arXiv subjects

Lei Yang

Publications and source records attributed to Lei Yang.

At least 523 records · Page 29Linked to original sources

arXiv2010

Quantum efficiency measurement of single photon detectors using photon pairs generated in optical fibers

Using the correlated signal and idler photon pairs generated in a dispersion shifted fiber by a pulsed pump, we measure the quantum efficiency of a InGaAs/InP avalanche photodiode-based single photon detector. Since the collection efficiency of photon pairs is a key parameter to correctly deduce the quantum efficiency, we carefully characterize the collection efficiency by studying correlation dependence of photon pairs upon the spectra of pump, signal and idler photons. This study allows us to obtain quantum efficiency of the single photon detector by using photon pairs with various kinds of bandwidths.

quant-ph↗

arXiv2010

Proton-neutron pairing correlations in the nuclear shell model

A shell-model study of proton-neutron pairing in f - p shell nuclei using a parametrized hamiltonian that includes deformation and spin-orbit effects as well as isoscalar and isovector pairing is reported. By working in a shell-model framework we are able to assess the role of the various modes of proton-neutron pairing in the presence of nuclear deformation without violating symmetries. Results are presented for $^{44}$Ti, $^{46}$Ti and $^{48}$Cr.

nucl-th↗

arXiv2010

Cross-Band Interference Considered Harmful in OFDM Based Distributed Spectrum Sharing

In the past few years we have witnessed the paradigm shift from static spectrum allocation to dynamic spectrum access/sharing. Orthogonal Frequency-Division Multiple Access (OFDMA) is a promising mechanism to implement the agile spectrum access. However, in wireless distributed networks where tight synchronization is infeasible, OFDMA faces the problem of cross-band interference. Subcarriers used by different users are no longer orthogonal, and transmissions operating on non-overlapping subcarriers can interfere with each other. In this paper, we explore the cause of cross-band interference and analytically quantify its strength and impact on packet transmissions. Our analysis captures three key practical artifacts: inter-link frequency offset, temporal sampling mismatch and power heterogeneity. To our best knowledge, this work is the first to systematically analyze the cause and impact of cross-band interference. Using insights from our analysis, we then build and compared three mitigating methods to combat cross-band interference. Analytical and simulation results show that placing frequency guardband at link boundaries is the most effective solution in distributed spectrum sharing, while the other two frequency-domain methods are sensitive to either temporal sampling mismatch or inter-link frequency offset. We find that the proper guardband size depends heavily on power heterogeneity. Consequently, protocol designs for dynamic spectrum access should carefully take into account the cross-band interference when configuring spectrum usage.

cs.IT↗

arXiv2010

On the pinning strategy of complex networks

In pinning control of complex networks, a tacit believing is that the system dynamics will be better controlled by pinning the large-degree nodes than the small-degree ones. Here, by changing the number of pinned nodes, we find that, when a significant fraction of the network nodes are pinned, pinning the small-degree nodes could generally have a higher performance than pinning the large-degree nodes. We demonstrate this interesting phenomenon on a variety of complex networks, and analyze the underlying mechanisms by the model of star networks. By changing the network properties, we also find that, comparing to densely connected homogeneous networks, the advantage of the small-degree pinning strategy is more distinct in sparsely connected heterogenous networks.

physics.soc-ph↗

arXiv2009

An all fiber source of frequency entangled photon pairs

We present an all fiber source of frequency entangled photon pairs by using four wave mixing in a Sagnac fiber loop. Special care is taken to suppress the impurity of the frequency entanglement by cooling the fiber and by matching the polarization modes of the photon pairs counter-propagating in the fiber loop. Coincidence detection of signal and idler photons, which are created in pair and in different spatial modes of the fiber loop, shows the quantum interference in the form of spatial beating, while the single counts of the individual signal (idler) photons keep constant. When the production rate of photon pairs is about 0.013 pairs/pulse, the envelope of the quantum interference reveals a visibility of $(95\pm 2)%$, which is close to the calculated theoretical limit 97.4%

quant-ph↗

arXiv2008

Topological Symmetry And Existence of Partial Synchronization

We study the relationship between the partial synchronous (PaS) state and the coupling structure in general dynamical systems. By the exact proof, we find the sufficient and necessary condition of the existence of PaS state for the coupling structure. Our result shows that the symmetry of the coupling structure is not the equivalent condition which is supposed before but only the sufficient condition. Furthermore, for the existence of the PaS state, the general structure is the equal-degree random.

