The research conducted by Cameron Simmons and Ross Donaldson from the Institute of Photonics and Quantum Sciences, Heriot-Watt University, focuses on addressing noise issues in satellite quantum key distribution (QKD). The study proposes a novel approach using a multicore fiber beacon system to significantly reduce back-reflection noise, which is critical for enhancing the performance and reliability of satellite QKD systems.


1. Back-Reflection Noise Reduction:

  • The study highlights that conventional methods, such as wavelength and time-division multiplexing, are insufficient in minimizing back-reflection noise.
  • Using a 2-by-2 multicore fiber as an uplink beacon source spatially separates the QKD channel from the uplink beacon, achieving up to a 50 dB improvement in noise rejection compared to purely spectral division systems​​.

2. System Design:

  • The proposed system uses a dichroic mirror to separate outgoing uplink beacon light from incoming quantum signal light, mitigating back-reflection issues in the QKD receiver module.
  • A Fibercore SM4C1500 4-core fiber was utilized in the experiments, demonstrating significant reductions in back-reflected power when compared to single-core systems​​

3. Experimental Setup:

  • The experiments used a Photodigm PH852DBR laser as the uplink beacon source, compatible with QKD signals spectrally separated from 852nm.
  • The setup showed that back-reflected power coupled into the QKD receiving fiber is minimized when using multicore fibers, particularly with receiving fiber core diameters of 25µm or less​​.

4. Quantum Bit Error Rate (QBER):

  • The reduction in back-reflected power significantly impacts QBER, with the multicore fiber system achieving QBER <1% for fiber core below 10µm diameter using 10ps time gating.
  • Further spectral filtering and optimized fiber splitter systems are suggested for enhancing the system's performance, potentially allowing practical QKD operations with existing single-photon detector technologies​​.

5. Simulation and Validation:

  • Simulations validated by experimental results showed that the proposed multicore fiber system significantly reduces back-reflected power and associated QBER.
  • The geometry of the multicore fiber, such as core spacing and arrangement, plays a crucial role in optimizing noise reduction​​.


Fibercore provided the multicore fiber used in the experiments, which was crucial in demonstrating the feasibility and effectiveness of the proposed noise reduction system. Their SM4C1500 4-core fiber was instrumental in achieving the observed improvements in noise performance​​.


The research presents a promising solution for reducing back-reflection noise in satellite QKD systems using multicore fiber technology. This approach not only improves noise performance but also maintains system simplicity and cost-effectiveness, paving the way for more reliable and scalable quantum communication systems in space applications​​.


  • Cameron Simmons and Ross Donaldson, "Multicore fiber beacon system for reducing back-reflection in satellite quantum key distribution," Optics Express, Vol. 31, No. 14, 3 Jul 2023, pp. 23382-23392. DOI: 10.1364/OE.493295.

For more detailed information, please refer to the full research article published in Optics Express:

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