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Physical Review A

Polarization entanglement with highly nondegenerate photon pairs enhanced by an effective walk-off-compensation method

Sungeun Oh, Thomas D. Jennewein

Published Dec 12, 2024• 6 citations
Publisher

Abstract

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 $1.0\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{W}$ 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.

DOI
10.1103/physreva.110.063515

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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±\else\textpm\fi0.05)kHz(33.33\ifmmode\pm\else\textpm\fi{}0.05)\mathrm{kHz}(33.33\ifmmode±\else\textpm\fi0.05)kHz, a significant improvement compared to the rate of (4.20\ifmmode±\else\textpm\fi0.01)kHz(4.20\ifmmode\pm\else\textpm\fi{}0.01)\text{kHz}(4.20\ifmmode±\else\textpm\fi0.01)kHz without spatial compensation. Additionally, we observed an estimated pair generation rate of (2.92\ifmmode±\else\textpm\fi0.12)MHz(2.92\ifmmode\pm\else\textpm\fi{}0.12)\text{MHz}(2.92\ifmmode±\else\textpm\fi0.12)MHz and an entanglement visibility of (97.1\ifmmode±\else\textpm\fi0.3)(97.1\ifmmode\pm\else\textpm\fi{}0.3)%(97.1\ifmmode±\else\textpm\fi0.3), making this a promising source for long-distance quantum communication in ground-to-satellite and fiber optic 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 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.
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.
We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures.
Photon-based entanglement sources, crucial for obtaining entangled states, are typically generated via spontaneous parametric down-conversion. However, birefringence in nonlinear crystals causes spatial or temporal walk-off, reducing entangled photon quality. The theoretical analysis attributes birefringent walk-off to dispersive materials curbed by pre- and post-compensation. We experimentally validate this method, enhancing polarization-entangled photon quality. We measured the Bell state along with a violation of CHSH-Bell's inequality by ∼366 standard deviations (S = 2.623 ± 0.0017). Our approach, simple and stable, does not complicate setups. Its applicability extends to optimizing various entanglement source schemes, aiding optical quantum technologies such as computation, sensing, and communication.
Recent advances in guided-wave optics bring quantum key distribution (QKD) to a new level where less complex, yet more robust, schemes have been proposed. Through continuous research in this field, newer QKD systems achieve higher bit rate, distance, and reliability. Entanglement-based QKD receives enormous attention from the scientific community, in order to increase the communication distance and to go to device-independent systems. In this framework, photons are generally the preferred qubit carrier, taking advantage of low-loss fibre optical networks, and high-performance standard telecom components. Future QKD networks will demand sources exhibiting high brightness, reliability, and photons in the standard telecom ITU channels for compatibility and easy integration with current technology. One of the most exploited approaches towards genera- ting photonic entanglement is based on the process of spontaneous parametric down-conversion (SPDC) in non-linear crystals. Usually employed entanglement observables are energy-time and polarization. The latter one is, without doubt, the easiest to analyze thanks to interferometer-free setups. Polarization entanglement is conveniently generated via the so-called type-II SPDC interaction, possibly in a waveguide non-linear structure for increasing the efficiency. Nevertheless, with a type-II source, there are some precautions that must be taken for achieving high entanglement qualities. In particular, the generated paired photons need to be spectrally and temporally indistinguishable. Typically, this requires spectral filtering, and temporal walk-off compensation stages. So far, these issues have been addressed by using bulk optics arrangements. We demonstrate a fully fibred approach based on a type-II periodically poled lithium niobate waveguide (PPLN/W) generating paired photons at 1540 nm (telecom C-band wavelength). We highlight the use of a standard polarization maintaining fibre (PMF) as an essentially loss-free, guided-wave solution to the temporal walk-off problem. By using standard telecom DWDMs, we perform, at the same time, the tasks of spectral filtering and deterministic photon pair separation in two ITU channels. By the violation of the Bell's inequalities with more than 100 standard deviations, we demonstrate excellent entanglement qualities and high coincidence rates. Our fully fibred approach offers very low losses, high reliability, and would be ready for QKD field tests.