Spectrally pure heralded single photons by spontaneous four-wave mixing in a fiber: reducing impact of dispersion fluctuations

We model the spectral quantum-mechanical purity of heralded single photons from a photon-pair source based on nondegenerate spontaneous four-wave mixing taking the impact of distributed dispersion fluctuations into account. The considered photon-pair-generation scheme utilizes pump-pulse walk-off to produce pure heralded photons and phase matching is achieved through the dispersion properties of distinct spatial modes in a few-mode silica step-index fiber. We show that fiber-core-radius fluctuations in general severely impact the single-photon purity. Furthermore, by optimizing the fiber design we show that generation of single photons with very high spectral purity is feasible even in the presence of large core-radius fluctuations. At the same time, contamination from spontaneous Raman scattering is greatly mitigated by separating the single-photon frequency by more than 32 THz from the pump frequency

Topology and Phases in Fermionic Systems

There can exist topological obstructions to continuously deforming a gapped Hamiltonian for free fermions into a trivial form without closing the gap. These topological obstructions are closely related to obstructions to the existence of exponentially localized Wannier functions. We show that by taking two copies of a gapped, free fermionic system with complex conjugate Hamiltonians, it is always possible to overcome these obstructions. This allows us to write the ground state in matrix product form using Grassman-valued bond variables, and show insensitivity of the ground state density matrix to boundary conditions.Comment: 4 pages, see also arxiv:0710.329

Observation of Entangled States of a Fully Controlled 20-Qubit System

We generate and characterise entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighbouring groups of qubits. We characterise entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighbouring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.Comment: 20 pages, 4 figure

Fruit anatomy of the genus Bupleurum (Apiaceae) in northeastern China and notes on systematic implications

The fruit anatomy of all eight species of the genus Bupleurum L. mentioned in the most recent literature from northeastern China was studied for the first time. The eight species were divided into two groups based on the shape of the mericarps in transverse section. The first group (round mericarps without prominent ribs) includes B. longiradiatum and B. komarovianum, and the second group (angular mericarps with prominent ribs) includes B. angustissimum, B. bicaule, B. chinense, B. euphorbioides, B. scorzonerifolium and B. sibiricum. The second group can be further subdivided into group 2a (large vittae, very small vascular bundles) — B. euphorbioides and B. sibiricum, and group 2b (small vittae, large vascular bundles) — B. angustissimum, B. bicaule, B. chinense and B. scorzonerifolium. This result closely supports the traditional classification system which was based on gross morphology (plant height; leaf shape and size; involucre number, size and shape). The only exception is that B. komarovianum should not be close to the second group, but close to B. longiradiatum. Group 1 corresponds with Bupleurum sect. Longifolia and group 2 with sect. Eubupleura; group 2a with ser. Ranunculoides and group 2b with ser. Falcata. Fruit anatomical characters seem to have potential for evaluating infrageneric relationships in the genus Bupleurum

Lorentz invariance of entanglement classes in multipartite systems

We analyze multipartite entanglement in systems of spin-1/2 particles from a relativistic perspective. General conditions which have to be met for any classification of multipartite entanglement to be Lorentz invariant are derived, which contributes to a physical understanding of entanglement classification. We show that quantum information in a relativistic setting requires the partition of the Hilbert space into particles to be taken seriously. Furthermore, we study exemplary cases and show how the spin and momentum entanglement transforms relativistically in a multipartite setting.Comment: v2: 5 pages, 4 figures, minor changes to main body, journal references update