Tutorial 5 — Hybrid Satellite-Fiber Link

Objective: Model a satellite-to-ground quantum link with atmospheric conditions (visibility and weather) that degrade the effective generation rate.

Satellite links are essential for long-distance quantum communication where fiber attenuation becomes prohibitive. Atmospheric conditions significantly impact the effective photon arrival rate, which this tutorial demonstrates.

Build the Network

from qnet_core import QNetEngine, NodeDefinition, LinkDefinition

def build_satellite_network(sat_conditions=None):
    """Build a network with one satellite uplink and two fiber segments."""
    engine = QNetEngine()

    nodes = [
        NodeDefinition(id="Sat",   memory_lifetime_t2=0.3),  # satellite node (limited coherence)
        NodeDefinition(id="G1",    memory_lifetime_t2=1.0),   # ground station 1
        NodeDefinition(id="G2",    memory_lifetime_t2=1.0),   # ground station 2
    ]

    links = [
        LinkDefinition(
            "Sat", "G1", distance_km=500.0, base_fidelity=0.85,
            generation_rate_hz=100.0, link_type="Satellite",
            satellite_conditions=sat_conditions,
        ),
        LinkDefinition("G1", "G2", distance_km=50.0, base_fidelity=0.93,
                       generation_rate_hz=1_000.0),
    ]

    engine.define_network(nodes, links)
    return engine

Simulate Clear Sky Conditions

# No satellite conditions → default visibility=1.0, weather_factor=1.0
engine_clear = build_satellite_network(sat_conditions=None)

stats = engine_clear.simulate(
    from_node="Sat", to="G2",
    fidelity_target=0.80, max_latency_ms=30_000.0, runs=500,
    seed=42,
)

print("=== Clear Sky ===")
print(f"Success rate:   {stats.empirical_success_rate:.1%}")
print(f"Mean latency:   {stats.mean_latency_ms:.1f} ms")
print(f"Mean fidelity:  {stats.mean_fidelity:.4f}")

Simulate Degraded Conditions

from qnet_core import SatelliteConditions

# Partial cloud cover
cloudy = SatelliteConditions(visibility=0.7, weather_factor=0.8)
engine_cloudy = build_satellite_network(cloudy)

stats_cloudy = engine_cloudy.simulate(
    from_node="Sat", to="G2",
    fidelity_target=0.80, max_latency_ms=30_000.0, runs=500,
    seed=42,
)

print("\n=== Partial Cloud Cover (vis=0.7, weather=0.8) ===")
print(f"Effective rate: {cloudy.effective_rate(100.0):.1f} Hz")
print(f"Success rate:   {stats_cloudy.empirical_success_rate:.1%}")

# Heavy storm
storm = SatelliteConditions(visibility=0.4, weather_factor=0.5)
engine_storm = build_satellite_network(storm)

print("\n=== Heavy Storm (vis=0.4, weather=0.5) ===")
print(f"Effective rate: {storm.effective_rate(100.0):.1f} Hz")
print(f"Success rate:   {engine_storm.simulate('Sat', 'G2', 0.80, 30_000.0, 500, seed=42).empirical_success_rate:.1%}")

Key Concepts

Parameter	Range	Effect
visibility	0.0–1.0	Atmospheric clarity: 1.0 = crystal clear, 0.0 = completely obscured
weather_factor	0.0–1.0	Weather attenuation multiplier: clouds, rain, fog reduce photon arrival
effective_rate(base)	—	Returns base × visibility × weather_factor

What's next

• QKD Key Exchange — run BB84 over a satellite link for secure key distribution
• Built-in Topology Generators — use the built-in hybrid_satellite_fiber generator