Continuous variable quantum key distribution channel emulator for the SPOQC mission

Abstract

In a free space optical (FSO) communication link from satellite to ground, the losses in the channel will be dynamic. Thus, the characterization of the FSO channel is of great importance and this can be emulated in the lab to evaluate the realistic performance of a satellite payload. In this work, we introduce a novel optical channel emulator capable of replicating these dynamics, especially for Low Earth Orbit based CubeSats. We demonstrate its ability to accurately emulate a satellite-to-ground optical communications channel under various atmospheric turbulence strengths, satellite trajectories, and optical ground station parameters at a given optical wavelength of interest. Our satellite channel emulator was designed to test and benchmark the performance of the continuous variable quantum key distribution payload for the Satellite Platform for Optical Quantum Communications mission - an in-orbit demonstrator for the UK's Quantum Communication Hub, to be launched in early 2026.

Figures (8)

• Figure 1: Visual representation of the first 15 Zernike polynomials, generated using AO tools townson2019aotools.
• Figure 2: Experimental set-up of the satellite-to-ground channel emulator as a diagram in (a) and photograph in (b). Here the VOA emulates the loss due to diffraction and atmospheric attenuation, the FSM emulates the pointing error and beam wander due to turbulence, GBE expands the beam, DM emulates the beam aberrations due to atmospheric turbulence affects, OA (optical attenuator) for addition beam attenuation and PM (power meter) measures the losses the signal experiences through this channel.
• Figure 3: Loss at the VOA given a satellite at altitude of $700~\text{km}$, telescope aperture at the satellite of $8~\text{cm}$ and OGS telescope aperture of $60~\text{cm}$ with $30\%$ obstruction. Emulated for three different wavelengths, $1550~\text{nm}$, $850~\text{nm}$ and $630~\text{nm}$. (a) comparison emulated and simulated loss from VOA and (b) probability distributions of emulated loss from the VOA.
• Figure 4: (a) FSM emulated loss, with input parameters as follows: a satellite at an altitude of $700~\text{km}$, a ground level turbulence strength of $C_N^2(h_0)=10^{-12}~\text{m}^{-2/3}$, a pointing error of $4~\mu\text{rad}$ and a OGS receiver telescope of $60~\text{cm}$ with $30\%$ obstruction. This was emulated for three different wavelengths, $1550~\text{nm}$, $850~\text{nm}$ and $630~\text{nm}$. (b) is the portability distribution of FSM emulated loss.
• Figure 5: Single shot phase screen at the zenith given ground level turbulence of $C_N^2(h_0)=10^{-12}~\text{m}^{-2/3}$, $1550~\text{nm}$ light and $60~\text{cm}$ with $30\%$ obstruction OGS telescope aperture. (a) the randomly generated phase screen and (b) the Zernike polynomials of said phase screen compared to Noll's turbulence theory noll1976Zernike. The DM can take the first 15 Zernike polynomials as an input to replicate these phase screens.