Application
The ability to generate high rates of entangled photon pairs is a critical requirement for quantum key distribution (QKD) and quantum information processing (QIP) systems. QKD offers prospects for a secure society, including the protection of critical infrastructure and valuable data, while QIP opens the route to fault-tolerant universal quantum computing delivering faster drug discovery and complex system optimisation.
Key Objectives
- Develop periodically poled, lithium niobate (PPLN) waveguides for highly efficient photon pair generation.
- Design an optical system capable of high-rate, entangled photon generation by combining the high efficiency capabilities of the waveguide with a high power pump source.
- Demonstrate a high generation rate (>1GHz, as per the European Space Agency defined market requirement) of polarisation entangled photons at telecoms wavelengths (1560nm).
- Provide a route to market for both the enabling waveguide crystals and integrated entangled photon source.
Project Success
PPLN waveguides have been developed that perform high efficiency second harmonic generation (SHG) and spontaneous parametric down conversion (SPDC). The waveguides have been optimised for single mode operation at the 1560nm fundamental wavelength with a MFD well matched to standard (PM1550) fiber patch cord enabling high coupling efficiency for fiber integration [1] .
An optical bench demonstrator has been designed and built using PPLN waveguides. The system comprises three stages;
- Stage 1, SPDC pump source: 780nm source based upon SHG of an amplified (1560nm) telecoms laser. High efficiency SHG is provided by a PPLN waveguide.
- Stage 2, Photon pair generation: A PPLN waveguide generates photon pairs at 1560nm via SPDC. Polarisation entanglement is achieved through placing the SPDC waveguide in a Sagnac interferometer.
- Stage 3, Photon pair detection: The signal and idler photons are separated into two paths, analysed individually using polarization optics, and measured with superconducting nanowire single-photon detectors (SNSPD’s) for time correlated single-photon counting (TCSPC).
Measurement of the CHSH (Clauser-Horne-Shimony-Holt) parameter (2 < S = 2.73 <2.83) demonstrates photon entanglement, a CHSH S > 2 is proof of entanglement. Measurement of the pair generation rate, in the low-gain regime, demonstrates a maximum pair generation rate of 1.48GHz at a 0.1 mean photon number. These measurements of the performance of the demonstration source can be translated into quantum key distribution (QKD) parameters. For the case of a BB84 protocol with heralded single photons the maximum secret key rate one could obtain with this source is calculated to be 0.633 Gbps for a polarization discrimination fidelity of 98%.
The performance of the demonstrator system is summarised in Table 1and more detailed information can be found in [2].
Pair Generation Rate
Exploitation
The SHG/SPDC waveguide operating at 1560/780nm has been released as a commercial product. Covesion offer the waveguide in a fiber coupled package with PM1550 or PM850 fiber on the input and output making the package suitable for SHG and/or SPDC conversions.
Further work is planned in order to develop the entangled source so that it is suitable for manufacture and release as a commercial product. This work includes miniaturisation of the source, integration of the pump laser source, use of a fully fiberized geometry including fiber coupled waveguide modules and an overall reduction in SWaP-c.
Overall the project has successfully demonstrated a polarisation entangled photon source operating at >1GHz generation rate. Thereby providing a route to practical implementation of a photon source suitable for QKD and quantum networking market requirements.
References
- Lewis G. Carpenter, Sam A. Berry, Rex H. S. Bannerman, Alan C. Gray, and Corin B. E. Gawith, "ZnO indiffused MgO:PPLN ridge waveguides," Opt. Express 27, 24538-24544 (2019)
- B. Ndagano et al., "A Gigahertz-Rate Source of Polarization-Entangled Photons Using Type-0 PPLN Waveguides," 2024 Conference on Lasers and Electro-Optics (CLEO), Charlotte, NC, USA, 2024, pp. 1-2.
