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We report on theoretical and experimental demonstration of high-efficiency coupling of two-photon entangled states produced in the nonlinear process of spontaneous parametric down conversion into a single-mode fiber. We determine constraints for the optimal coupling parameters. This result is crucial for practical implementation of quantum key distribution protocols with entangled states.c 2008 Optical Society of America OCIS codes: 000.1600, 220.4830.Entangled-photon pairs generated in the nonlinear process of spontaneous parametric down conversion (SPDC) are proven to be a highly desirable means 1 for practical quantum cryptography 2 . The main difficulty of practical utilization of such system usually stems from a relatively low photon collection efficiency because of the complex spatial distribution of SPDC radiation and due to the broad spectral width of entangled-photon wave packets.The problem of coupling entangled photons into a fiber has been considered before by Kurtsiefer et al.3 . Assuming the pump to be a plane wave the emission angle of the SPDC has been calculated as a function of the wavelength. The waist of the focused pump beam has been chosen to maximally overlap the "impression" of the Gaussian mode of a single-mode fiber on the crystal 3 . It has been pointed out that the coupling efficiency may be significantly affected by transverse walk-off.In this letter we present a significantly modified approach allowing us to achieve a high-efficiency coupling of the SPDC pairs into single mode fibers. In particular, we demonstrate how the pump beam waist, crystal length, optical system magnification and the fiber mode field diameter (MFD) must obey a precise joint relation in order to ensure high coupling efficiency. We describe a The function Φ(q o , ω o ; q e , ω e ) = E p (q o + q e , ω o + ω e ) χ (2) (q o , ω o ; q e , ω e ) accounts for the phase matching conditions. E p (·) represents the amplitude of the plane-wave expansion of the pump field andInside the crystal the zcomponent of the wave-vector is defined as k z (q, ω) = [ω n(q, ω)/c] 2 − |q| 2 . All the information on the state is given by the amplitude A 1,2 (x 1 , t 1 ; x 2 , t 2 ) 5,6 of detecting the SPDC two-photons in space-time events at (x 1 , t 1 ) and (x 2 , t 2 ). The Fourier transform with respect to t 1 and t 2 of the two-photon amplitude A 1,2 (x 1 , t 1 ; x 2 , t 2 ) is given by A 1,2 (x 1 , ω o ; x 2 , ω e ) = dq o dq e Φ(q o , ω o ; q e , ω e ) · ·H 1 (x 1 ; q o , ω o ) H 2 (x 2 ; q e , ω e ) (2)

This paper proposes a new protocol for quantum dense key distribution. This protocol embeds the benefits of a quantum dense coding and a quantum key distribution and is able to generate shared secret keys four times more efficiently than BB84 one. We hereinafter prove the security of this scheme against individual eavesdropping attacks, and we present preliminary experimental results, showing its feasibility.

In this paper a system for analysis and automatic indexing of imaged documents for high-volume applications is described. This system, named STRETCH (STorage and RETrieval by Content of imaged documents), is based on an Archiving and Retrieval Engine, which overcomes the bottleneck of document profiling bypassing some limitations of existing pre-defined indexing schemes. The engine exploits a structured document representation and can activate appropriate methods to characterise and automatically index heterogeneous documents with variable layout. The originality of STRETCH lies principally in the possibility for unskilled users to define the indexes relevant to the document domains of their interest by simply presenting visual examples and applying reliable automatic information extraction methods (document classification, flexible reading strategies) to index the documents automatically, thus creating archives as desired. STRETCH offers ease of use and application programming and the ability to dynamically adapt to new types of documents. The system has been tested in two applications in particular, one concerning passive invoices and the other bank documents. In these applications, several classes of documents are involved. The indexing strategy first automatically classifies the document, thus avoiding pre-sorting, then locates and reads the information pertaining to the specific document class. Experimental results are encouraging overall; in particular, document classification results fulfill the requirements of high-volume applications . Integration into production lines is under execution.

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