nlin.CD↗

arXiv2008

Fiber based source of photon pairs at telecom band with high temporal coherence and brightness for quantum information processing

We experimentally demonstrate a bright pulsed source of correlated photon pairs at 1550 nm telecom band by pumping 300 m dispersion shifted fiber with a 4 ps pulse train. We investigate the coherence property of the source by measuring the second order intensity correlation function $g^{(2)}$ of individual signal (idler) photons. A preliminary Hong-Ou-Mandel type two-photon interference experiment with two such sources confirms the high temporal and spatial coherence of the source. The source is suitable for multi-photon quantum interference of independent sources, required in quantum information processing.

quant-ph↗

arXiv2007

Enhancing Robustness and Immunization in geographical networks

We find that different geographical structures of networks lead to varied percolation thresholds, although these networks may have similar abstract topological structures. Thus, the strategies for enhancing robustness and immunization of a geographical network are proposed. Using the generating function formalism, we obtain the explicit form of the percolation threshold $q_{c}$ for networks containing arbitrary order cycles. For 3-cycles, the dependence of $q_c$ on the clustering coefficients is ascertained. The analysis substantiates the validity of the strategies with an analytical evidence.

physics.soc-ph↗

arXiv2006

Galois representations arising from twenty-seven lines on a cubic surface and the arithmetic associated with Hessian polyhedra

In the present paper, we will show that three apparently disjoint objects: Galois representations arising from twenty-seven lines on a cubic surface (number theory and arithmetic algebraic geometry), Picard modular forms (automorphic forms), rigid Calabi-Yau threefolds and their arithmetic (Diophantine geometry) are intimately related to Hessian polyhedra and their invariants. We construct a Galois representation whose image is a proper subgroup of $W(E_6)$, the Weyl group of the exceptional Lie algebra $E_6$. We give a conjecture about the identification of two different kinds of $L$-functions which can be considered as a higher dimensional counterpart of the Langlands-Tunnell theorem.

math.NT↗

arXiv2005

Asymmetric Properties of Heat Conduction in a One-Dimensional Frenkel-Kontorova Model

In this Letter, we show numerically that the rectifying effect of heat flux in a one-dimensional two-segment Frenkel-Kontorova chain demonstrated in recent literature is merely available under the limit of the weak coupling between the two constituent segments. Surprisingly, the rectifying effect will be reversed when the properties of the interface and the system size change. The two types of asymmetric heat conduction are dominated by different mechanisms, which are all induced by the nonlinearity. We further discuss the possibility of the experimental realization of thermal diode or rectifier devices.

cond-mat.stat-mech↗

arXiv2005

Interdependence of dynamical signals and topology: Detecting the influential nodes in networks

By studying varies dynamical processes, including coupled maps, cellular automata and coupled differential equations, on five different kinds of known networks, we found a positive relation between signal correlation and node's degree. Thus a method of identifying influential nodes in dynamical systems is proposed, its validity is studied, and potential applications on real systems are discussed.

cond-mat.stat-mech↗

arXiv2005

Geographical effects on cascading breakdowns of scale-free networks

Cascading breakdowns of real networks are severe accidents in recent years, such as the blackouts of the power transportation networks in North America. In this paper, we study the effects of geographical structure on the cascading phenomena of load-carried scale-free networks, find that more geographically constrained networks tend to have larger cascading breakdowns. Explanations by the effects of circles and large betweenness of small degree nodes are provided.

physics.soc-ph↗

arXiv2004

Geometry and arithmetic associated to Appell hypergeometric partial differential equations

