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We present a technique for rapid site-selective control of the quantum state of particles in a large array using a combination of a fast deflector (e.g. an acousto-optic deflector) and a relatively slow spatial light modulator. The use of spatial light modulators for site-selective quantum state manipulation has been limited due to slow transition times preventing rapid, consecutive quantum gates. By partitioning the spatial light modulator into multiple segments, and using a fast deflector to transition between them, it is possible to substantially reduce the average time increment between scanner transitions by increasing the number of gates that can be performed for a single spatial light modulator full frame setting. We analyze the performance of this device in two different configurations: in configuration 1, each segment of the spatial light modulator addresses the full qubit array; in configuration 2, each segment of the spatial light modulator addresses a sub-array and an additional fast deflector positions that sub-array with respect to the full qubit array. With these hybrid scanners we calculate qubit addressing rates that are tens to hundreds of times faster than using an SLM alone., Comment: 8 figures, minor revisions

Development of a network for remote entanglement of quantum processors is an outstanding challenge in quantum information science. We propose and analyze a two-species architecture for remote entanglement of neutral atom quantum computers based on integration of optically trapped atomic qubit arrays with fast optics for photon collection. One of the atomic species is used for atom-photon entanglement, and the other species provides local processing. We compare the achievable rates of remote entanglement generation for two optical approaches: free space photon collection with a lens and a near-concentric, long working distance resonant cavity. Laser cooling and trapping within the cavity removes the need for mechanical transport of atoms from a source region, which allows for a fast repetition rate. Using optimized values of the cavity finesse, remote entanglement generation rates $> 10^3~\rm s^{-1}$ are predicted for experimentally feasible parameters., Comment: v3: minor revisions

We modify the Kuramoto model for synchronization on complex networks by introducing a gauge term that depends on the edge betweenness centrality (BC). The gauge term introduces additional phase difference between two vertices from 0 to $\pi$ as the BC on the edge between them increases from the minimum to the maximum in the network. When the network has a modular structure, the model generates the phase synchronization within each module, however, not over the entire system. Based on this feature, we can distinguish modules in complex networks, with relatively little computational time of $\mathcal{O}(NL)$, where $N$ and $L$ are the number of vertices and edges in the system, respectively. We also examine the synchronization of the modified Kuramoto model and compare it with that of the original Kuramoto model in several complex networks., Comment: 10 pages, 7 figures

We investigated the inhomogeneous electronic properties at the surface and interior of VO_{2} thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a VO_{2} MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature dependence of the p$^{XRD}$ was very closely correlated with the dc conductivity near the MIT temperature, and fit the percolation theory predictions quite well: $\sigma$ $\sim$ (p - p_{c})^{t} with t = 2.0$\pm$0.1 and p_{c} = 0.16$\pm$0.01. This agreement demonstrates that in our VO$_{2}$ thin film, the MIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectroscopy (STS) to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results, STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate of change was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity near the surface differs greatly from that inside the film. We investigated several possible origins of this discrepancy, and postulated that the variety in the strain states near the surface plays an important role in the broad MIT observed using STS. We also explored the possible involvement of such strain effects in other correlated electron oxide systems with strong electron-lattice interactions., Comment: 27 pages and 7 figures

Measurements of optical turbulence time series data using unattended instruments over long time intervals inevitably lead to data drop-outs or degraded signals. We present a comparison of methods using both Principal Component Analysis, which is also known as the Karhunen--Loeve decomposition, and ARIMA that seek to correct for these event-induced and mechanically-induced signal drop-outs and degradations. We report on the quality of the correction by examining the Intrinsic Mode Functions generated by Empirical Mode Decomposition. The data studied are optical turbulence parameter time series from a commercial long path length optical anemometer/scintillometer, measured over several hundred metres in outdoor environments., Comment: 8 pages, 9 figures, submitted to ICOLAD 2007, City University, London, UK

We investigate the synchronization transition of the modified Kuramoto model where the oscillators form a scale-free network with degree exponent $\lambda$. An oscillator of degree $k_i$ is coupled to its neighboring oscillators with asymmetric and degree-dependent coupling in the form of $\couplingcoeff k_i^{\eta-1}$. By invoking the mean-field approach, we determine the synchronization transition point $J_c$, which is zero (finite) when $\eta > \lambda-2$ ($\eta < \lambda-2$). We find eight different synchronization transition behaviors depending on the values of $\eta$ and $\lambda$, and derive the critical exponents associated with the order parameter and the finite-size scaling in each case. The synchronization transition is also studied from the perspective of cluster formation of synchronized vertices. The cluster-size distribution and the largest cluster size as a function of the system size are derived for each case using the generating function technique. Our analytic results are confirmed by numerical simulations., Comment: 11 pages, 3 figures and two tables

