multipartite entanglement for Gaussian states generated by the parametric down-conversion of a femtosecond frequency comb. Using a recently introduced method for constructing optimal entanglement criteria, a family of tests is formulated for mode decompositions that extends beyond the traditional bipartition analyses. A numerical optimization over this family is performed to achieve maximal significance of entanglement verification. For experimentally prepared 4-, 6-, and 10-mode states, full entanglement is certified for all of the 14, 202, and 115974 possible nontrivial partitions, respectively.

quant-ph↗

arXiv2014

Robust MMSE Precoding for Multiuser MIMO Relay Systems using Switched Relaying and Side Information

This study proposes a novel precoding scheme for multiuser multiple-input multiple-output (MIMO) relay systems in the presence of imperfect channel state information (CSI). The base station (BS) and the MIMO relay station (RS) are both equipped with the same codebook of unitary matrices. According to each element of the codebook, we create a latent precoding matrix pair, namely a BS precoding matrix and an RS precoding matrix. The RS precoding matrix is formed by multiplying the appropriate unitary matrix from the codebook by a power scaling factor. Based on the given CSI and a block of transmit symbols, the optimum precoding matrix pair, within the class of all possible latent precoding matrix pairs derived from the various unitary matrices, is selected by a suitable selection mechanism for transmission, which is designed to minimize the squared Euclidean distance between the pre-estimated received vector and the true transmit symbol vector. We develop a minimum mean square error (MMSE) design algorithm for the construction of the latent precoding matrix pairs. In the proposed scheme, rather than sending the complete processing matrix, only the index of the unitary matrix and its power scaling factor are sent by the BS to the RS. This significantly reduces the overhead. Simulation results show that compared to other recently reported precoding algorithms the proposed precoding scheme is capable of providing improved robustness against the effects of CSI estimation errors and multiuser interference.

cs.IT↗

arXiv2014

Robust Multi-Branch Tomlinson-Harashima Precoding in Cooperative MIMO Relay Systems

This paper proposes the design of robust transceivers with Tomlinson-Harashima precoding (THP) for multiple-input multiple-output (MIMO) relay systems with amplify-and-forward (AF) protocols based on a multi-branch (MB) strategy. The MB strategy employs successive interference cancellation (SIC) on several parallel branches which are equipped with different ordering patterns so that each branch produces transmit signals by exploiting a certain ordering pattern. For each parallel branch, the proposed robust nonlinear transceiver design consists of THP at the source along with a linear precoder at the relay and a linear minimum-mean-squared-error (MMSE) receiver at the destination. By taking the channel uncertainties into account, the source and relay precoders are jointly optimised to minimise the mean-squared-error (MSE). We then employ a diagonalization method along with some attributes of matrix-monotone functions to convert the optimization problem with matrix variables into an optimization problem with scalar variables. We resort to an iterative method to obtain the solution for the relay and the source precoders via Karush-Kuhn-Tucker (KKT) conditions. An appropriate selection rule is developed to choose the nonlinear transceiver corresponding to the best branch for data transmission. Simulation results demonstrate that the proposed MB-THP scheme is capable of alleviating the effects of channel state information (CSI) errors and improving the robustness of the system.

cs.IT↗

arXiv2014

Low-Complexity Variable Forgetting Factor Constrained Constant Modulus RLS Algorithm for Adaptive Beamforming

In this paper, a recursive least squares (RLS) based blind adaptive beamforming algorithm that features a new variable forgetting factor (VFF) mechanism is presented. The beamformer is designed according to the constrained constant modulus (CCM) criterion, and the proposed adaptive algorithm operates in the generalized sidelobe canceler (GSC) structure. A detailed study of its operating properties is carried out, including a convexity analysis and a mean squared error (MSE) analysis of its steady-state behavior. The results of numerical experiments demonstrate that the proposed VFF mechanism achieves a superior learning and tracking performance compared to other VFF mechanisms.

cs.OH↗

arXiv2014

Archimedean solid-like superconducting framework in phase-separated K0.8Fe1.6+xSe2 (0<=x<=0.15)

The superconducting (SC) phase in the phase-separated (PS) K0.8Fe1.6+xSe2 (0<=x<=0.15) materials is found to crystallize on Archimedean solid-like frameworks, this structural feature originate from a spinodal phase separation (SPS) at around Ts~540K depending slightly on the Fe concentration. Two stable phases in K0.8Fe1.6+xSe2 are demonstrated to be the SC K0.5Fe2Se2 and antiferromagnetic (AFM) K0.8Fe1.6Se2. The spinodal waves go along the systematic [113] direction and result in notable lamellar structure as illustrated by using the strain-field theoretical simulation. The 3-dimentional SC framework is constructed by hollow truncated octahedra similar with what discussed for Archimedean solids. Based on this structural model, we can efficiently calculate the volume fraction of SC phase in this type of PS SC materials.

