25 October 2011 Quantum security in homodyne reception using weak coherent states
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We present an experimental 8-port Balanced Homodyne Detector at 1550 nm wavelength, operating in free space, implemented with polarization devices to produce a circularly polarized local oscillator, splitting its In-Phase and Quadrature components to beat separately with the weak coherent incoming signal. This allows the simultaneous measurements of the 2 quadratures at the price of an additional noise due to the vacuum fields that leak via the unused ports, resulting in a modified Husimi function for joint probability distribution for the quadrature components. These schemes require the proper optical phase synchronization between the local oscillator and the incoming field, which constitutes a challenge for weak coherent state reception. To achieve this we designed and implemented an optical Costas loop; the feedback loop (especially the loop filter) which is a result of the optimal design has an impact on the mutual information between transmitter and receiver, being this parameter a condition to generate the cryptographic key. We present experimental and theoretical results on the performance of the mutual information between the transmitter and the receiver due the phase error for different photon numbers.
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E. García, E. García, J. A. López, J. A. López, F. J. Mendieta, F. J. Mendieta, A. Arvizu, A. Arvizu, } "Quantum security in homodyne reception using weak coherent states", Proc. SPIE 8011, 22nd Congress of the International Commission for Optics: Light for the Development of the World, 80113K (25 October 2011); doi: 10.1117/12.902191; https://doi.org/10.1117/12.902191

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