Telecom-band entanglement generation in standard fibers Jun Chen, Xiaoying Li, Paul L. Voss and Prem Kumar Center for Photonic Communication and Computing, ECE Department Northwestern University, Evanston, IL 60208-3118 ABSTRACT We review on-going progress in the development of fiber-based telecom-band entanglement sources. Two different schemes (a Sagnac-loop scheme and a counter-propagating scheme) for generating polarization entanglement are reviewed and the pros and cons of each are summarized. A new scheme, called the double-loop scheme is proposed, which is theoretically shown to be capable of combining the benefits and avoiding the pitfalls of each previous scheme. Keywords: Quantum entanglement, Four-photon scattering, Raman scattering, Four-wave mixing, Fiber non- linear optics 1. INTRODUCTION Recent years have seen rapid advances in the field of quantum information processing (QIP). 1 Entangled qubits (e-bits), the fundamental building blocks of many quantum communication and cryptographic protocols, are needed to be transmitted over long distances while preserving high fidelity. 2 Spontaneous parametric down- conversion (SPDC) in second-order (χ (2) ) nonlinear crystals has proven to be a very convenient tool for producing these photonic “flying” e-bits, and has therefore been most widely used in nearly-all photonic QIP applications thus far. 3 However, high coupling losses occur when these χ (2) -generated photon pairs are launched into optical fibers for long-distance transmission, which is due to the mismatch between the SPDC photonic spatial mode and the propagation mode of standard optical fiber. This severely limits the practical prospect for such a source to develop into an efficient fiber-based quantum network transmitter/distributor. Formidable engineering problems still remain when coupling photons from a SPDC source into standard optical fiber, despite the much progress made in this direction. 4–6 Fortunately, it turns out that by utilizing the third-order (χ (3) ) nonlinearity inside optical fiber itself, photonic e-bits can be generated via a process called four-photon scattering (FPS). This approach effectively removes the need of having to couple light from free space to optical fiber. Dispersion-shifted fibers (DSF) with zero-dispersion wavelength in the telecom band (i.e., near 1550 nm) can be employed to make the resulting e-bits suitable for long-distance transmission in standard optical fiber, because the e-bits themselves also reside in the telecom band where the transmission loss is minimum. The excellent modal purity of single-mode optical fibers makes fiber-based entanglement even more attractive: it promises effective multiple quantum interactions as required by a lot of QIP applications. The broadband nature of FPS makes it possible, at least in principle, to wavelength multiplex several different entanglement channels from a single source. The only drawback of this approach has been identified to be spontaneous Raman scattering (SRS), during which uncorrelated photons are generated into the detection bands, leading to a degradation of the entanglement purity. 7 Various efforts have been made to mitigate the negative effect that SRS imposes. 8 In this paper, we will review the on-going progress in the development of our fiber-based telecom-band entan- glement sources. Three different schemes to generate polarization entanglement will be described and compared. We will start with the so-called Sagnac-loop (SL) scheme, with which we have successfully demonstrated efficient generation, 9 transmission, and storage 10 of entangled-photon pairs. Some recent improvements made within the context of this scheme will be highlighted. We then compare the results with those from a more simplified scheme — the counter-propagating (CP) scheme. 11, 12 The pros and cons of each scheme will be discussed. Finally we summarize with a new proposed scheme, viz., the double-loop (DL) scheme, which we show theoretically to be capable of combining the benefits of both previous schemes. kumarp@ece.northwestern.edu Invited Paper Quantum Optics and Applications in Computing and Communications II, edited by G.-C. Guo, H.-K. Lo, M. Sasaki, S. Liu, Proc. of SPIE Vol. 5631 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.581917 51