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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Efficient generation of CV entanglement by triply resonant non-degenerate OPA

A. Porzio, Virginia D’Auria, Stefano Fornaro, Salvatore Solimeno, Stefano Olivares, Matteo G. A. Paris

Published May 4, 2007• 0 citations
Publisher

Abstract

CV entangled CW bright beams are experimentally generated by a Non-degenerate "triply resonant" Optical Parametric Amplifier (NOPA) based on PPKTP type-II crystal below threshold. Operating the OPA at frequency degeneracy makes the down-converted entangled beams able to be optically manipulated for generating different combination of the entangled system. Particular care has been required by the triply resonance condition essential to obtain a suffcient degree of entanglement. The triply resonance condition is pursued by combining temperature phase-matching and crystal tilting in an optical cavity so to optimize the triple resonance condition acting on different independent parameters. Exploiting the dependence of the bipartite system covariance matrix on different combination of this continuous variable two-mode states it can be directly measured using a single homodyne detector and few linear optical elements. The system is actually operating at the University of Napoli-Quantum Optics Laboratory. Preliminary measurements show that CV entanglement is present at the OPO output. This source of CV entanglement can be used for quantum communication purposes. The technical tricks herein implemented are interesting also for realizing triply resonant CW OPOs whose spectral properties and conversion efficiencies are better compared to single and double resonant devices. The quality of entanglement is also improved thanks to the use of a single cavity instead of dual cavities often employed in triply resonant devices. The measurement method can be applied to generic bipartite states made of frequency degenerate but orthogonally polarized modes.

