⚛️ quantum physics

Continuous variable quantum key distribution channel emulator for the SPOQC mission

This paper presents a novel optical channel emulator designed to replicate dynamic free-space optical link conditions for Low Earth Orbit CubeSats, specifically to test and benchmark the continuous variable quantum key distribution payload for the UK's upcoming SPOQC satellite mission.

Original authors: Emma Tien Hwai Medlock, Vinod N. Rao, Ry Render, Timothy Spiller, Rupesh Kumar

Published 2026-03-02

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Emma Tien Hwai Medlock, Vinod N. Rao, Ry Render, Timothy Spiller, Rupesh Kumar

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to send a secret, unbreakable message from a satellite zooming high above the Earth down to a telescope on the ground. This is the goal of the SPOQC mission, a project by the UK's Quantum Communications Hub set to launch in 2026.

But here's the problem: The space between the satellite and the ground isn't empty. It's filled with our atmosphere, which is like a giant, chaotic ocean of air. As the light carrying your secret message travels through this "ocean," it gets bumped around, stretched out, and scattered by wind, heat, and turbulence. It's like trying to shine a laser pointer through a foggy, windy night; the beam wobbles, dims, and sometimes disappears entirely.

If you build a real satellite and launch it, only to find out your laser can't get through the "fog," you've wasted millions of dollars and years of work. You can't just "turn it off and try again" in space.

The Solution: A "Virtual Reality" for Light

This paper introduces a brilliant solution: a Satellite Channel Emulator. Think of this device as a high-tech flight simulator for light beams.

Instead of launching a satellite, the researchers built a "wind tunnel" for light in their lab at the University of York. They created a system that mimics the exact conditions a laser beam would face traveling from a satellite to Earth, allowing them to test their equipment on the ground before it ever leaves the atmosphere.

How the "Flight Simulator" Works

The emulator uses three main "actors" to recreate the chaos of the sky:

The Dimmer Switch (Variable Optical Attenuator):
- The Real World: As light travels through the atmosphere, it gets weaker due to scattering and the sheer distance.
- The Emulator: A special filter acts like a dimmer switch on a light bulb. It automatically turns the brightness down to match exactly how much light would be lost in a real 700km journey from space to Earth.
The Shaky Hand (Fine Steering Mirror):
- The Real World: The satellite might wobble slightly, or the wind might push the beam off-center. This is called "beam wandering."
- The Emulator: A tiny, super-fast mirror acts like a shaky hand holding the laser. It moves the beam back and forth randomly, simulating the jitter and misalignment that happens in the real sky.
The Wavy Mirror (Deformable Mirror):
- The Real World: Hot air rising and cold air sinking creates invisible "lenses" in the sky that distort the shape of the light beam, making it look like it's passing through a funhouse mirror.
- The Emulator: A high-tech mirror that can physically bend and warp its own surface. It changes shape thousands of times a second to twist and distort the light beam, perfectly copying the "bumpy" atmosphere.

Why This Matters

The researchers used this simulator to test a specific type of secure communication called Continuous Variable Quantum Key Distribution (CV-QKD).

Think of CV-QKD as a way to create a secret code that is physically impossible to hack without breaking the laws of physics. The team needed to know: "If we send this code from space, will the atmosphere scramble it too much to be useful?"

By running their equipment through this "virtual sky," they found:

It works! They successfully simulated the trip from a satellite to the ground under various weather conditions (from calm to very turbulent).
Wavelength matters: They tested different colors of light (like red, near-infrared, and far-infrared). They found that while shorter wavelengths (like red) travel tighter beams, they get more scrambled by the wind. Longer wavelengths are more stable but spread out more.
It's a safety net: They can now tweak their satellite design, adjust their lasers, and perfect their software in the lab before spending a fortune on a rocket launch.

The Big Picture

In simple terms, this paper describes building a test track for space lasers. Just as car manufacturers crash-test cars on a track before selling them to the public, this team is "crash-testing" their quantum communication system against a simulated atmosphere.

This ensures that when the SPOQC satellite actually launches in 2026, it won't just be a piece of metal floating in space; it will be a proven, working machine capable of sending unbreakable secret codes from the stars to our doorstep.

1. Problem Statement

Free Space Optical (FSO) communication links between satellites and the ground are subject to highly dynamic channel conditions that degrade signal integrity. These conditions include:

Atmospheric Turbulence: Causing beam wandering, scintillation, and wavefront aberrations.
Beam Divergence and Diffraction: Leading to significant power loss over long distances.
Atmospheric Attenuation: Due to scattering (Rayleigh and aerosol).
Pointing Errors: Misalignment between the satellite transmitter and the ground receiver.

Testing satellite payloads (specifically for Quantum Key Distribution - QKD) via actual orbital launches is expensive, risky, and inflexible for preliminary protocol validation. There is a critical need for a laboratory-based satellite channel emulator that can accurately replicate these dynamic losses and aberrations to benchmark the performance of quantum payloads before launch. This paper addresses the development of such an emulator specifically for the Satellite Platform for Optical Quantum Communications (SPOQC) mission, a UK-led CubeSat mission scheduled for launch in 2026.