Fractal scaling--a power-law behavior of the number of boxes needed to tile a given network with respect to the lateral size of the box--is studied. We introduce a new box-covering algorithm that is a modified version of the original algorithm introduced by Song et al. [Nature (London) 433, 392 (2005)]; this algorithm enables effective computation and easy implementation. Fractal networks are viewed as comprising a skeleton and shortcuts. The skeleton, embedded underneath the original network, is a special type of spanning tree based on the edge betweenness centrality; it provides a scaffold for the fractality of the network. When the skeleton is regarded as a branching tree, it exhibits a plateau in the mean branching number as a function of the distance from a root. Based on these observations, we construct a fractal network model by combining a random branching tree and local shortcuts. The scaffold branching tree can be either critical or supercritical, depending on the small-worldness of a given network. For the network constructed from the critical (supercritical) branching tree, the average number of vertices within a given box grows with the lateral size of the box according to a power-law (an exponential) form in the cluster-growing method. The distribution of box masses, i.e., the number of vertices within each box, follows a power law P_m(M) sim M^{-eta}. The exponent eta depends on the box lateral size ell_B. For small values of ell_B, eta is equal to the degree exponent gamma of a given scale-free network, whereas eta approaches the exponent tau=gamma/(gamma-1) as ell_B increases, which is the exponent of the cluster-size distribution of the random branching tree. We also study the perimeter of a given box as a function of the box mass., Comment: 14 pages, 18 figures, 2 tables

With the advent of digital media, people are increasingly resorting to online channels for commercial transactions. Online auction is a prototypical example. In such online transactions, the pattern of bidding activity is more complex than traditional online transactions; this is because the number of bidders participating in a given transaction is not bounded and the bidders can also easily respond to the bidding instantaneously. By using the recently developed network theory, we study the interaction patterns between bidders (items) who (that) are connected when they bid for the same item (if the item is bid by the same bidder). The resulting network is analyzed by using the hierarchical clustering algorithm, which is used for clustering analysis for expression data from DNA microarrays. A dendrogram is constructed for the item subcategories; this dendrogram is compared with a traditional classification scheme. The implication of the difference between the two is discussed., Comment: 8 pages and 11 figures

We study the synchronization transition (ST) of a modified Kuramoto model on two different types of modular complex networks. It is found that the ST depends on the type of inter-modular connections. For the network with decentralized (centralized) inter-modular connections, the ST occurs at finite coupling constant (behaves abnormally). Such distinct features are found in the yeast protein interaction network and the Internet, respectively. Moreover, by applying the finite-size scaling analysis to an artificial network with decentralized inter-modular connections, we obtain the exponent associated with the order parameter of the ST to be $\beta \approx 1$ different from $\beta_{\rm MF} \approx 1/2$ obtained from the scale-free network with the same degree distribution but the absence of modular structure, corresponding to the mean field value., Comment: 9pages, 6figures. PRE (in press)

We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in In$_x$Ga$_{1-x}$N/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction $x$ and unlike other coherent phonon oscillations, both \textit{amplitude} and \textit{period} are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, and as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe-wavelength as well as epilayer thickness., Comment: 11 pages, 14 figures

Scale-free (SF) networks exhibiting a power-law degree distribution can be grouped into the assortative, dissortative and neutral networks according to the behavior of the degree-degree correlation coefficient. Here we investigate the betweenness centrality (BC) correlation for each type of SF networks. While the BC-BC correlation coefficients behave similarly to the degree-degree correlation coefficients for the dissortative and neutral networks, the BC correlation is nontrivial for the assortative ones found mainly in social networks. The mean BC of neighbors of a vertex with BC $g_i$ is almost independent of $g_i$, implying that each person is surrounded by almost the same influential environments of people no matter how influential the person is., Comment: 4 pages, 4 figures, 1 table

We investigated the temperature-dependence of the Raman spectra of a nine-layer BaRuO$_3$ single crystal and a four-layer BaRuO$_3$ epitaxial film, which show pseudogap formations in their metallic states. From the polarized and depolarized spectra, the observed phonon modes are assigned properly according to the predictions of group theory analysis. In both compounds, with decreasing temperature, while $A_{1g}$ modes show a strong hardening, $E_g$ (or $E_{2g}$) modes experience a softening or no significant shift. Their different temperature-dependent behaviors could be related to a direct Ru metal-bonding through the face-sharing of RuO$_6$. It is also observed that another $E_{2g}$ mode of the oxygen participating in the face-sharing becomes split at low temperatures in the four layer BaRuO$_3 $. And, the temperature-dependence of the Raman continua between 250 $\sim $ 600 cm$^{-1}$ is strongly correlated to the square of the plasma frequency. Our observations imply that there should be a structural instability in the face-shared structure, which could be closely related to the pseudogap formation of BaRuO$_3$ systems., Comment: 8 pages, 6 figures. to be published in Phys. Rev. B

While the emergence of a power law degree distribution in complex networks is intriguing, the degree exponent is not universal. Here we show that the betweenness centrality displays a power-law distribution with an exponent \eta which is robust and use it to classify the scale-free networks. We have observed two universality classes with \eta \approx 2.2(1) and 2.0, respectively. Real world networks for the former are the protein interaction networks, the metabolic networks for eukaryotes and bacteria, and the co-authorship network, and those for the latter one are the Internet, the world-wide web, and the metabolic networks for archaea. Distinct features of the mass-distance relation, generic topology of geodesics and resilience under attack of the two classes are identified. Various model networks also belong to either of the two classes while their degree exponents are tunable., Comment: 6 Pages, 6 Figures, 1 table