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arXiv:1307.7939 · Optics Express

Post-selection free, integrated optical source of non-degenerate, polarization entangled photon pairs

Harald Herrmann, Xu Lei Yang, Abu Thomas, Andreas Poppe, W. Sohler, Christine Silberhorn

Published Nov 7, 2013• 66 citations
arXivHTMLPDF

Abstract

We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.

DOI
10.1364/oe.21.027981

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We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.
We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures.
Entangled photon pair sources are of key importance in the emerging field of quantum information technology. A common method to generate entangled pairs is based on type II phase-matched spontaneous parametric down conversion (SPDC) followed by a spatial separation of the pair using a conventional beam splitter (see e.g.). This scheme can be used to generate entangled pairs at degeneracy with the drawback that half of the generated pairs are “lost” if both photons leave the beam splitter at the same output port. Recently, we have demonstrated a new scheme providing non-degenerated pairs by employing a sophisticated poling pattern. In this contribution we report a source of non-degenerated polarization entangled photon pairs using such a PPLN waveguide together with a polarization maintaining fiber as quantum eraser and a commercially available fiber-based coarse WDM coupler.
Abstract Polarisation-entangled photon pairs generated through nonlinear optical processes are crucial ingredients for a range of quantum information processing applications, such as quantum sensing and communication. In this work, we report an optimised design of a lithium niobate on insulator waveguide that is capable of directly generating polarisation-entangled photon pairs enabled through two different spontaneous parametric down-conversion processes simultaneously in the telecommunication wavelength band with a single poling period, based on Type II phase matching. The generated photon pairs cover wavelengths in telecom O (1310 nm) and telecom C (1550 nm) bands with the computed spectral brightness of ≈ 2 × 10 9 pairs sec − 1 mW − 1 . We also study the effect of temporal delay on the quality of entanglement, which is minimised by selecting an appropriate length of the waveguide and pump bandwidth. The optimised integrated quantum light source, presented here, eliminates the need for complex domain engineering or intermodal dispersion and will serve as a valuable resource for various quantum-optical experiments.
In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.
We demonstrate a compact, practical all-fiber source of non-degenerate telecom-band polarization-entangled photon pairs assembled from off-the-shelf components. The compact fiber-integrated architecture is designed to be affordable, straightforward to assemble and align, and compatible with existing optical-fiber infrastructure. Photon pairs are generated through type-0 spontaneous parametric down-conversion in a pigtailed periodically poled lithium-niobate waveguide embedded in a Sagnac interferometer. At 0.5 mW pump power, quantum-state tomography yields a Bell-state fidelity FΦ−=0.850F_{Φ^-}=0.850FΦ−​=0.850, purity P=0.836P=0.836P=0.836, and concurrence C=0.782C=0.782C=0.782. A separate CHSH measurement gives S=2.3301±0.0075S=2.3301\pm0.0075S=2.3301±0.0075. The measured Bell-state fidelity is sufficient for the reconstructed polarization state to support qubit teleportation above the classical fidelity limit under ideal protocol operations. Analysis of the density matrix shows that approximately 96.7% of the population remains in the desired ∣HH⟩|HH\rangle∣HH⟩-∣VV⟩|VV\rangle∣VV⟩ subspace, with a normalized coherence of approximately 0.86. A residual relative-phase offset indicates that local phase compensation could increase the Bell-state fidelity to approximately 0.8865. We further characterize the brightness--quality trade-off and obtain a corrected pair-rate slope of (0.918±0.040)(0.918\pm0.040)(0.918±0.040) Mcps mW−1^{-1}−1. The broadband emission and fiber interfaces offer a route to multiuser entanglement distribution through off-the-shelf multichannel wavelength-division multiplexers with passbands matched to the signal and idler wavelengths.
We report the realization of a new polarization entangled photon-pair source based on a titanium-indiffused waveguide integrated on periodically poled lithium niobate pumped by a CW laser at 655 nm. The paired photons are emitted at the telecom wavelength of 1310 nm within a bandwidth of 0.7 nm. The quantum properties of the pairs are measured using a two-photon coalescence experiment showing a visibility of 85%. The evaluated source brightness, on the order of 10(5) pairs s(-1)GHz(-1)mW(-1), associated with its compactness and reliability, demonstrates the source's high potential for long-distance quantum communication.