cond-mat.supr-con↗

arXiv2013

A High Luminosity e+e- Collider to study the Higgs Boson

A strong candidate for the Standard Model Scalar boson, H(126), has been discovered by the Large Hadron Collider (LHC) experiments. In order to study this fundamental particle with unprecedented precision, and to perform precision tests of the closure of the Standard Model, we investigate the possibilities offered by An e+e- storage ring collider. We use a design inspired by the B-factories, taking into account the performance achieved at LEP2, and imposing a synchrotron radiation power limit of 100 MW. At the most relevant centre-of-mass energy of 240 GeV, near-constant luminosities of 10^34 cm^{-2}s^{-1} are possible in up to four collision points for a ring of 27km circumference. The achievable luminosity increases with the bending radius, and for 80km circumference, a luminosity of 5 10^34 cm^{-2}s^{-1} in four collision points appears feasible. Beamstrahlung becomes relevant at these high luminosities, leading to a design requirement of large momentum acceptance both in the accelerating system and in the optics. The larger machine could reach the top quark threshold, would yield luminosities per interaction point of 10^36 cm^{-2}s^{-1} at the Z pole (91 GeV) and 2 10^35 cm^{-2}s^{-1} at the W pair production threshold (80 GeV per beam). The energy spread is reduced in the larger ring with respect to what is was at LEP, giving confidence that beam polarization for energy calibration purposes should be available up to the W pair threshold. The capabilities in term of physics performance are outlined.

physics.acc-ph↗

arXiv2013

Adaptive Reduced-Rank MBER Linear Receive Processing for Large Multiuser MIMO Systems

In this work, we propose a novel adaptive reduced-rank strategy based on joint interpolation, decimation and filtering (JIDF) for large multiuser multiple-input multiple-output (MIMO) systems. In this scheme, a reduced-rank framework is proposed for linear receive processing and multiuser interference suppression according to the minimization of the bit error rate (BER) cost function. We present a structure with multiple processing branches that performs dimensionality reduction, where each branch contains a group of jointly optimized interpolation and decimation units, followed by a linear receive filter. We then develop stochastic gradient (SG) algorithms to compute the parameters of the interpolation and receive filters along with a low-complexity decimation technique. Simulation results are presented for time-varying environments and show that the proposed MBER-JIDF receive processing strategy and algorithms achieve a superior performance to existing methods at a reduced complexity.

cs.IT↗

arXiv2012

Probing and modelling the localized self-mixing in a GaN/AlGaN field-effect terahertz detector

In a GaN/AlGaN field-effect terahertz detector, the directional photocurrent is mapped in the two-dimensional space of the gate voltage and the drain/source bias. It is found that not only the magnitude, but also the polarity, of the photocurrent can be tuned. A quasistatic self-mixing model taking into account the localized terahertz field provides a quantitative description of the detector characteristics. Strongly localized self-mixing is confirmed. It is therefore important to engineer the spatial distribution of the terahertz field and its coupling to the field-effect channel on the sub-micron scale.

cond-mat.mes-hall↗

arXiv2011

Breathing oscillations of a trapped impurity in a Bose gas

Motivated by a recent experiment [J. Catani et al., arXiv:1106.0828v1 preprint, 2011], we study breathing oscillations in the width of a harmonically trapped impurity interacting with a separately trapped Bose gas. We provide an intuitive physical picture of such dynamics at zero temperature, using a time-dependent variational approach. In the Gross-Pitaevskii regime we obtain breathing oscillations whose amplitudes are suppressed by self trapping, due to interactions with the Bose gas. Introducing phonons in the Bose gas leads to the damping of breathing oscillations and non-Markovian dynamics of the width of the impurity, the degree of which can be engineered through controllable parameters. Our results reproduce the main features of the impurity dynamics observed by Catani et al. despite experimental thermal effects, and are supported by simulations of the system in the Gross-Pitaevskii regime. Moreover, we predict novel effects at lower temperatures due to self-trapping and the inhomogeneity of the trapped Bose gas.

cond-mat.quant-gas↗

arXiv2006

Combined phase space characterization at the PEP-II IP using single-beam and luminous-region measurements

