Quantum Internet in the Sky

• Quantum Internet in the Sky is a global-scale quantum communication network integrating space and terrestrial platforms using free-space optical links.
• It leverages a layered architecture with satellites, HAPS, and UAVs, applying advanced channel modeling and quantum protocols for high-fidelity entanglement distribution.
• Engineering challenges such as atmospheric turbulence, precise pointing, and synchronization are addressed through adaptive optics and innovative routing algorithms.

A quantum internet in the sky is a global-scale quantum communication infrastructure based on free-space optical quantum channels linking non-terrestrial platforms—satellites, high-altitude platform stations (HAPS), and unmanned aerial vehicles (UAVs)—with terrestrial networks. The overarching goal is ubiquitous distribution of quantum entanglement and quantum keys among distant network nodes via atmospheric and near-vacuum links, leveraging the limited attenuation and geometric losses of space to overcome the fundamental distance limitations inherent to fiber-based quantum networks. Functional realization combines quantum terminals, quantum memories, photonic sources, advanced pointing-acquisition-tracking systems, quantum error correction, and entanglement routing, with layered architectures spanning low-altitude relays up to LEO satellites and integrating with ground-based fiber/freespace infrastructures (Trinh et al., 11 Dec 2025).

1. Layered Architecture and Physical Platform Stack

The quantum internet in the sky comprises a heterogeneous, multi-layer topology:

• Ground Layer: Classical fiber and terrestrial free-space QKD interconnect metropolitan quantum nodes.
• Low-Altitude Relays: LAPS (<5 km, e.g., UAVs/drones) provide flexible, mobile regional relay and mesh capabilities (Jha et al., 29 May 2025).
• Medium and High-Altitude Platforms: HAPS (~20 km) offer extended-area regional links with stable operation above most atmospheric turbulence and weather.
• Satellite Layer: LEO satellites (500–2,000 km) and, for timing/control, GEO satellites (36,000 km) serve as global backbone nodes for entanglement distribution, long-range teleportation, and inter-continental routing (Trinh et al., 11 Dec 2025, Conti et al., 16 Oct 2024, Khatri et al., 2019, Shabani, 12 May 2025).
• Non-terrestrial terminal elements: Quantum FSO payloads are equipped with transmit/receive telescopes, coarse (gimbal) and fine (fast-steering mirror + position-sensing detector) pointing, polarization control via automatic wave-plates, quantum sources (decoy-state BB84, entangled-photon pair sources), optional quantum memories (buffering for entanglement swapping), and high-efficiency single-photon detectors or coherent-state receivers (Trinh et al., 11 Dec 2025, Simon, 2017).

Intra-layer and inter-layer links form a 3D mesh topology, with intra-satellite, HAPS–HAPS, drone swarms, LEO↔HAPS, HAPS↔LAPS, LEO↔ground, and ground–fiber/FSL cross-connections (Jha et al., 29 May 2025). Topological modularity enables efficient network growth and multi-scale quantum services.

2. Free-Space Quantum Channel Modeling

Channel performance in non-terrestrial quantum links is dictated by:

• Atmospheric Attenuation: $$\eta_{\mathrm{atm}}(h,\lambda) = \exp[-\alpha(\lambda, h) \cdot d]$$, with altitude- and wavelength-dependent loss coefficient $\alpha(\lambda, h)$ and atmospheric path length $d$ (Trinh et al., 11 Dec 2025).
• Geometric (Aperture + Divergence) Loss: $$\eta_{\mathrm{geo}} = \left( \frac{D_r}{D_t + \theta d} \right)^2$$, where $D_t$/$D_r$ are the transmitter/receiver apertures, $\theta$ the full-angle divergence, and $d$ the link distance (Trinh et al., 11 Dec 2025).
• Turbulence Parameters: The Fried parameter $r_0(\theta)$, Greenwood frequency $f_G(\theta)$, and Rytov variance $\sigma_R^2$ define spatial/temporal scales relevant for adaptive optics and guide the design of turbulence mitigation systems (Trinh et al., 11 Dec 2025, Jha et al., 29 May 2025).
• Total Channel Transmissivity: $$\eta = \eta_{\mathrm{atm}} \cdot \eta_{\mathrm{geo}} \cdot \eta_{\mathrm{det}}$$ (where $\eta_{\mathrm{det}}$ is detector efficiency).

