Pulsed source of spectrally uncorrelated and indistinguishable photons at telecom wavelengths

We report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter- propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g^(2) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols

The geometrical non-linear response of some pre-tensioned cable structures

A geometrical non-linear analysis of pre-tensioned cable structures is presented. Fully consistent methods of analysis are developed from earlier work involving minimum energy search techniques. The descent direction in the iterative process is determined by the method of conjugate gradients. The algorithm is suitable for both cable trusses and cable net structures and can include the effect of fully flexible boundaries. Convergence is obtained for all of the structural forms investigated for loadings up to the point where significant instability in the structure caused by member slackening occurs. An initial linear approximate analysis is shown to be of computational value as an initial starting point for the iterative process. Economic form finding routines are also presented including the flexible boundary case which are of general value. Two and three dimensional cable model experiments are devised to provide data for comparison with the theoretical analysis. The three dimensional net is constructed with fully flexible boundaries to provide a stringent test for the theory. Suitable measurement techniques are developed to record cable tensions and nodal deflections. Reservations of other writers about the vibrating wire method of tension measurement are overcome by a rigorous treatment of the theoretical analysis and a careful development of a demountable vibrating wire gauge. Comparison between experimental and theoretical results gives generally good agreement particularly for the non-symmetrical load cases which exhibit the greatest degree of non-linear behaviour

The geometrical non-linear response of some pre-tensioned cable structures

A geometrical non-linear analysis of pre-tensioned cable structures is presented. Fully consistent methods of analysis are developed from earlier work involving minimum energy search techniques. The descent direction in the iterative process is determined by the method of conjugate gradients. The algorithm is suitable for both cable trusses and cable net structures and can include the effect of fully flexible boundaries. Convergence is obtained for all of the structural forms investigated for loadings up to the point where significant instability in the structure caused by member slackening occurs. An initial linear approximate analysis is shown to be of computational value as an initial starting point for the iterative process. Economic form finding routines are also presented including the flexible boundary case which are of general value. Two and three dimensional cable model experiments are devised to provide data for comparison with the theoretical analysis. The three dimensional net is constructed with fully flexible boundaries to provide a stringent test for the theory. Suitable measurement techniques are developed to record cable tensions and nodal deflections. Reservations of other writers about the vibrating wire method of tension measurement are overcome by a rigorous treatment of the theoretical analysis and a careful development of a demountable vibrating wire gauge. Comparison between experimental and theoretical results gives generally good agreement particularly for the non-symmetrical load cases which exhibit the greatest degree of non-linear behaviour

High coherence photon pair source for quantum communication

This paper reports a novel single mode source of narrow-band entangled photon pairs at telecom wavelengths under continuous wave excitation, based on parametric down conversion. For only 7 mW of pump power it has a created spectral radiance of 0.08 pairs per coherence length and a bandwidth of 10 pm (1.2 GHz). The effectively emitted spectral brightness reaches 3.9*10^5 pairs /(s pm). Furthermore, when combined with low jitter single photon detectors, such sources allow for the implementation of quantum communication protocols without any active synchronization or path length stabilization. A HOM-Dip with photons from two autonomous CW sources has been realized demonstrating the setup's stability and performance.Comment: 12 pages, 4 figure

Waveguide-based OPO source of entangled photon pairs

In this paper we present a compact source of narrow-band energy-time entangled photon pairs in the telecom regime based on a Ti-indiffused Periodically Poled Lithium Niobate (PPLN) waveguide resonator, i.e. a waveguide with end-face dielectric multi-layer mirrors. This is a monolithic doubly resonant Optical Parametric Oscillator (OPO) far below threshold, which generates photon pairs by Spontaneous Parametric Down Conversion (SPDC) at around 1560nm with a 117MHz (0.91 pm)- bandwidth. A coherence time of 2.7 ns is estimated by a time correlation measurement and a high quality of the entangled states is confirmed by a Bell-type experiment. Since highly coherent energy-time entangled photon pairs in the telecom regime are suitable for long distance transmission and manipulation, this source is well suited to the requirements of quantum communication.Comment: 13 page

Energy-Time Entangled Two-Photon Molecular Absorption

Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident flux rates than in conventional two-photon absorption (TPA) schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dependence of the ETPA rate with the photon-pair flux, a clear signature of ETPA, and estimate the first values for the concentration-dependent ETPA cross-section for Rhodamine 6G.We then investigate the signature of energy-time entanglement and polarization dependence in the ETPA fluorescence rate and demonstrate a strong dependence of the signal on the inter-photon delay that reflects the coherence time of the entangled two-photon wave-packet

Universal quantum computation and simulation using any entangling Hamiltonian and local unitaries

What interactions are sufficient to simulate arbitrary quantum dynamics in a composite quantum system? We provide an efficient algorithm to simulate any desired two-body Hamiltonian evolution using any fixed two-body entangling n-qubit Hamiltonian and local unitaries. It follows that universal quantum computation can be performed using any entangling interaction and local unitary operations.Comment: Added references to NMR refocusing and to earlier work by Leung et al and Jones and Knil

Entangled Bessel beams

Orbital angular momentum (OAM) entanglement is investigated in the Bessel-Gauss (BG) basis. Having a readily adjustable radial scale, BG modes provide a more favourable basis for OAM entanglement over Laguerre-Gaussian (LG) modes. The OAM bandwidth in terms of BG modes can be increased by selection of particular radial modes and leads to a flattening of the spectrum. The flattening of the spectrum allows for higher entanglement. We demonstrate increased entanglement in terms of BG modes by performing a Bell-type experiment and violating the appropriate Clauser Horne Shimony Holt (CHSH) inequality. In addition, we reconstruct the quantum state of BG modes entangled in high-dimensions.Comment: 8 pages, 4 figure