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arXiv:1208.2592 · Physical Review Letters

Experimental Realization of Three-Color Entanglement at Optical Fiber Communication and Atomic Storage Wavelengths

Xiaojun Jia, Zhihui Yan, Zhiyuan Duan, Xiaolong Su, Hai Wang, Changde Xie, Kunchi Peng

Published Dec 18, 2012• 79 citations
arXivHTMLPDF

Abstract

Entangled states of light including low-loss optical fiber transmission and atomic resonance frequencies are essential resources for future quantum information networks. We present the experimental achievement on the three-color entanglement generation at 852, 1550, and 1440 nm wavelengths for optical continuous variables. The entanglement generation system consists of two cascaded nondegenerated optical parametric oscillators (NOPOs). The flexible selectivity of nonlinear crystals in the two NOPOs and the tunable property of NOPO provide large freedom for the frequency selection of three entangled optical beams. The presented system will hopefully be developed as a practical entangled source to be used in quantum-information networks with atomic storage units and long fiber transmission lines.

DOI
10.1103/physrevlett.109.253604

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Entangled states of light including low-loss optical fiber transmission and atomic resonance frequencies are essential resources for future quantum information networks. We present the experimental achievement on the three-color entanglement generation at 852, 1550, and 1440 nm wavelengths for optical continuous variables. The entanglement generation system consists of two cascaded nondegenerated optical parametric oscillators (NOPOs). The flexible selectivity of nonlinear crystals in the two NOPOs and the tunable property of NOPO provide large freedom for the frequency selection of three entangled optical beams. The presented system will hopefully be developed as a practical entangled source to be used in quantum-information networks with atomic storage units and long fiber transmission lines.
We propose a generation system of continuous-variable (CV) three-color entangled state of bright optical beams based on two cascaded standard nondegenerate optical parametric oscillators (NOPOs) above the threshold. One of signal and idler beams produced by the first NOPO is used for the pump light of the second NOPO. The three-color entanglement among signal and idler beams produced by the second NOPO and the retained another beam of the first NOPO is theoretically demonstrated. The symplectic eigenvalues of the partial transposition covariance matrix of the generated optical entangled state are numerically calculated in terms of experimentally reachable system parameters. The optimal operation conditions of the cascaded NOPOs system for obtaining high entanglement are found. The calculated results explicitly demonstrate that the OPO action can transfer entanglement. Due to that the cavity parameters and the nonlinear crystals of the two NOPOs can be freely chosen, the flexibility of the proposed protocol is relatively good and the system can be also extended to prepare entangled states with more parts easily.
Quantum entangled states, especially those having particular properties, are key resources for quantum information and quantum computation. In this paper, we put forward a new scheme to produce 31 continuous–variable (CV) tripartite entanglement fields based on three optical frequency combs via cascade nonlinear processes in an optical parametric cavity, and investigate the spectral characteristics of three frequency combs. The center wavelengths of the three combs are designed as 852 nm, 780 nm (atomic transition lines), and 1550 nm (fiber communication wavelength). The positivity under partial transposition (PPT) criterion, which is sufficient and necessary, is used to evaluate the entanglement in each group of comb lines. This scheme is experimentally feasible and valuable for constructing quantum information networks in future.
Continuous variable (CV) quantum entanglement is an essential resource for quantum computation and communication protocols. The use of CV quantum entanglement at a telecommunication wavelength of 1.5m in combination with existing fiber telecommunication networks offers the possibility to implement long-distance quantum communication protocols like quantum key distribution (QKD) and applications such as quantum repeaters, quantum teleportation in the future. In spite of the fact that the optical power attenuation of light in a standard telecommunication fiber is lowest at a wavelength of 1.5m, the entangled states will interact with fiber channels and the disentanglement will occur. It is one of the important factors restricting the development of long distance quantum information. In this paper, CV entangled state at 1.5m telecommunication band is obtained by using a type-II periodically poled KTP (PPKTP) crystal inside a nondegenerate optical parametric amplifier (NOPA). A wedged PPKTP is used for implementing frequency-down-conversion of the pump field to generate the optically entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. By controlling the temperature and the length of the PPKTP crystal, a triply resonant optical parametric oscillator with a threshold of 80 mW is realized. Einstein-PodolskyRosen (EPR)-entangled beams with quantum correlation of 8.3 dB for both the amplitude and phase quadratures are experimentally generated by using a single NOPA at a pump power of 40 mW and an injected signal power of 10 mW when the relative phase between the pump and injected signal is locked to . The generated entangled state is coupled into a single-mode optical fiber, and the transmission characteristics of the generated EPR entangled beams through standard single-mode fibers are investigated experimentally and theoretically. A fiber polarization controller is used to compensate for the polarization state variation induced by random fluctuations of birefringence of the single mode fiber when the light propagates along the fiber, and to keep the polarization of light linear at the fiber output. A 0.21 dB quantum entanglement could still be observed for the EPR-entangled beams transmitted through a 50-km-long single-mode fiber. The theoretical prediction considering the excess noise in fiber channel is in good agreement with the experimental result. The generated CV quantum entanglement is highly suitable for the required experiments, such as CV measurement-device-independence QKD based on standard fibers, owing to the fact that the tolerance of the excess noise in the quantum channel can be enhanced significantly with respect to a coherent state if EPR-entangled beams are used.
The direct generation of discrete-variable three-photon polarization entanglement has been recently demonstrated [Nat. Photonics8, 801 (2014)10.1038/nphoton.2014.218NPAHBY1749-4885]. Here, we propose a feasible scheme to generate continuous-variable (CV) three-color polarization entangled states of light in a deterministic way. The method initially prepares a tripartite quadrature entanglement based on two cascaded nondegenerate optical parametric oscillators. The entangled modes are then coupled, respectively, with three strong coherent optical beams on polarized beam splitters, leading to the generation of the three-color CV polarization entanglement. The polarization entanglement is then verified by inseparability criterion and positive partial transposition criterion. Additionally, optimal parameters in practical conditions are obtained through numerical simulations, which could provide useful guidance for experimental implementations. The demonstrated CV three-color polarization entanglement is suitable in quantum information processing tasks relying on the direct interaction between spin of atoms and polarization of light.
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.