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arXiv:2309.12447 · Physical Review Applied

Fiber-based double-pass single-crystal photon-pair source for quantum key distribution in a network

Maximilian Tippmann, Erik Fitzke, Oleg Nikiforov, Philipp Kleinpaß, Till Dolejsky, Maximilian Mengler, Thomas Walther

Published Mar 10, 2025• 4 citations
arXivHTMLPDF

Abstract

Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.

DOI
10.1103/physrevapplied.23.034017

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Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.
Spontaneous parametric down-conversion (SPDC) in nonlinear crystals is the most commonly used means to generate single photon pairs for quantum key distribution systems [1]. By using waveguides instead of bulk crystals, highly efficient devices can be obtained. Both, type-I and type-II phase-matched integrated devices have been demonstrated in periodically poled lithium niobate (PPLN) [2–4]. Although the type-II process is less efficient, it has two favourable properties: the spectrum of down-converted photons is narrower compared to type-I and orthogonally polarised photons are emitted.
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Ultrabright sources of entangled photon pairs with high heralding efficiency are an important step in the pursuit of high-bit-rate entanglement-base quantum key distribution, as well as an enabling tool for loss-sensitive quantum optics experiments. Here, we present a robust single-mode fiber-coupled source of polarization-entangled photons around 810 nm with both high brightness and heralding efficiency. Our approach is based on quasi-phase-matched spontaneous parametric downconversion (SPDC) from periodically poled KTiOPO4 in a Sagnac loop configuration. We demonstrate a high degree of two-photon polarization entanglement for single-frequency-pumped type-0 and type-II SPDC, and conduct a conclusive comparison in terms of efficiency and spectral characteristics. In light of applications outside a laboratory environment, we evaluate the feasibility of pumping nonlinear crystals with free-running laser diodes.
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.
As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.