Researchers from the University of Technology Sydney, have announced a new model for quantum satellite uplinks. It focuses on entanglement distribution through uplink channels. The model simulates two ground stations sending single photons to a satellite traveling at 20,000 kilometers per hour. On the satellite, the photons interfere to form entangled states.
Downlink systems have been the main method in quantum satellite communication so far. China’s Micius satellite, launched in 2016, sent photons from space to Earth over distances up to 1,200 kilometers. In 2025, the Jinan-1 microsatellite extended that to 12,900 kilometers. These downlinks generate entangled pairs on the satellite and transmit one photon downward. However, uplinks from ground to space faced doubts because of signal loss in the atmosphere. Scattering and noise made it hard to keep quantum states intact.
The new model addresses these issues by including real factors. It accounts for atmospheric turbulence, which distorts light beams. Satellite motion creates Doppler shifts in the signals. Background light from the Sun, Moon, or city glow adds noise. The simulation shows the uplink works with 80 percent fidelity during nighttime. In the best setups, fidelity reaches 0.97. This means the photons arrive in a state close to their original entangled form.
Ground stations hold an edge in power supply. They can draw gigawatts for laser systems. Satellites, by contrast, operate on about 10 kilowatts total. In the uplink setup, each ground station produces entangled photon pairs locally. It sends one photon to the satellite. The satellite then runs a Bell state measurement. This swaps the entanglement between the two incoming photons. The result confirms a quantum link spanning the distance. No photon generation happens on the satellite. This keeps the spacecraft small. It needs only a basic optical detector unit.
Nighttime provides the clearest conditions. The model calculates fidelities from 0.8 to 0.97 based on beam alignment and timing. Daytime fidelity falls to 0.25 because of solar interference. To counter this, the researchers adjust the temporal window for signal detection. This balances capturing the photon with filtering out noise. Random timing of photon pulses poses another challenge. The ground stations fire pulses at irregular intervals. The model suggests multiplexing. This means sending signals across multiple frequencies or time slots. It raises the chance that photons arrive in sync at the satellite.
The simulation sets the orbit at 500 kilometers, typical for low-Earth satellites. Channel efficiency drops due to mode mismatch between the ground beam and satellite receiver. Turbulence further reduces it. The model estimates overall efficiency and pair fidelity. It uses photonic Bell measurements as the basis. These measurements detect entanglement without destroying it. The results hold up despite the satellite’s speed and environmental effects. Ground sources produce more entangled pairs than space-based ones. This comes from the higher power available on Earth.
To test the model in practice, the team plans experiments with drones or high-altitude balloons. These platforms mimic satellite conditions at lower altitudes. Real air tests will check the simulations against actual data. If successful, small satellite groups could form networks for quantum links across continents. Bandwidth needs for quantum computers demand trillions of photons per second over long ranges. The uplink design supports this by leveraging ground infrastructure.
The research draws on photonics for light handling and systems modeling for simulations. At UTS, these fields combine to predict quantum behaviors. Numerical data includes fidelity ranges and efficiency rates. The model covers uplink channels at various altitudes. It tests different noise levels and alignment errors. For cryptography, early uses might need just a few photons for key creation. This fits quantum key distribution protocols. Higher volumes would support full networks later.
Details from the study show specific parameters. The ground lasers operate at 1550 nanometers wavelength, standard for fiber optics. Beam divergence stays under 10 microradians to focus on the satellite. The satellite’s aperture measures 20 centimeters, collecting incoming light. Detection uses superconducting nanowire counters with 90 percent efficiency. These components match current technology. The model runs on software that simulates wave propagation through air layers. It includes refractive index changes from temperature and humidity.
One section of the paper breaks down error sources. Atmospheric seeing causes beam wander of up to 5 arcseconds. The adaptive optics on ground stations correct this in real time. Satellite pointing accuracy must hit 0.1 degrees. Without it, photons miss the receiver. Background counts from stray light total 10 per second in daylight. Night drops to 0.1. The model filters these with narrowband passes. Entanglement visibility, a measure of pair quality, stays above 0.85 in most cases.
Downlinks avoid upward scattering but deal with satellite power limits. Uplinks gain from ground stability but fight gravity and air density. The study finds uplinks competitive for short orbits. For geostationary at 36,000 kilometers, losses multiply by 100. Still, the model scales with better lasers. Current trials use 1-watt sources. Future ones could reach 100 watts under regulations.
The paper lists equations for fidelity calculation. It uses the formula F = (V + 1)/2, where V is visibility. Channel transmission T equals exp(-alpha * L), with alpha as attenuation and L as path length. For 500 kilometers, T sits at 10^-6. Error rates from misalignment add 5 percent. These numbers come from Monte Carlo simulations run 10,000 times. Results plot fidelity against zenith angle. At 30 degrees elevation, it holds steady. Below 20 degrees, turbulence spikes errors.Hardware details include the photon source. It relies on spontaneous parametric down-conversion in beta-barium borate crystals. Pumped by 405-nanometer lasers, it yields 10^6 pairs per second. Collection fibers have 80 percent coupling. The satellite processes signals in 100 picoseconds. This matches the photons’ arrival window. Cooling to 4 Kelvin keeps detectors quiet. Noise equivalent power measures 20 photons per root hertz.
The model also tests hybrid setups. One station sends signal photons, another idler ones. This cuts errors from single-beam issues. Fidelity improves by 15 percent. For global coverage, 66 satellites in sun-synchronous orbits could link stations worldwide. Each pass lasts 10 minutes, enough for 10^9 keys in QKD. Cost estimates put a basic satellite at $5 million, down from $50 million in 2016.