Atmospheric turbulence, pointing errors (few $\mu$rad), Doppler-induced frequency shifts, polarization reference drift, background light (daylight, urban illumination), and cloud cover all degrade link performance and are more severe for uplinks than downlinks (Trinh et al., 11 Dec 2025). Case analyses show that for a LEO-HAPS link (810 nm, weak jitter, $D_t = 9$ cm, $D_r = 35$ cm), the total link loss is about 40 dB, yielding secret-key yields of $0.6$ Mbit per pass; strong jitter or adverse weather can reduce this by over 60% (Trinh et al., 11 Dec 2025).

3. Quantum Networking Protocols and Routing

Quantum internet in the sky supports a range of quantum communication tasks:

• Quantum Key Distribution (QKD): Satellite-based decoy-state BB84 (Micius, 848.6 nm/100 MHz), yielding kHz–MHz secure key rates over 1200 km distances and QBER $<3\%$ (Liao et al., 2017). CV-QKD is also deployable using LCT hardware on GEO satellites, with key rates $\sim$10–100 bits/s over 36,000 km (given $\eta\sim10^{-15}$, $\xi\sim0.01$ SNU) (Elser et al., 2015).
• Entanglement Distribution: Polarization- or time-bin-entangled pair sources onboard satellites, drones, or ground facilities. Multi-hop routing with entanglement swapping at repeater nodes (satellite, HAPS, ground) and optional quantum memories for synchronization (Simon, 2017, Conti et al., 16 Oct 2024).
• Teleportation and Distributed Quantum Computation: High-fidelity Bell-pair sharing enables teleportation of arbitrary qubits or quantum gates between remote quantum processors (demonstrated up to $\sim$0.82 fidelity in current satellite-ground scenarios; exceeding 0.9 requires improved systematics) (Parny et al., 15 Sep 2025).
• High-dimensional and Multipartite Protocols: Exploiting $d$-level photonic states (e.g., OAM) for increased per-photon key rates and multipartite entanglement (GHZ/W states) enables multi-user secret sharing and distributed quantum sensing (Trinh et al., 11 Dec 2025).

Network control relies on dedicated quantum routers (LEO), precise timing (GEO broadcasts), quantum clock synchronization ($<$1 ps RMS), and entanglement-aware routing algorithms (e.g., Dijkstra with $-\log \eta$ cost, schedule-based protocol for dynamic topologies) (Jha et al., 29 May 2025, Conti et al., 16 Oct 2024, Brito et al., 2020).

4. Scalability, Performance Benchmarks, and Comparative Analysis

Satellite-based quantum internet architectures outperform terrestrial fiber-based repeater chains for transcontinental distances due to:

• Loss Scaling: Free-space links scale as $1/d^2$ (quadratic), vs. exponential attenuation $e^{-\alpha L}$ in fiber (with $\alpha\sim$0.16–0.2 dB/km) (Goswami et al., 10 May 2025).
• Experimental Performance: Direct LEO satellite to ground links achieve sifted key rates of $12$ kbit/s at $645$ km and $1$ kbit/s at $1200$ km, with final secure keys per pass up to $0.3$ Mbit (Liao et al., 2017). European-scale satellite relay chains (5–8 satellites, $500$ km altitude) deliver MHz-class rates between major cities ($0.2$–$26$ MHz, depending on aperture and setup) (Shabani, 12 May 2025).
• Network-Theoretic Properties: Satellite-based quantum networks exhibit small-world characteristics—$\langle \ell\rangle \sim O(\ln N)$, diameter $O(\ln N)$, and log-normal degree distributions with fat tails. This structure enables low-latency, few-hop connectivity and high resilience to random node/link failures ($f_c\sim0.94$ for SBQI) compared to fiber-based topologies ($f_c\sim0.68$) (Brito et al., 2020).
• Hybrid and Balloon-based Networks: Balloon relay chains (24 km altitude, AO correction, optimized beam waist) can achieve $-21$ dB loss over $10,000$ km, outperforming satellite-only relay chains by $12$ dB in channel efficiency, and support sub-Hz entanglement distribution rates with Eu:YSO quantum memories (Liu et al., 21 Jul 2025).

