Spectral decomposition of Bell's operators for qubits

The spectral decomposition is given for the N-qubit Bell operators with two observables per qubit. It is found that the eigenstates (when non-degenerate) are N-qubit GHZ states even for those operators that do not allow the maximal violation of the corresponding inequality. We present two applications of this analysis. In particular, we discuss the existence of pure entangled states that do not violate any Mermin-Klyshko inequality for $N\geq 3$.Comment: 12 pages, 1 figure

From Cooperative Scans to Predictive Buffer Management

In analytical applications, database systems often need to sustain workloads with multiple concurrent scans hitting the same table. The Cooperative Scans (CScans) framework, which introduces an Active Buffer Manager (ABM) component into the database architecture, has been the most effective and elaborate response to this problem, and was initially developed in the X100 research prototype. We now report on the the experiences of integrating Cooperative Scans into its industrial-strength successor, the Vectorwise database product. During this implementation we invented a simpler optimization of concurrent scan buffer management, called Predictive Buffer Management (PBM). PBM is based on the observation that in a workload with long-running scans, the buffer manager has quite a bit of information on the workload in the immediate future, such that an approximation of the ideal OPT algorithm becomes feasible. In the evaluation on both synthetic benchmarks as well as a TPC-H throughput run we compare the benefits of naive buffer management (LRU) versus CScans, PBM and OPT; showing that PBM achieves benefits close to Cooperative Scans, while incurring much lower architectural impact.Comment: VLDB201

The mechanical properties and behavior of unidirectionally solified Mg-based alloys

Three systems investigated represent different morphologies and volume fractions of the constituent phases. Strength and fracture characteristics were studied in tension and compression. Metallography and fractography revealed that fracture initiated in the brittle intermetallic and that the integrity of the matrix-intermetallic interface was maintained

Interference due to Coherence Swapping

We propose a method called `coherence swapping' which enables us to create superposition of a particle in two distinct paths, which is fed with initially incoherent, independent radiations. This phenomenon is also present for the charged particles, and can be used to swap the effect of flux line due to Aharonov-Bohm effect. We propose an optical version of the experimental set-up to test the coherence swapping. The phenomenon, which is simpler than entanglement swapping or teleportation, raises some fundamental questions about true nature of wave-particle duality, and also opens up the possibility of studying the quantum erasure from a new angle.Comment: Latex file, 10 pages, Two figure

Nonsequential positive-operator-valued measurements on entangled mixed states do not always violate a Bell inequality

We present a local-hidden-variable model for positive-operator-valued measurements (an LHVPOV model) on a class of entangled generalized Werner states, thus demonstrating that such measurements do not always violate a Bell-type inequality. We also show that, in general, if the state $\rho'$ can be obtained from $\rho$ with certainty by local quantum operations without classical communication then an LHVPOV model for the state $\rho$ implies the existence of such a model for $\rho'$.Comment: 4 pages, no figures. Title changed to accord with Phys. Rev. A version. Journal reference adde

Full separability criterion for tripartite quantum systems

In this paper, an intuitive approach is employed to generalize the full separability criterion of tripartite quantum states of qubits to the higher-dimensional systems (Phys. Rev. A \textbf{72}, 022333 (2005)). A distinct characteristic of the present generalization is that less restrictive conditions are needed to characterize the properties of full separability. Furthermore, the formulation for pure states can be conveniently extended to the case of mixed states by utilizing the kronecker product approximate technique. As applications, we give the analytic approximation of the criterion for weakly mixed tripartite quantum states and investigate the full separability of some weakly mixed states.Comment: 5 pages. To be published in Europ. J.

Photon-bunching measurement after 2x25km of standard optical fibers

To show the feasibility of a long distance partial Bell-State measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecom wavelength are created by parametric down conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam-splitter and transmitted via two fibers of 25km. The wave-packets are relatively delayed and recombined on a second beam-splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the 25km long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long distance Bell state measurement with two independent sources, as e.g. required in an entanglement swapping configuration in the scale of tens of km.Comment: 6 pages, 5 figure

Comment on "Quantum key distribution without alternative measurements"

In a recent paper [A. Cabello, Phys. Rev. A 61, 052312 (2000)], a quantum key distribution protocol based on entanglement swapping was proposed. However, in this comment, it is shown that this protocol is insecure if Eve use a special strategy to attack.Comment: 2 Pages, 1 Figur

Comment on ``Manipulating the frequency entangled states by an acoutic-optical modulator''

A recent theoretical paper [1] proposes a scheme for entanglement swapping utilizing acousto-optic modulators without requiring a Bell-state measurement. In this comment, we show that the proposal is flawed and no entanglement swapping can occur without measurement.Comment: 6 pages, 2 figures submitted to Phys. Rev