2. Methodology

The authors developed a modular optical channel emulator capable of simulating a satellite-to-ground FSO link. The system emulates three primary sources of channel degradation using specific optical components:

System Architecture:
- Source: A continuous wave (CW) laser with a 2 mm collimated beam.
- Variable Optical Attenuator (VOA): Emulates atmospheric attenuation and beam divergence losses. The VOA is motorized to dynamically adjust loss based on the satellite's zenith angle and altitude.
- Fine Steering Mirror (FSM): Emulates pointing errors and turbulence-induced beam wandering. The FSM moves randomly within a defined range to displace the beam center relative to the receiver aperture.
- Deformable Mirror (DM): Emulates atmospheric turbulence-induced wavefront aberrations. The DM applies phase distortions based on the first 15 Zernike polynomials.
- Beam Expander (GBE): Expands the beam to 1 cm to match the aperture of the Deformable Mirror.
- Power Meter (PM): Measures the final signal loss.
Modeling and Simulation:
- Turbulence Modeling: The authors utilized the Hufnagel-Andrew-Phillips model to calculate the refractive index structure parameter ( $C_N^2$ ) as a function of altitude.
- Aberration Generation: Phase screens were generated using the AOtools Python package based on Kolmogorov turbulence theory. These screens were decomposed into Zernike polynomials to drive the DM.
- Loss Calculation: Total channel loss was calculated by integrating effects from atmospheric attenuation (Beer-Lambert law), beam divergence (diffraction limits), and pointing errors (Gaussian beam overlap with receiver aperture).
Experimental Parameters:
- Wavelengths: Tested at 630 nm, 850 nm, and 1550 nm.
- Orbit: Simulated a Low Earth Orbit (LEO) satellite at 700 km altitude.
- Apertures: Satellite transmitter (8 cm) and Ground Station receiver (60 cm with 30% obstruction).
- Turbulence: Strong ground-level turbulence ( $C_N^2(h_0) = 10^{-12} m^{-2/3}$ ).

3. Key Contributions

Novel Emulator Design: Introduction of a flexible, laboratory-based emulator that can replicate arbitrary satellite passes (LEO to GEO) and varying turbulence strengths.
Dynamic Synchronization: The system synchronizes the VOA (loss), FSM (displacement), and DM (aberrations) to mimic the time-varying nature of a real satellite overpass.
CV-QKD Specific Optimization: The emulator is specifically tuned for the Continuous Variable (CV) QKD payload of the SPOQC mission. It accounts for the specific requirements of Gaussian-modulated coherent states, including the use of a Transmitting Local Oscillator (TLO) which mitigates the need to emulate Doppler shifts and polarization drifts (as both signal and LO co-propagate).
Comprehensive Characterization: The paper provides a detailed breakdown of how individual channel parameters (wavelength, aperture size, turbulence strength) contribute to total link loss and secret key rates.

4. Results

Loss Accuracy: The emulator successfully replicated theoretical loss models.
- VOA: Accurately matched simulated loss curves for beam divergence and atmospheric attenuation across all tested wavelengths.
- FSM: The loss distribution due to pointing error and beam wandering followed a Weibull distribution, consistent with theoretical predictions.
- DM: The wavefront aberrations introduced by the DM correlated strongly with input Zernike modes (verified via Shack-Hartmann wavefront sensor). The loss distribution due to turbulence followed a lognormal distribution (scintillation).
Wavelength Dependence:
- Shorter Wavelengths (630 nm): Experienced higher loss variance due to stronger turbulence effects but lower total average loss due to reduced beam divergence.
- Longer Wavelengths (1550 nm): Experienced lower loss variance but higher total average loss due to increased beam divergence.
Key Rate Performance:
- The emulator was used to calculate the achievable secret key rate for the SPOQC mission.
- Results: For a single satellite pass, the average secret key bits per pass were calculated as:
  - 1550 nm: $3.1 \times 10^7$ bits
  - 850 nm: $1.9 \times 10^8$ bits
  - 630 nm: $3.6 \times 10^8$ bits
- The results indicate that despite higher turbulence sensitivity, shorter wavelengths offer higher potential key rates in this specific configuration due to lower divergence losses.

5. Significance

Mission Readiness: This emulator is a critical tool for the SPOQC mission, allowing the University of York team to test, verify, and benchmark their CV-QKD payload under realistic, dynamic channel conditions prior to the 2026 launch.
Cost and Risk Reduction: It eliminates the need for expensive and risky preliminary orbital tests for protocol validation.
Versatility: While designed for CV-QKD, the architecture is adaptable for Discrete Variable (DV) QKD and classical optical communications. Future iterations plan to include polarization and Doppler effect emulation for broader applicability.
Scientific Validation: The work validates the theoretical models of FSO channel losses (turbulence, divergence, pointing) against experimental data, confirming the feasibility of high-rate QKD from CubeSats in turbulent atmospheric conditions.

In conclusion, this paper presents a robust, high-fidelity optical channel emulator that bridges the gap between theoretical FSO channel modeling and practical satellite payload testing, directly enabling the success of the upcoming UK SPOQC quantum communication mission.