Comparison with repeater chains shows satellite-based networks require orders-of-magnitude fewer quantum memories for equivalent throughput and can provide continuous, on-demand continental entanglement at practical satellite counts (O(100)) (Khatri et al., 2019, Gu et al., 26 Jan 2025).

5. Engineering Challenges and Solutions

Critical implementation bottlenecks and their mitigations include:

• Pointing, Acquisition, and Tracking (PAT): Mitigated by dual-stage ATP systems—coarse gimbal, fast FSM/PSD fine tracking, and beacon-based feedback. Residual losses can be held to $<$3 dB per link (Trinh et al., 11 Dec 2025, Jha et al., 29 May 2025).
• Atmospheric Turbulence: Adaptive optics with bandwidth $>$Greenwood frequency, and spatial filtering (SMF coupling, SNSPDs) improve link coherence and detection yield (Trinh et al., 11 Dec 2025).
• Polarization and Timing Drift: Real-time polarization control via motorized wave plates, quantum and classical frame synchronization extracted from beacon or quantum frames (Trinh et al., 11 Dec 2025, Liao et al., 2017).
• Background Noise: Nighttime operation, narrowband filtering, timing gate optimization (sub-ns), and high-performance detectors (SNSPDs with dark counts $<$100 Hz) are essential for maintaining low-QBER long baseline operation (Shabani, 12 May 2025).
• Finite-key and Cloud Blockage: Site diversity, ground-station networks, and statistical modeling of entanglement throughput over satellite passes address intermittency and finite window statistics (Trinh et al., 11 Dec 2025).
• Resource Scheduling: Distributed entanglement schedulers and heuristic routing with $O(N \log N)$ complexity enable multi-user, multi-pass traffic management (Jha et al., 29 May 2025, Gu et al., 26 Jan 2025).

6. Future Directions and Roadmap

Emergent trajectories include:

• High-dimensional Quantum Links: Encoding in $d$-level Hilbert spaces (OAM, time-bin) yielding log$_2(d)$ bits per photon and boosting resilience to noise (Trinh et al., 11 Dec 2025).
• Multipartite and Multi-user Networking: Upgrading protocols to GHZ/W-state distribution, enabling quantum secret sharing and collaborative computing across large networks (Trinh et al., 11 Dec 2025).
• Quantum Sensing and Intelligence Integration: Embedding co-located quantum sensors (gravity, magnetism, weather), and onboard quantum-enhanced algorithms for adaptive network control, dynamic beamforming, and autonomous resource allocation (Trinh et al., 11 Dec 2025).
• Standardized Quantum Control Planes: Interoperability with terrestrial 6G NTN systems and standardized quantum-packet formats, with dynamic addressing and routing (Trinh et al., 11 Dec 2025, Conti et al., 16 Oct 2024).
• Near-term Milestones: Deployment of SWaP-optimized quantum FSO payloads on UAVs and CubeSats, HAPS payload demonstration, integration with site-diverse AO ground-station networks, and scaling to small LEO quantum repeater constellations with multi-hop, multi-user service (Trinh et al., 11 Dec 2025).

A representative high-level roadmap:

Stage	Milestone
1	Miniaturized FSO terminals on UAVs/CubeSats with ATP, polarization/timing sync
2	HAPS deployment for regional entanglement/QKD
3	LEO constellations with quantum repeaters/routers; multi-hop swapping
4	AO-enabled, site-diverse ground integration (SMF+SNSPDs)
5	Standardization/integration with classical NTN (6G) frameworks
6	High-dimensional, multipartite upgrades; IQCSCI embedding

7. Perspectives and Open Research Directions

Open challenges include realization of room-temperature, low-SWaP quantum memories for space deployment, extensible scheduling and routing in large constellations, integration of hybrid discrete-variable/continuous-variable protocols, and standardization of channel models and protocol parameters (Trinh et al., 11 Dec 2025, Conti et al., 16 Oct 2024). Practical deployment will require co-design across optics, photonics, quantum information, and satellite engineering.

Cumulative advances in quantum photonics, control algorithms, integration with classical communication, and cross-layer system design now provide a concrete, technically grounded path for deploying a resilient, high-throughput quantum internet in the sky (Trinh et al., 11 Dec 2025, Liao et al., 2017, Gu et al., 26 Jan 2025).