We report a Sagnac-type source based on a MgO:PPLN non-linear crystal pumped with a 532 nm continuous-wave laser, which generates spectrally non-degenerate, polarization-entangled photon pairs of 785 nm signal and 1651 nm idler wavelength, via the type-0 spontaneous parametric downconversion (SPDC) process. We use standard off-the-shelf components; hence, no custom-designed multi-wavelength polarization optics are required. At the same time, long-term phase stability can be achieved due to the geometry of our Sagnac-type design. The source's brightness is 6.17(4)×106 pairs/sec/mW, which is comparable to the brightest state-of-the-art non-degenerate polarization-entangled photon-pair source. The spectral bandwidth of the single photon is approximately 195 GHz (root mean square). The fidelity of the entangled state is measured to be 96.72(1)%, the concurrence is 94.7(2)%, and the Bell-CHSH value S=2.71(6). This source has the potential to be applied in long-distance quantum communication using both free-space and fiber links simultaneously.
We report a Sagnac-type source based on a MgO:PPLN non-linear crystal pumped with a 532 nm continuous-wave laser, which generates spectrally non-degenerate, polarization-entangled photon pairs of 785 nm signal and 1651 nm idler wavelength, via the type-0 spontaneous parametric downconversion (SPDC) process. We use standard off-the-shelf components; hence, no custom-designed multi-wavelength polarization optics are required. At the same time, long-term phase stability can be achieved due to the geometry of our Sagnac-type design. The source's brightness is 6.17(4)×106 pairs/sec/mW, which is comparable to the brightest state-of-the-art non-degenerate polarization-entangled photon-pair source. The spectral bandwidth of the single photon is approximately 195 GHz (root mean square). The fidelity of the entangled state is measured to be 96.72(1)%, the concurrence is 94.7(2)%, and the Bell-CHSH value S=2.71(6). This source has the potential to be applied in long-distance quantum communication using both free-space and fiber links simultaneously.
We develop a Sagnac-type source of ultrabright, non-degenerate, polarization-entangled photon pair that is highly stable and compact simultaneously. We use a 20mm long PPLN bulk crystal which, upon pumping continuously with 532nm wavelength, produces polarization-entangled photon-pairs of 785nm signal and 1651nm idler wavelengths via the type-0 spontaneous parametric down-conversion (SPDC) process. All optical components used in the setup are off-the-shelf components, readily available commercially; hence, no custom-designed or costly multi-wavelength polarization optics are required. At the same time, long-term phase stability is achieved without any additional active stabilization; due to the geometry of our Sagnac-type design. We also report one of the highest brightness of non-degenerate polarization-entangled photon pairs available in the literature. Even with a very low pump power of 0.034mW, we detect a coincidence rate of (6.96±0.03)×104 entangled pairs/sec/mW (averaged over three polarization-basis measurements: H/V, D/A, and R/L basis). The source's brightness is calculated to be (6.17±0.04)×106 entangled pairs/sec/mW for the signal line width of 0.4nm. From the raw coincidence counts (without any background coincidence correction), the fidelity of the entangled state is measured to be (96.72±0.01)% with a concurrence of (94.68±0.20)%. Bell-CHSH inequality violation is reported as S=2.71±0.06.
We present an ultrabright polarization-entangled photon source that is optimally coupled into single-mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down-conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, we measure the collinear spatial modes of photon-pairs and collection optics under various beam waist conditions and analyze them using a collinear Gaussian approximation model. By employing a simple mode-matching optical setup, we optimize the SMF coupling and heralding efficiencies of the photon-pairs. Consequently, we achieve a spectral brightness exceeding 2.0 MHz/mW/nm from a fiber-coupled entangled photon source, utilizing a 30-mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a CW laser pumped bulk crystal to date. Polarization entanglement was verified by a quantum state tomography and a polarization-correlation measurement. The fidelity of the entangled state is measured to be 97.8 % and the Bell-CHSH value S = 2.782 +- 0.04. The results obtained here provide practical insights for designing high-performance SPDC sources for satellite-based communication and long-distance optical links with extremely high-photon loss.
We report on a compact, simple and robust high brightness entangled photon source at room temperature. Based on a 30-mm-long periodically-poled potassium titanyl phosphate crystal, the source produces non-collinear, type-0, phase-matched, degenerate photons at 810 nm with spectral brightness as high as ~0.41 ± 0.02 (~0.025 ± 0.02) MHz/mW/nm for multi (single) mode fiber coupling. So far, this is the highest number of degenerate photons generated using a continuous-wave laser pumped bulk crystal and detected using multimode fiber. We have studied the dependence of pump focusing on the brightness of the generated photons collected using both multimode, and single mode fibers. For a fixed pump power and crystal parameters, the SPDC source has an optimum pump waist radius producing maximum number of paired photons. Combining the crystal in a novel system architecture comprised with Sagnac interferometer and polarizing optical elements, the source produces polarization entangled photon states with high spectral brightness. Even in the absence of any phase compensation, the entangled photon states detected using single mode fiber have a Bell's parameter, S = 2.63 ± 0.02, violating the Bell's inequality by nearly 32 standard deviations and fidelity of 0.975. The compact footprint, robust design, and room temperature operation, make our source ideal for various quantum communication experiments.
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.
Entangled photon sources (EPS), the major building block for a variety of quantum communication protocols, are commonly developed by utilizing the spontaneous parametric down-conversion (SPDC) in χ2 nonlinear bulk optical materials. While high nonlinearity and long interaction length have established the superiority of the periodically poled crystals for EPSs, the phase-matching condition of such crystals is very sensitive to the fluctuation of the crystal temperature and the laser wavelength. As a result, deploying such sources outside the laboratory, for example, satellite-based applications, demands a stringent mass and power budget, thus enhancing the mission complexity and cost. We report a bright, stable entangled photon source with a relaxed requirement of crystal temperature and laser wavelength stabilization. Using a periodically poled KTP crystal inside a polarization Sagnac interferometer producing degenerate, type-0 phase-matched entangled photon pairs at 810 nm in an annular ring, we have transformed the SPDC ring into a "perfect" ring with the help of two common optical elements, axicon, and lens. Despite the variation of the SPDC ring size from Gaussian to annular of different diameters due to the change of crystal temperature over 7oC, and laser wavelength over 300 GHz, we observe the size of the "perfect" ring to be constant. The new EPS, having a spectral brightness as high as 22.58 ± 0.15 kHz/mW collected using single-mode fiber with a Bell's parameter, S = 2.64 ± 0.05, and quantum state fidelity of 0.95 ± 0.02, requires a crystal temperature stability of ±0.8oC, almost five times relaxation as compared to the previous EPS. The generic scheme can be used for non-collinear SPDC photons in all crystals to develop EPS at any wavelength and timescales for resource-constrained applications.
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−=0.850, purity P=0.836, and concurrence C=0.782. A separate CHSH measurement gives S=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⟩-∣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) Mcps mW−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.