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arXiv:2606.24036

Spectrally engineered collinear type-0 SPDC source with enhanced spectral brightness for entanglement distribution

Dong-Gil Im, Jungmo Lee, Kyungdeuk Park, Dongkyu Kim, Yonggi Jo, Yong Sup Ihn

quant-phSubmitted Jun 23, 2026• Announced Jun 24, 2026• 0 citations
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Abstract

Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.

Comments
9 pages, 5 figures

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Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.
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.
Spontaneous parametric down-conversion (SPDC) is a reliable and robust source of photons for quantum information applications. For applications that involve operations such as entanglement swapping or single-photon heralding, two-photon states are required to be factorable (uncorrelated) in their spectral and spatial degrees of freedom. We report the design and experimental characterization of an SPDC source that has been optimized for high spectral and spatial purity. The source is pumped by the 776 nm output of a mode-locked Ti:Sapphire laser and consists of a periodically-poled Potassium Titanyl Phosphate (PPKTP) crystal phase-matched for collinear type-II SPDC. The dispersive properties of PPKTP at these wavelengths is such that it is possible to minimize the spectral entanglement by matching the widths of the pump to the spectral phase-matching function. The spatial entanglement is minimized through careful control of the pump focus, yielding nearly single-mode emission. An advantage of this approach is that the emission rate into the collection modes is very high, resulting in a very bright SPDC source. We also report a scheme that employs the output of collinear sources such as these to produce polarization-entangled photon pairs. The scheme, which requires only simple polarization elements, can be scaled to N-photon GHZ states.
This study presents 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, this study measures the collinear spatial modes of photon‐pairs and collection optics under various beam waist conditions and analyzes them using a collinear Gaussian approximation model. By employing a simple mode‐matching optical setup, it optimizes the SMF coupling and heralding efficiencies of the photon‐pairs. Consequently, it achieves a spectral brightness exceeding 2.0 MHz mW −1 nm −1 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 continuous‐wave (CW) laser pumped bulk crystal to date. Polarization entanglement is 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‐Clauser‐Horne‐Shimony‐Holt 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 present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.
Photon pairs generated via spontaneous parametric down-conversion (SPDC) can exhibit entanglement in the Laguerre-Gaussian (LG) mode basis, which enables high-dimensional free-space quantum communication by exploiting the high-dimensional space spanned by the LG modes. For such free-space quantum communication, the brightness of the quantum light source plays an important role due to the atmospheric turbulence and photon loss. A variety of studies have analyzed the SPDC brightness by decomposing biphoton states into LG modes, but they have often relied on a degenerate state, a narrow spectral bandwidth approximation, or a thin crystal approximation. However, these approaches are unsuitable for non-degenerate type-0 SPDC with a periodicallypoled nonlinear crystal, which offers higher brightness due to its superior nonlinear coefficients. In this study, we examine the spectrum of photon pairs in specific LG modes generated by a type-0 ppKTP crystal whileavoiding the constraints imposed by the aforementioned assumptions. In addition, we investigate the optimal focal parameters of the pump, signal, and idler to maximize the brightness for a given LG mode. Our findings show that it is not feasible to simultaneously optimize the brightness for different LG modes with a single pump focal parameter. The results of this study provide a comprehensive framework for developing highbrightness quantum light sources and contribute to the advancement of high-dimensional free-space quantum communication.
We demonstrate a polarization-entangled photon-pair source obtained via a type-II non-collinear quasi-phase-matched spontaneous parametric down-conversion process with a 10-mm periodically poled KTiOPO4 crystal, which is as stable and wavelength-tunable as the well-known Sagnac configuration scheme. A brightness of 4.2 kHz/mW is detected and a concurrence of 0.975 is estimated using quantum state tomography. Without loss of entanglement and brightness, the photon-pair wavelengths are tunable through control of the crystal temperature. This improvement is achieved using the non-collinear configuration and a stable interferometric distinguishability compensator.