We present a fiber based source of polarization-entangled photon pairs that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally-polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either the signal or the idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the CHSH form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.

Abstract-In this letter, we present a source of quantum-correlated photon pairs based on parametric fluorescence in a fiber Sagnac loop. The photon pairs are generated in the 1550-nm fiber-optic communication band and detected with InGaAs-InP avalanche photodiodes operating in a gated Geiger mode. A generation rate 10 3 pairs/s is observed, which is limited by the detection electronics at present. We also demonstrate the nonclassical nature of the photon correlations in the pairs. This source, given its spectral properties and robustness, is well suited for use in fiber-optic quantum communication and cryptography networks.Index Terms-Fiber four-wave mixing, parametric amplifiers, photon counting, quantum communication, quantum cryptography. EFFICIENT generation and transmission of quantum-correlated photon pairs, especially in the 1550-nm fiber-optic communication band, is of paramount importance for practical realization of the quantum communication and cryptography protocols [1]. The workhorse source employed in all implementations, thus far [2] has been based on the process of spontaneous parametric down-conversion in second-order [ ] nonlinear crystals. Such a source, however, is not compatible with optical fibers as large coupling losses occur when the pairs are launched into the fiber. This severely degrades the correlated photon-pair rate coupled into the fiber, since the rate depends quadratically on the coupling efficiency. From a practical standpoint, it would be advantageous if a photon-pair source could be developed that not only produces photons in the communication band but also can be spliced to standard telecommunication fibers with high efficiency. Over the past few years, various attempts have been made to develop more efficient photon-pair sources, but all have relied on the down-conversion process [3]- [6]. Of particular note is [4], in which the effective of periodically poled silica fibers was used. In this letter, we report the first, to the best of our knowledge, photon-pair source that is based on the Kerr nonlinearity ( ) of standard fiber. Quantum-correlated photon pairs are observed and characterized in the parametric fluorescence of four-wave mixing (FWM) in dispersion-shifted fiber (DSF).The FWM process takes place in a nonlinear-fiber Sagnac interferometer (NFSI), shown schematically in Fig. 1. Previ- ously, we have used this NFSI to generate quantum-correlated twin beams in the fiber [7]. The NFSI consists of a fused-silica 50/50 fiber coupler spliced to 300 m of DSF having zero-dispersion wavelength nm. It can be set as a reflector with proper adjustment of the intraloop FPC to yield a transmission coefficient 30 dB. When the injected pump wavelength is slightly greater than , FWM in the DSF is phase matched [8]. Two pump photons of frequency scatter into a signal photon and an idler photon of frequencies and , respectively, where . Signal-idler separations of 20 nm can be easily obtained with use of commercial DSF [7]. The pump is a mode-locked train of 3-ps-long pulses that a...

We demonstrate greatly improved results for the production of correlated photon-pairs using the four-photon scattering process in silica fiber. We achieve a true-coincidence-count to accidental-coincidence-count ratio greater than 10, when the photon-pair production rate is about 0.04 /pulse. This represents a four-fold improvement over our previous results. The contribution of spontaneous Raman scattering, the primary cause of uncorrelated photons that degrades the fidelity of this source, is reduced by decreasing the wavelength detuning between the correlated photons and the pump photons and by using polarizers to remove the cross-polarized Raman-scattered photons. Excess Raman scattering could be further suppressed by cooling the silica fiber. Even without cooling the fiber, the achieved 10 to 1 ratio of true-coincidence to accidental-coincidence makes the fiber source of correlated photon-pairs a useful tool for realizing various quantum-communication protocols.

This article reports on two-dimensional (2D) layered hexagonal BN (h-BN) grown on sapphire by metalorganic vapor phase epitaxy (MOVPE). The highly oriented lattice and hexagonal phase of the epitaxial layers were confirmed by X-ray diffraction, Raman spectrum, and cross-section scanning transmission electron microscopy. The surface of BN over a 2-in. wafer exhibits specific 2D material morphology features for different BN thicknesses, from an atomically flat surface to a honeycomb wrinkle network. The grown epitaxial layers demonstrate a large absorption coefficient (∼10 6 cm −1 ) above the bandgap energy of 5.87 eV with direct band transition behavior. Near-bandgap luminescence at 216.5 nm (5.73 eV) and characteristic defect band recombination at longer wavelengths were observed by cathodoluminescence at 77 K. This wafer-scale MOVPE-grown layered h-BN with different 2D morphology and with near bandgap emission can facilitate applications such as graphene-based electronics, advanced van der Waals heterostructures, and deep UV photonics.