Abstract High-dimensional entanglement is a valuable resource for quantum information processing and quantum communication. All-fiber implementations are particularly attractive for fiber-based quantum networks, as they offer compactness, intrinsic stability, and direct compatibility with existing fiber infrastructure. Here we demonstrate an all-fiber source of high-dimensional time-bin entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in a single double-pass PPLN waveguide. By precisely matching the delay of an asymmetric Faraday-Michelson interfer-ometer to the pump-pulse spacing, we obtain a two-photon interference visibility of 99.23% ± 0.31%. We obtain a lower bound on the entanglement of formation (EoF) and certify at least 4 × 4-dimensional entanglement by applying semidefinite-programming-based certification to sparse measurement data. We further demonstrate that optimizing the encoding dimension enables more efficient utilization of entanglement resources across different source brightness levels, highlighting a distinct advantage of high-dimensional sources over two-dimensional entangled sources. Our work offers a compact and stable all-fiber route to high-dimensional entanglement for fiber-based quantum communication.
Abstract High-dimensional entanglement is a valuable resource for quantum information processing and quantum communication. All-fiber implementations are particularly attractive for fiber-based quantum networks, as they offer compactness, intrinsic stability, and direct compatibility with existing fiber infrastructure. Here we demonstrate an all-fiber source of high-dimensional time-bin entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in a single double-pass PPLN waveguide. By precisely matching the delay of an asymmetric Faraday-Michelson interfer-ometer to the pump-pulse spacing, we obtain a two-photon interference visibility of 99.23% ± 0.31%. We obtain a lower bound on the entanglement of formation (EoF) and certify at least 4 × 4-dimensional entanglement by applying semidefinite-programming-based certification to sparse measurement data. We further demonstrate that optimizing the encoding dimension enables more efficient utilization of entanglement resources across different source brightness levels, highlighting a distinct advantage of high-dimensional sources over two-dimensional entangled sources. Our work offers a compact and stable all-fiber route to high-dimensional entanglement for fiber-based quantum communication.
The time-energy entangled photon source is a core resource in quantum key distribution, quantum teleportation, and quantum metrology. The output of orthogonally polarized photon pairs can enhance the effective coincidence counts of the entangled source in these applications. Additionally, an all-fiber entangled source offers small size and high mobility advantages. However, most current orthogonally polarized all-fiber time-energy entangled sources are limited to a bandwidth of around 1 nm, which is detrimental to the effective utilization of channel resources in fiber networks and restricts precision in quantum metrology. To address this issue, we employ a fiber-coupled 775 nm narrow linewidth pump laser in a periodically poled lithium niobate (PPLN) waveguide to achieve type-II spontaneous parametric down-conversion (SPDC). This results in an all-fiber orthogonally polarized time-energy entangled photon source with a bandwidth greater than 7 nm. We measured the single counts, coincidence counts, coincidence to accidental coincidence ratio (CAR), single photon spectra, and joint spectral intensity of the entangled source, calculating the Schmidt number S = 7.8. Furthermore, we measured Hong-Ou-Mandel (HOM) interference by inserting two 2 nm bandpass filters, and the interference visibility remained around 95.8% after continuous operation for 3 hours. The experimental results indicate that while maintaining an all-fiber structure, this source possesses a broadband range, low noise, and high stability, providing efficient and reliable biphoton resources for applications in quantum communication, quantum networks, and precision metrology.
We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). We use a fibered birefingent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ+⟩ = 1/√2 [|s,l⟩ − |l,s⟩], where s and l represent the short and long time-bins, respectively.
We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement.
We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
Many quantum communication protocols rely on the distribution of entanglement between the different participating parties. One example is quantum key distribution (QKD), an application that has matured to commercial use in recent years. However, difficulties remain, especially with noise resilience and channel capacity in long-distance communication. One way to overcome these problems is to use high-dimensional entanglement, which has been shown to be more robust to noise and enables higher secret-key rates. It is therefore important to have access to certifiable high-dimensional entanglement sources to confidently implement these advanced QKD protocols. Here we develop a method for certifying high-dimensional time-bin entanglement in fiber-loop systems. In these systems, entanglement creation and detection can utilize the same physical components, and the number of time bins, and thus the entanglement dimension, can be adapted without making physical changes to the setup. Our certification method builds on previous proposals for the certification of angular-momentum entanglement in photon pairs. In particular, measurements in only two experimentally accessible bases are sufficient to obtain a lower bound on the entanglement dimension for both two- and multiphoton quantum states. Numerical simulations show that the method is robust against typical experimental noise effects and works well even with limited measurement statistics, thus establishing time-bin encoded photons as a promising platform for high-dimensional quantum-communication protocols.
We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). Here, we use a birefringent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ ⟩ = 1/√2 [|s,l⟩ + |l,s⟩], where s and l represent the short and long time-bins, respectively.