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Free-space laser communication technologies. Conference

A space-suitable, high brilliant entangled photon source for satellite based quantum key distribution

Erik Beckert, Oliver de Vries, Rupert Ursin, Fabian Steinlechner, Markus Gräfe, Marta Gilaberte Basset

Published Mar 4, 2019• 6 citations
Publisher

Abstract

Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.

DOI
10.1117/12.2507603

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 5, 2003•11 citations
Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.
Within the EU-funded consortium QuDice technologies and systems for space-based Entanglement distributions are developed, amongst them also a compact and space-suitable entangled photon source. The source’s architecture is based on a high-performant linear Mach-Zehnder Interferometer in a beam-displacer configuration, using a ppKTP nonlinear material for spontaneous-parametric down-conversion to the C-Band. The source will be tested by means of thermal vacuum and mechanical vibration cycling. Its performance on the laboratory level proved 800..900k counts/s/mW for Signal and Idler and 300k coincidences, an overall heralding efficiency of >33% and visibilities >98%. The form factor of the Engineering Qualification Model fits into a volume of ca. 100x80x50 mm3, excluding the pump laser and electronics for the nonlinear material temperature levelling.
We report on improvements in an entangled free-space quantum key distribution (QKD) system by replacing the original non-collinear type-II spontaneous parametric down-conversion (SPDC) polarization entangled photon source with a new brighter Sagnac interferometric entangled photon source. While the SPDC source was integral to the initial setup of the system, it was limited in photon pair production rate and entanglement quality. Initial experiments with the new Sagnac source have already yielded substantially higher entangled photon rates and improved visibilities. In order to examine the integration of the new source with the QKD system, a local QKD experiment is performed where the source is pumped with 5 mW of power yielding an average raw key rate of 9,423 bits/s and an average final secret key rate of 2,695 bits/s, with an observed average QBER of 2.48%. Initial experiments distributing entangled photons over a single 1,305 m free-space link have seen entangled photon pair coincident detection rates as high as 3,000 cps. Extrapolating based on these initial numbers and previous experiments, we hope to obtain an average secret key rate of 715 bits/s for a two free-space link QKD experiment running the source at full power which will represent an order of magnitude increase over our previous experiments. An additional benefit of the new source is that it has a much narrower bandwidth which will aid in making the system compatible with daylight experiments. However, one drawback of the source is an appreciable double pair emission rate which initial experiments indicate.
Entangled photon sources (EPSs) are essential building blocks for scalable quantum communication and quantum key distribution (QKD). We present a stable, highly nondegenerate EPS based on type-0 spontaneous parametric down-conversion (SPDC) in a crossed-crystal configuration, generating photon pairs at 680~nm and 1550~nm when pumped by a 473~nm laser. This wavelength combination, reported here for the first time, simultaneously benefits from the peak detection efficiency of the most of the Si-SPADs in the visible/near-infrared spectral range and the low-loss fiber transmission of the telecom C-band. This configuration provides the most favorable balance between performance and cost for detection using Si-SPADs and InGaAs detectors. The source exhibits a measured spectral bandwidth of 300~GHz, corresponding to a spectral brightness of up to 1.9×1031.9\times 10^31.9×103~pairs~s−1^{-1}−1~mW−1^{-1}−1~GHz−1^{-1}−1. Heralding efficiencies reach 18~\% (signal) and 34~\% (idler) with Si-SPAD and superconducting nanowire single-photon detectors (SNSPD) detection. The entangled state achieves visibilities of (97.3±1.0) %(97.3\pm 1.0)\,\%(97.3±1.0)% in the H/V basis and (94.9±1.6) %(94.9\pm1.6)\,\%(94.9±1.6)% in the D/A basis, yielding a fidelity of ≥(96.1±1.3) %\geq(96.1\pm1.3)\,\%≥(96.1±1.3)%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.
Polarization-Entangled Photon Sources (EPS) are an important enabling technology in the fields of quantum sensing, quantum communication, and quantum computing. Recently, a need has arisen for efficient sources of entangled photons with high brightness and phase stability, for use in free space and fiber-based quantum communication links. In this work, we present a prototype of EPS based on commercial bulk opto-mechanical components, generating photon pairs via type-0 parametric down-conversion (SPDC). The source is configured in a linear interferometer, where a dual beam displacement is performed by symmetrically disposed birefringent components. The pairs emission can be prepared as an N00N state for quantum sensing, or as a Bell state for entanglement-based Quantum Key Distribution (QKD) protocols. We show a maximal Bell inequality violation, on ⪆99% average visibility, proving the high quality of the generated entanglement. The unique geometry of this interferometer is intrinsically symmetric, thus completely removing any temporal walk-off and decoherence between the two components of the Bell state and enhancing its suitability for various on-field quantum applications.
Entangled-photon sources are indispensable components in free-space quantum key distribution (QKD) systems. Here, we present a compact, lightweight, and robust airborne entangled-photon source (AEPS) based on a Sagnac loop structure with single-mode fiber coupling. To meet the drone requirements for miniaturization, lightweight design, and high robustness, we developed a highly integrated entangled-photon source using customized miniature optical components and an adhesive bonding technique. The total volume and weight are only 38 × 40 × 24 mm3 and 58 g, respectively. Entangled-photon pairs at 810 nm are generated via Type-II spontaneous parametric down-conversion (SPDC) in a periodically poled KTiOPO4 (PPKTP) crystal. We achieve a quantum state fidelity of F = 0.986 ± 0.0017, a photon-pair generation rate of 3.03 × 106 pairs/s/mW, and a CHSH Bell parameter of S = 2.764 ± 0.082. Owing to its excellent size, weight, performance, and stability, the proposed entangled-photon source is particularly well suited for drone-based free-space mobile quantum communication.
We report on the construction of polarization-entangled photon sources for quantum key distribution (QKD) testbed using nonlinear optical process of spontaneous parametric down-conversion (SPDC) pumped by a 266-nm and 351-nm continuous wave (cw) and a 415-nm femtosecond-pulsed laser sources. The efficient coupling of down-converted photons at 702 nm and 830 nm into optical single-mode fiber has enabled us to increase the rate of entangled-photon pairs available for transmission over communication channels with a high degree of polarization entanglement. The detection and characterization of the entangled-photon-state properties has been performed using commercially available Silicon avalanche photodiodes (APD) as well as using a novel photon-number-resolving cryogenic photodetector, which has been developed by our colleagues at the Boulder Division of NIST and brought to BU for tests with elements of the future DARPA Quantum Network.