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Polarization entangled photon-pair source in a dual displacement interferometric configuration

Giacomo Paganini, Álvaro Cuevas, Robin Camphausen, Alexander Demuth, Valerio Pruneri

Published Apr 5, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3017509

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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 (EPSs) are pivotal in advancing quantum communication, computing, and sensing. The demand for deploying efficient, robust EPSs in the field, characterized by exceptional (phase) stability, has become increasingly apparent. This work introduces a polarization-entangled photon source, leveraging type-0 spontaneous parametric downconversion and constructed using commercial bulk optomechanical components. Our system is versatile, enabling the generation of N00N states for sensing applications or Bell states for quantum key distribution protocols. We attained a maximal Bell inequality violation, with the average entanglement visibility exceeding 99%. The potential for further performance enhancements is also explored.
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.
We demonstrate polarization entanglement in highly non-degenerate photon pairs, generated through Type-0 spontaneous parametric down conversion (SPDC) using bulk periodically poled Lithium Niobate (PPLN) crystals. Through the utilization of both a beam displacer interferometer scheme and a Sagnac interferometer, we ensure high polarisation contrast and stable interference of the highly non-degenerate photon pairs, which however causes substantial spatial and temporal walk-offs of the photon paths which poses a formidable challenge. We introduce an effective compensation method using birefringent crystal wedges to eliminate spatial and temporal walkoffs simultaneously. This method is implemented in our entangled photon source (EPS) designed specifically for testing entanglement-based quantum key distribution (EBQKD) between ground and satellite, as part of the Quantum Encryption and Science Satellite (QEYSSat) mission funded by the Canadian Space Agency (CSA). We observed a coincidence rate of N = (33.33+-0.05)kHz, a significant improvement compared to the absence of the spatial compensation. We also observed an estimated pair generation rate of (2.92+-0.12)MHz and an entanglement visibility of (96.6+-0.3)% from only 1.0mW of pump power, making it a promising source for long-distance quantum communication for ground-to-satellite and fiber optic links.
We present an entangled photon source that produces polarization entanglement in highly nondegenerate photon pairs, generated through Type-0 spontaneous parametric down conversion using bulk periodically poled lithium niobate. This source is specifically designed for entanglement-based quantum key distribution between ground and satellite, as part of the Quantum Encryption and Science Satellite mission funded by the Canadian Space Agency. Through the utilization of birefringent beam displacers in a Sagnac interferometer scheme, we ensure high polarization contrast and stable interference of highly nondegenerate photon pairs. However, this approach causes substantial spatial and temporal walk-offs of the photon paths, posing a formidable challenge. We introduce an effective compensation method using birefringent crystal wedges to eliminate spatial and temporal walk-offs simultaneously. With only 1.0mW1.0\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{W}1.0mW of pump power, we observed a coincidence rate of $(33.33\ifmmode\pm\else\textpm\fi{}0.05)\mathrm{kHz}$, a significant improvement compared to the rate of $(4.20\ifmmode\pm\else\textpm\fi{}0.01)\text{kHz}$ without spatial compensation. Additionally, we observed an estimated pair generation rate of $(2.92\ifmmode\pm\else\textpm\fi{}0.12)\text{MHz}$ and an entanglement visibility of $(97.1\ifmmode\pm\else\textpm\fi{}0.3)%$, making this a promising source for long-distance quantum communication in ground-to-satellite and fiber optic links.
We demonstrate a fiber-based source of polarization-entangled photon pairs at visible wavelengths suitable for integration with local quantum-processing schemes. The photons are created through birefringent phase-matching in spontaneous four-wave mixing inside a Sagnac interferometer. We address entanglement due to temporal distinguishability of the photons to enable the generation of a spectrally unfiltered polarization-entangled photon-pair state with 95.86±0.10% fidelity to a maximally entangled Bell state, evaluated with a tomographic state reconstruction without applying any corrections or background subtractions. Owing to the large birefringence of the fiber, photons are created far detuned from the pump, where Raman contamination is negligible. This source’s spatial mode and ability to produce spectrally uncorrelated photons make it suitable for implementing quantum information protocols over free-space and fiber-based networks.
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.