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

Higher fidelity source of entangled photon-pairs from a photonic crystal fiber via mode expansion and Bragg filtering

Alexander Ling, Jun Chen, Jingyun Fan, Alan L. Migdall

Published Jul 21, 2009• 0 citations
Publisher

Abstract

We report the development of a fiber-based single-spatial-mode source of photon-pairs where the efficiency of extracting photon pairs is 14× higher than a previous implementation [16]. This critical improvement in efficiency enabled a spectrally bright and pure photon-pair source having a small second-order correlation function (0.03) and a raw spectral brightness of 44,700 pairs/(s nm mW). The source can be configured to generate entangled photon-pairs, characterized via optimal and minimal quantum state tomography, to have a fidelity of 97% and tangle of 92%, without correcting for accidentals.

DOI
10.1117/12.824362

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We report the development of a fiber-based single-spatial-mode source of photon-pairs where the efficiency of extracting photon pairs is 14× higher than a previous implementation [16]. This critical improvement in efficiency enabled a spectrally bright and pure photon-pair source having a small second-order correlation function (0.03) and a raw spectral brightness of 44,700 pairs/(s nm mW). The source can be configured to generate entangled photon-pairs, characterized via optimal and minimal quantum state tomography, to have a fidelity of 97% and tangle of 92%, without correcting for accidentals.
We report the development of a fiber-based single spatial-mode source of photon-pairs where the efficiency of extracting photon-pairs is increased through the use of fiber-end expansion and Bragg filters. This improvement in efficiency enabled a spectrally bright and pure photon-pair source having a small second-order correlation function (0.03) and a raw spectral brightness of 44,700 pairs s(-1)nm(-1)mW(-1). The source can be configured to generate entangled photon-pairs, characterized via optimal and minimal quantum state tomography, to have a fidelity of 97% and tangle of 92%, without subtracting any background.
Entangled photon pairs play a crucial role in emerging quantum technologies, acting as tamper-proof padlocks in secure quantum communication, ultra-precise probes in quantum metrology and high-fidelity information carriers in photonic quantum computing. Amongst the many technological possibilities for generating photon pairs, sources based on spontaneous parametric down-conversion (SPDC) in second-order nonlinear crystals are still the workhorse tool in quantum optics laboratories, and more recently even in long-distance quantum communication with satellites. SPDC is a widely used method to create entangled photon pairs. The number of photon pairs that can be detected in a particular choice of collection modes, typically being optical single-mode fibers, depends critically on the spatial characteristics of the multi-mode SPDC emission as well as the particular beam waist of the collection modes. While several studies have already addressed the issue of optimal fiber coupling of SPDC photons in theory and experiment, the results of these studies arrive at different conclusions. Here, we present the results of a comprehensive experimental study on the optimal collection of photon pairs into single-mode optical fibers. Our approach is based on quasi-phase-matched type II SPDC from a periodically poled KTiOPO4 (ppKTP) crystal. We discuss the influence of pump and collection focal parameters on the spectral brightness and heralding efficiency, as well as practical issues of alignment tolerances into an optical single mode fiber. Further, Using two-photon interference (Hong-Ou-Mandel interference), we assess the spectral bandwidth of the photon pairs for variable crystal lengths. The results are in good agreement with our theoretical model, thus providing the recipe of building ultra bright and stable entangled photon sources.
We demonstrate a wavelength-tunable, fiber-coupled source of polarization- entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs (s mW nm)(-1), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T = 0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.
We demonstrate a wavelength-tunable, fiber-coupled source of polarization-entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs/(s mW nm), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T=0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.
From quantum computation to quantum key distribution, many quantum-enhanced applications rely on the ability to generate pure single photons. Even though the process of spontaneous parametric downconversion (SPDC) is widely used as the basis for photon-pair sources, the conditions for pure heralded single-photon generation, taking into account both spectral and spatial degrees of freedom, have not been fully described. We present an analysis of the spatio-temporal correlations present in photon pairs produced by type-I, non-collinear SPDC. We derive a set of conditions for full factorability in all degrees of freedom—required for the heralding of pure single photons—between the signal and idler modes. In this paper, we consider several possible approaches for the design of bright, fiber-coupled and factorable photon-pair sources. We show through numerical simulations of the exact equations that sources based on: (i) the suppression of spatio-temporal entanglement according to our derived conditions and (ii) a tightly focused pump beam together with optimized fiber-collection modes and spectral filtering of the signal and idler photon pairs, lead to a source brightness of the same order of magnitude. Likewise, we find that both of these sources lead to a drastically higher factorable photon-pair flux, compared to an unengineered source.