In this paper, we study the monodromy of Appell hypergeometric partial differential equations, which lead us to find four derivatives which are associated to the group GL(3). Our four derivatives have the remarkable properties. We find that Appell hypergeometric partial differential equations can be reduced to four nonlinear partial differential equations by the use of our four derivatives. We investigate these four nonlinear partial differential equations. We are interested in some particular cases. In these cases, the system of linear partial differential equations has the remarkable properties. We find the $η$-functions associated to the unitary group $U(2, 1)$, which has the interesting properties. Our $η$-functions lead us to study the arithmetic of Picard curves, especially the transform problems (moduli problems) and the corresponding modular equations. We give a rational transformation between two irrational integrals which are associated to some algebraic surfaces of degree seven. We find a modular equation which give the relation between the moduli of two Picard curves.

math.NT↗

arXiv2004

Hessian polyhedra, invariant theory and Appell hypergeometric partial differential equations

It is well-known that Klein's lectures on the icosahedron and the solution of equations of fifth degree is one of the most important and influential books of 19th-century mathematics. In the present paper, we will give the complex counterpart of Klein's book, i.e., a story about complex regular polyhedra. We will show that the following four apparently disjoint theories: the symmetries of the Hessian polyhedra (geometry), the resolution of some system of algebraic equations (algebra), the system of partial differential equations of Appell hypergeometric functions (analysis) and the modular equation of Picard modular functions (arithmetic) are in fact dominated by the structure of a single object, the Hessian group ${\frak G}^{\prime}_{216}$. There are two finite unitary groups generated by reflections corresponding to this collineation group. One, of order 648, is the symmetry group of the complex regular polyhedron $3 \{3 \} 3 \{3 \} 3$, and the other, of order 1296, is the symmetry group of the regular complex polyhedron $2 \{4 \} 3 \{3 \} 3$ or its reciprocal $3 \{3 \} 3 \{4 \} 2$.

math.NT↗

arXiv2003

Are there really phase transitions in 1-d heat conduction models?

Recently, it has been claimed (O. V. Gendelman and A. V. Savin, Phys. Rev. Lett. {\bf 84}, 2381 (2000); A.V.Savin and O.V.Gendelman, arXiv: cond-mat/0204631 (2002)) that two nonlinear classical 1-d lattice models show transitions, at finite temperatures, where the heat conduction changes from being finite to being infinite. These are the well known Frenkel-Kontorova (FK) model and a model for coupled rotators. For the FK model we give strong theoretical arguments why such a phase transition is not to be expected. For both models we show numerically that the effects observed by Gendelman {\it et al.} are not true phase transitions but are rather the expected cross-overs associated to the conductivity divergence as $T\to 0$ and (for the FK model) $T\to\infty$.

cond-mat.stat-mech↗

arXiv2002

Heat Conduction and Entropy Production in a One-Dimensional Hard-Particle Gas

We present large scale simulations for a one-dimensional chain of hard-point particles with alternating masses. We correct several claims in the recent literature based on much smaller simulations. Both for boundary conditions with two heat baths at different temperatures at both ends and from heat current autocorrelations in equilibrium we find heat conductivities kappa to diverge with the number N of particles. These depended very strongly on the mass ratios, and extrapolation to N -> infty resp. t -> infty is difficult due to very large finite-size and finite-time corrections. Nevertheless, our data seem compatible with a universal power law kappa ~ N^alpha with alpha approx 0.33. This suggests a relation to the Kardar-Parisi-Zhang model. We finally show that the hard-point gas with periodic boundary conditions is not chaotic in the usual sense and discuss why the system, when kept out of equilibrium, leads nevertheless to energy dissipation and entropy production.

nlin.CD↗

arXiv2001

Trace formulas for a class of compact complex surfaces

We give the trace formulas of weight $k$ for cocompact, torsion-free discrete subgroups of $SU(2, 1)$ and prove the analogue of the Riemann hypothesis on compact complex surfaces $M$ with $c_1^2(M)=3 c_2(M)$, where $c_i(M)$ is the $i$-th Chern class of $M$, $c_2(M)$ is a multiple of three and $c_2(M)>0$.

math.NT↗

arXiv2001

Finite Heat conduction in 2D Lattices

This paper gives a 2D hamonic lattices model with missing bond defects, when the capacity ratio of defects is enough large, the temperature gradient can be formed and the finite heat conduction is found in the model. The defects in the 2D harmonic lattices impede the energy carriers free propagation, by another words, the mean free paths of the energy carrier are relatively short. The microscopic dynamics leads to the finite conduction in the model.

nlin.CD↗