Seminar: Quantum Internet in the Sky (Vision, Challenges, Solutions)
This is a seminar I prepared for a graduate course at SoICT, Hanoi University of Science and Technology, class CH2025A, analyzing the paper “Quantum Internet in the Sky: Vision, Challenges, Solutions & Future Directions” by Phuc V. Trinh and Shinya Sugiura, published in IEEE Communications Magazine in 2024.
The paper envisions a layered Quantum Internet built on satellites and airborne platforms, HAPS and LAPS, combining Free-Space Optics with Quantum Key Distribution. The topic sits exactly at the intersection of two directions I already work in: satellite QKD and FSO at PTIT, and satellite technology through the Master in Space program at USTH.
Foundational concepts
The paper opens with a set of core concepts. A qubit is the unit of quantum information, existing in superposition until measured. Superposition is a linear combination of basis states, allowing parallel computation across 2ⁿ states at once. Entanglement is a non-local correlation between two qubits, where measuring one instantly determines the other’s state regardless of distance. The No-Cloning Theorem states that an unknown qubit cannot be copied without destroying the original.
These four ideas lead directly to QKD, key distribution using quantum states with unconditional security. Two metrics come with it: QBER, the quantum bit error rate that rises abnormally whenever an eavesdropper is present, and Secret Key Rate, the rate at which a usable secret key is produced after error correction and privacy amplification. I explain each of these in more depth in From Qubit to Secret Key Rate.
The paper’s infrastructure section introduces a second set of concepts around Free-Space Optics and non-terrestrial networks. FSO transmits an optical signal through air or vacuum and requires line-of-sight. Pointing error is the angular misalignment of the laser beam, measured in microradians. Atmospheric turbulence is the refractive-index fluctuation that causes beam wander and scintillation. Three platform layers, LEO, HAPS, and LAPS, span from 500 kilometers down to below 5 kilometers, and every one of them is bound by SWaP-C: size, weight, power, and cost. Entanglement swapping connects two nodes with no direct physical link through a Bell-state measurement at an intermediate node, while finite-key effects remind us that a short overpass yields only a limited number of photons, pushing statistical error above the asymptotic theoretical limit. I cover this full set of concepts in FSO and Non-Terrestrial Networks.
Background and motivation
The paper’s second part asks why this topic is urgent now, rather than a purely academic interest. The timeline runs from Feynman and Deutsch in the 1980s, through Shor’s algorithm in 1994 proving RSA could be broken in polynomial time, to Google’s Sycamore in 2019 demonstrating quantum supremacy. Current roadmaps place 2030 to 2035 as the point where quantum computers may become powerful enough to exploit this in practice. The most pressing motivation, though, is the harvest-now-decrypt-later strategy: an attacker can collect encrypted data today, store it, and decrypt it once the technology matures. I go through this timeline in detail in Why We Need Quantum-Safe Security Today.
Facing that threat, two lines of defense are being developed in parallel. PQC, post-quantum cryptography, operates at the software layer, relies on computational hardness, and can already be deployed using NIST’s 2024 standards, CRYSTALS-Kyber and Dilithium. QKD sits at the hardware layer, relies on physical law, and reaches information-theoretic security at the cost of needing new infrastructure. The paper focuses on QKD while explicitly stating that PQC and QKD complement rather than replace each other. I compare the two in more detail in PQC vs. QKD.
Finally, the paper explains why FSO is the natural platform for QKD. Compared to RF and terrestrial fiber, FSO carries almost no thermal noise, handles single-photon counting well, and, most importantly, needs no trusted relay, which is fiber-based QKD’s central weakness: a single compromised node can collapse the security of an entire link. A single LEO satellite link can span thousands of kilometers with no intermediate station at all, making true end-to-end quantum security achievable. I lay out this argument fully in Why FSO Is the Natural Platform for Satellite QKD.
Why this matters for my research
The paper reinforces the theoretical framework behind what I am actually doing at PTIT: modeling atmospheric loss and turbulence for tropical satellite FSO channels, working toward a SIKD system described in Quantum Communication for Satellite Technology. The paper’s three-layer LEO, HAPS, LAPS framework also suggests a natural extension for the three-state weather model I am developing, particularly the LEO-to-HAPS link, where HAPS flies above the cloud layer and avoids cloud-induced loss entirely, facing only the effects of the lower atmosphere.
Source: Phuc V. Trinh, Shinya Sugiura, “Quantum Internet in the Sky: Vision, Challenges, Solutions & Future Directions,” IEEE Communications Magazine, 2024.