Mixed H^2/H^∞ Estimation: Preliminary Analytic Characterization And A Numerical Solution

We introduce and motivate the problem of mixed H^2/H∞ estimation by studying the stochastic and deterministic approaches of H^2 and H^∞ estimation. Mixed H^2/H^∞ estimators have the property that they have the best average performance over all estimators that achieve a certain worst-case performance bound. They thus allow a tradeoff between average and worst-case performances. In the finite horizon case, we obtain a numerical solution (based on convex optimization methods) for the optimal mixed H^2/H^∞ estimator. We also give some analytic characterizations, both on this optimal solution, and on the set of all estimators achieving a guaranteed worst-case bound. A numerical example is also provided

Design of optimal mixed H_2/H_∞ static state feedback controllers

Despite advances in robust control theory, the robust performance problem formulated in the mixed H_2/H_∞ framework largely remains an open problem. In this approach, one seeks a controller that minimizes the H_2 norm of a closed-loop map over all admissible controllers while satisfying an H_∞ constraint on another closed-loop map. Unlike the optimal H_2 problem or the γ-level sub-optimal H_∞ problem, the mixed H_2/H_∞ problem does not have a readily computable solution. In the paper we restrict consideration to static state feedback controllers and propose an efficient iterative algorithm for computing the optimal H_2/H_∞ solution

Integrated optical source of polarization entangled photons at 1310 nm

We report the realization of a new polarization entangled photon-pair source based on a titanium-indiffused waveguide integrated on periodically poled lithium niobate pumped by a CW laser at $655 nm$. The paired photons are emitted at the telecom wavelength of $1310 nm$ within a bandwidth of $0.7 nm$. The quantum properties of the pairs are measured using a two-photon coalescence experiment showing a visibility of 85%. The evaluated source brightness, on the order of $10^5$ pairs $s^{-1} GHz^{-1} mW^{-1}$, associated with its compactness and reliability, demonstrates the source's high potential for long-distance quantum communication.Comment: There is a typing mistake in the previous version in the visibility equation. This mistake doesn't change the result

Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40*pi rad/s are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres

Polarization entangled photon-pair source based on a type-II PPLN waveguide emitting at a telecom wavelength

We report the realization of a fiber coupled polarization entangled photon-pair source at 1310 nm based on a birefringent titanium in-diffused waveguide integrated on periodically poled lithium niobate. By taking advantage of a dedicated and high-performance setup, we characterized the quantum properties of the pairs by measuring two-photon interference in both Hong-Ou-Mandel and standard Bell inequality configurations. We obtained, for the two sets of measurements, interference net visibilities reaching nearly 100%, which represent important and competitive results compared to similar waveguide-based configurations already reported. These results prove the relevance of our approach as an enabling technology for long-distance quantum communication.Comment: 13 pages, 4 figures, to appear in New Journal of Physic

Nonclassical 2-photon interference with separate intrinsically narrowband fibre sources

In this paper, we demonstrate a source of photon pairs based on four-wave-mixing in photonic crystal fibres. Careful engineering of the phase matching conditions in the fibres enables us to create photon pairs at 597 nm and 860 nm in an intrinsically factorable state showing no spectral correlations. This allows for heralding one photon in a pure state and hence renders narrow band filtering obsolete. The source is narrow band, bright and achieves an overall detection efficiency of up to 21% per photon. For the first time, a Hong-Ou-Mandel interference with unfiltered photons from separate fibre sources is presented.Comment: 10 pages, 6 figure

Reciprocal space mapping of magnetic order in thick epitaxial MnSi films

We report grazing incidence small angle neutron scattering (GISANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit. Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a magnetic modulation perpendicular to the films under magnetic fields parallel and perpendicular to the film, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of field orientation, our data does not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct difference with bulk samples of MnSi of similar thickness under perpendicular field, in which a skyrmion lattice dominates the phase diagram. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell control the magnetic state, representing the main difference between the films and thin bulk samples

Evaluation and exploration of a social prescribing initiative: study protocol

Attention is being given to healthcare initiatives with the potential to save money and improve lives. One example is Social Prescribing (SP), which supports patients whose ill-health is exacerbated by loneliness. While evidence has accumulated attesting to SP’s efficacy, one limitation has been the lack of a theoretical framework, which limits understanding of how tackling loneliness improves health. In our evaluation of an SP pathway, we adopt a 'Social Cure' approach, which posits that social relations affect health. Our study will evaluate the efficacy of the pathway and determine the extent to which group processes are responsible for health improvements