DOI
10.1117/12.722940

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Physical Review A•Aug 2, 2011•14 citations
CV entangled CW bright beams are experimentally generated by a Non-degenerate "triply resonant" Optical Parametric Amplifier (NOPA) based on PPKTP type-II crystal below threshold. Operating the OPA at frequency degeneracy makes the down-converted entangled beams able to be optically manipulated for generating different combination of the entangled system. Particular care has been required by the triply resonance condition essential to obtain a suffcient degree of entanglement. The triply resonance condition is pursued by combining temperature phase-matching and crystal tilting in an optical cavity so to optimize the triple resonance condition acting on different independent parameters. Exploiting the dependence of the bipartite system covariance matrix on different combination of this continuous variable two-mode states it can be directly measured using a single homodyne detector and few linear optical elements. The system is actually operating at the University of Napoli-Quantum Optics Laboratory. Preliminary measurements show that CV entanglement is present at the OPO output. This source of CV entanglement can be used for quantum communication purposes. The technical tricks herein implemented are interesting also for realizing triply resonant CW OPOs whose spectral properties and conversion efficiencies are better compared to single and double resonant devices. The quality of entanglement is also improved thanks to the use of a single cavity instead of dual cavities often employed in triply resonant devices. The measurement method can be applied to generic bipartite states made of frequency degenerate but orthogonally polarized modes.
We report the first complete experimental reconstruction of the covariance matrix (CM) relative to a bi-partite continuous variable (CV) entangled state outing a non degenerate optical parametric oscillator (OPO) below threshold. The covariance matrix CM has been reconstructed following the method reported in V. D'Auria et al., J. Opt. B 7, S750 (2005). The two entangled beams (signal (a) and idler (b)) are produced by a by a continuous wave (CW) optical parametric oscillator (OPO) via type-II (same frequency but with orthogonal polarizations), phase matching and working below threshold. Our experimental setup makes use of a single homodyne detector and of a compact source of entangled beams. The quadratures values and other relevant quantities are reconstructed by quantum tomography, without making any a priori assumption on the state under evalutation.
Entangled state of light is one of the essential quantum resources in quantum information science and technology. Especially, when the fundamental principle experiments have been achieved in labs and the applications of continuous variable quantum information in the real world are considered, it is crucial to design and construct the generation devices of entangled states with high entanglement and compact configuration. We have designed and built an efficient and compact light source of entangled state, which is a non-degenerate optical parametric amplifier (NOPA) with the triple resonance of the pump and two subharmonic modes. A wedged type-II KTP crystal inside the NOPA is used for implementing frequency-down-conversion of the pump field to generate the optical entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. The EPR entangled state of light with quantum correlations of 8.4 dB for both amplitude and phase quadratures are experimentally produced by a single NOPA under the pump power of 75 mW.
Continuous variable (CV) quantum entanglement is a fundamental resource of CV quantum communication and quantum computation. It is useful in a wide variety of applications, including quantum teleportation, quantum dense coding, quantum key distribution, and high-precision quantum measurement. In this paper, we generate CV quantum entanglement at a telecommunication wavelength of 1342 nm by using a nondegenerate optical parametric amplifier (NOPA) with a type-Ⅱ periodically poled KTiOPO4 (PPKTP) crystal. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1342 nm) Nd:YVO4/LiB3O5 laser is achieved with output powers of 1.5 W (671 nm) and 1.3 W (1342 nm). Then a mode cleaner (MC1) with a fineness of 400 and linewidth of 0.75 MHz and a mode cleaner MC2 with a fineness of 400 and linewidth of 0.75 MHz are used to filter the noises of laser at 1342 nm and 671 nm, respectively. By using MCs, the intensity noise of laser reaches a shot noise level (SNL) for analysis frequencies higher than 1.0 MHz, and the phase noise of laser reaches an SNL for analysis frequencies higher than 1.3 MHz. Utilizing this kind of low noise single-frequency 671 nm laser as a pump, a doubly-resonant optical parametric oscillator with a threshold of 325 mW is realised. When the low noise single-frequency 1342 nm laser is injected as a signal and the relative phase between the pump and injected signal is locked to , the NOPA is operated at deamplification. After optimizing the temperature of the type-Ⅱ PPKTP crystal and at a pump power of 260 mW, Einstein-Podolsky-Rosen (EPR)-entangled beams with quantum correlation of 3.0 dB for both the amplitude and phase quadratures are experimentally generated. The strength of EPR-entangled beams is relatively low. It is maybe due to the low nonlinear conversion efficiency and large absorption of the type-Ⅱ PPKTP crystal at 671 nm and 1342 nm. The generated CV quantum entanglement at 1.34 m has lower transmission loss and smaller phase diffusion effect in a silica fiber. The research contributes to a high quality quantum source for the CV quantum communication based on existing telecommunication fiber networks.
Abstract Continuous variable quantum entanglement at a telecommunication wavelength of 1550 nm is experimentally generated using a single nondegenerate optical parametric amplifier based on a type-II periodically poled KTiOPO 4 crystal. The triply resonant of the nondegenerate optical parametric amplifier is adjusted by tuning the crystal temperature and tilting the orientation of the crystal in the optical cavity. Einstein–Podolsky–Rosen-entangled beams with quantum correlations of 8.3 dB for both the amplitude and phase quadratures are experimentally generated. This system can be used for continuous variable fibre-based quantum communication.
We present a scheme for generating and homodyne detecting of continuous-variable entanglement of bright optical beams with a large wavelength difference by utilizing an optical parametric oscillator (OPO) and an optical parametric amplifier (OPA) simultaneously. Entangled optical beams at 0.8 and 1.5 \ensuremath{\mu}m are generated from the OPA; the seed beams injected in the OPA as well as the local oscillators at the two wavelengths needed for homodyne detection are provided by the OPO. The entangler is a ring resonator involving a second-order nonlinear crystal that is pumped from two opposite directions. In one direction the pump power is above the oscillation threshold and the optical nonlinear resonator operates as an OPO. In the other direction the pump power is below the threshold and it operates as a phase-sensitive frequency nondegenerate optical parametric amplifier. Our scheme combines the advantages of both OPO and OPA quantum optical devices and opens another avenue for preparation and homodyne detection of high quality bright entangled light with a large wavelength difference.