We present a novel method to characterize the e+/- phase space at the IP of the SLAC B-factory, that combines single-beam measurements with a detailed mapping of luminous-region observables. Transverse spot sizes are determined in the two rings with synchrotron-light monitors and extrapolated to the IP using measured lattice functions. The specific luminosity, which is proportional to the inverse product of the overlap IP beam sizes, is continuously monitored using radiative-Bhabha events. The spatial variation of the luminosity and of the transverse-boost distribution of the colliding e+/-, are measured using e+ e- --> mu+ mu- events reconstructed in the BaBar detector. The combination of these measurements provide constraints on the emittances, horizontal and vertical spot sizes, angular divergences and beta functions of both beams at the IP during physics data-taking. Preliminary results of this combined spot-size analysis are confronted with independent measurements of IP beta-functions and overlap IP beam sizes at low beam current.

physics.acc-ph↗

arXiv2006

Parameters for a Super-Flavor-Factory

A Super Flavor Factory, an asymmetric energy e+e- collider with a luminosity of order 10^36 cm-2s-1, can provide a sensitive probe of new physics in the flavor sector of the Standard Model. The success of the PEP-II and KEKB asymmetric colliders in producing unprecedented luminosity above 10^34 cm-2s-1 has taught us about the accelerator physics of asymmetric e+e- colliders in a new parameter regime. Furthermore, the success of the SLAC Linear Collider and the subsequent work on the International Linear Collider allow a new Super-Flavor collider to also incorporate linear collider techniques. This note describes the parameters of an asymmetric Flavor-Factory collider at a luminosity of order 10^36 cm-2s-1 at the Upsilon(4S) resonance and about 10^35 cm-2s-1 at the Tau production threshold. Such a collider would produce an integrated luminosity of about 10,000 fb-1 (10 ab-1) in a running year (10^7 sec) at the Upsilon(4S) resonance.

physics.acc-ph↗

arXiv2006

Evidence for non-conventional pairing in Na$\_x$CoO$\_2$$\cdot$yH$\_2$O

We report on first investigation of the lattice dynamics in the novel superconducting material Na$\_{0.35}$CoO$\_2$$\cdot$1.3H$\_2$O and the non-hydrated parent compound Na$\_{0.7}$CoO$\_2$ by inelastic x-ray scattering. The measured phonon dispersion along the $Γ-M$ direction show a marked softening with hole doping of two optical phonon branches close to the Brillouin zone boundary. The phonon spectra, dispersion, and softening are well reproduced by first-principle calculations. The calculations indicates that the soft branches are mainly composed of Co-vibration modes. The estimation of the critical temperature based on electron-phonon coupling mechanism undisputedly points to a non-conventional superconducting state in this material.

cond-mat.str-el↗

arXiv2005

SuperB: a linear high-luminosity B Factory

This paper is based on the outcome of the activity that has taken place during the recent workshop on "SuperB in Italy" held in Frascati on November 11-12, 2005. The workshop was opened by a theoretical introduction of Marco Ciuchini and was structured in two working groups. One focused on the machine and the other on the detector and experimental issues. The present status on CP is mainly based on the results achieved by BaBar and Belle. Estabilishment of the indirect CP violation in B sector in 2001 and of the direct CP violation in 2004 thanks to the success of PEP-II and KEKB e+e- asymmetric B Factories operating at the center of mass energy corresponding to the mass of the Y(4s). With the two B Factories taking data, the Unitarity Triangle is now beginning to be overconstrained by improving the measurements of the sides and now also of the angles alpha, and gamma. We are also in presence of the very intriguing results about the measurements of sin(2 beta) in the time dependent analysis of decay channels via penguin loops, where b --> s sbar s and b --> s dbar d. Tau physics, in particular LFV search, as well as charm and ISR physics are important parts of the scientific program of a SuperB Factory. The physics case together with possible scenarios for the high luminosity SuperB Factory based on the concepts of the Linear Collider and the related experimental issues are discussed.

physics.ins-det↗

arXiv2003

Tracking Simulations Near Half-Integer Resonance at PEP-II

Beam-beam simulations predict that PEP-II luminosity can be increased by operating the horizontal betatron tune near and above a half-integer resonance. However, effects of the resonance and its synchrotron sidebands significantly enhance betatron and chromatic perturbations which tend to reduce dynamic aperture. In the study, chromatic variation of horizontal tune near the resonance was minimized by optimizing local sextupoles in the Interaction Region. Dynamic aperture was calculated using tracking simulations in LEGO code. Dependence of dynamic aperture on the residual orbit, dispersion and distortion of beta function after correction was investigated.

physics.acc-ph↗