FSO | Truong Tuan Nghia

Why FSO Is the Natural Platform for Satellite QKD

Thu, 18 Jun 2026 00:00:00 +0000

If QKD is physically secure, why don’t we already have a global fiber-based QKD network? The answer lies in a structural weakness that only satellites and FSO can decisively fix: the trusted-relay problem.

Three channels, one shared limitation

RF, fiber, and FSO can all carry a signal over distance, but only one of them really suits QKD. RF is essentially ruled out because it does not practically support single-photon encoding, and it suffers from high thermal noise. Fiber handles single-photon counting well, but loss limits its range severely, with roughly 200 kilometers being the practical ceiling. FSO carries almost no thermal noise and handles single photons just as well as fiber, yet it can reach beyond 1000 kilometers without any signal amplification, something fiber cannot do since amplification would destroy the quantum state it is meant to carry.

Fiber’s range limit is the root of a much bigger problem.

The trusted-relay problem in fiber QKD

Fiber loss grows exponentially with distance, so extending fiber QKD beyond 200 kilometers currently means relying on trusted relays: intermediate stations that decrypt the incoming key and re-encrypt it for the next hop. The catch is that this is no longer end-to-end security in any meaningful sense. Each relay station has to be trusted completely, because at that moment the key exists as ordinary classical information, and classical information can be intercepted.

Put differently, a chain of relays is a chain of weak points. Compromise a single node anywhere along the chain and the entire link loses its security, no matter how flawless the QKD is on every other segment. This is exactly why large fiber QKD networks, such as China’s 4,600 kilometer network combining 700 fiber links and two satellites, still depend on a large number of trusted relay stations. That is an architectural weakness, not a temporary technical limitation waiting to be engineered away.

How satellites solve this decisively

A single LEO satellite link can cover thousands of kilometers with no intermediate station at all. The principle is simple: most of the path is vacuum, where loss is essentially zero, and meaningful loss only occurs over a short atmospheric segment near the ground. This turns the satellite into the only relay in the chain, but if it is designed correctly, it does not need to be trusted in the sense of holding the key in classical form. It can genuinely pass along the quantum state itself.

Five reasons satellite FSO wins

Terrain is the first reason. Mountains, oceans, and deserts are places where a ground-based fiber relay simply cannot be built, while satellites are unaffected by any of it. The second reason concerns ground-based FSO specifically: rain, fog, and the near-ground atmosphere are where turbulence and loss are strongest, so a purely terrestrial FSO link between two buildings or two ground stations is at the mercy of local weather. Satellites avoid most of this because the bulk of a photon’s journey lies outside the dense atmosphere, in a stratum with negligible loss or turbulence, which is the third reason.

The fourth reason comes down to control systems: modern satellite Attitude Determination and Control Systems achieve pointing accuracy on the order of microradians, precise enough to hold a stable FSO link over hundreds to thousands of kilometers. The fifth reason is that a satellite FSO link stacks two independent layers of security at once, a physical one from a narrow beam that is hard to intercept without detection, and a quantum one from the no-cloning theorem. Together, they form a defense in depth that ground-based fiber simply cannot match.

Satellite FSO is not just another option for QKD, then. It is the most natural platform for making end-to-end quantum security work at a global scale, and it is why I chose this research direction at PTIT, specifically the tropical ASEAN channel problem I describe in .

FSO and Non-Terrestrial Networks: The Physical Foundation for a Quantum Internet

Wed, 10 Jun 2026 00:00:00 +0000

Building a quantum internet that spans the globe does not start with choosing an algorithm. It starts with a more basic question: how do you send photons over long distances? Right now, the answer is Free-Space Optics, combined with platforms flying at several different altitudes.

What Free-Space Optics actually is

FSO sends an optical signal, usually a laser, directly through air or vacuum, with no physical waveguide involved. It requires line-of-sight: transmitter and receiver have to see each other, with nothing blocking the path. Compared to fiber, FSO has two properties that matter a great deal for quantum applications. First, the path carries almost no significant thermal noise, unlike fiber, where spontaneous Raman scattering inside the core adds background noise of its own. Second, FSO handles single-photon transmission and detection well, which is a hard requirement for encoding a qubit in a photon’s polarization or phase.

Two characteristic sources of error

The laser beam has to be aimed at the receiver with extreme precision. A misalignment of just a few microradians can send the beam missing the receive aperture entirely, especially over distances of hundreds or thousands of kilometers. With a satellite constantly in motion, precise tracking becomes an engineering problem in its own right, requiring a high-precision Attitude Determination and Control System. This is called pointing error.

The second source comes from the atmosphere itself. Local temperature and pressure fluctuations change the air’s refractive index, producing two effects: beam wandering, where the beam drifts off-target unpredictably over time, and scintillation, where the received signal intensity flickers randomly. Both are commonly modeled with the Gamma-Gamma statistical distribution, the standard used across FSO channel research.

Three non-terrestrial platform layers

A quantum internet in the sky does not rely on a single type of satellite. It combines three altitude layers, each playing a different role.

LEO, at 500 to 600 kilometers, gives wide coverage through Starlink-style constellations, but its fast motion means each overpass offers only a short communication window. HAPS, at 19 to 22 kilometers, flies above the cloud layer, avoiding cloud blockage entirely, experiencing less turbulence, covering a stable footprint of roughly 500 kilometers, and often running on solar power. LAPS, below 5 kilometers, is deployed on demand using drones, suited to last-mile key distribution or temporary operation such as disaster response.

The key insight sits with HAPS: flying above the clouds is a strategic advantage, because it eliminates entirely the largest source of atmospheric loss that a ground-based link has to endure, leaving only the much thinner high-altitude atmosphere to contend with.

The shared constraint: SWaP-C

Whatever the layer, every quantum device deployed on a satellite, a HAPS, or a drone runs into SWaP-C: size, weight, power, and cost. This is not a minor engineering footnote, it is a central design constraint. A single-photon source or detector that performs perfectly well on an optical bench is often far too large, too heavy, or too power-hungry to fit on a small satellite or strap onto a drone.

Two ways of extending the network

When two nodes have no direct physical link, they can still be connected through an intermediate node, by performing a Bell-state measurement there on two separately entangled photon pairs. The result is that the two end nodes become entangled with each other, even though they never interacted directly. This is entanglement swapping, and it extends a quantum network’s reach without classical signal amplification, which is fundamentally incompatible with quantum states under the no-cloning theorem.

Another real-world limit is finite-key effects. A LEO overpass lasts only a few minutes, so the number of detected photons is always finite, not infinite as asymptotic theory assumes. With a finite sample, statistical error in estimating QBER and SKR grows significantly, meaning the achievable key rate is always lower than the theoretical limit. Any serious satellite QKD performance analysis has to account for this rather than relying on idealized formulas.

These ideas form the physical foundation for the problem I am actually working on: modeling the tropical satellite FSO channel for a QKD system, covered in more depth in .

Quantum Communication for Satellite Technology: From QKD to the Tropical Weather Problem

Mon, 01 Jun 2026 00:00:00 +0000

Quantum Key Distribution lets two parties share an encryption key whose security is guaranteed by quantum physics, not by computational hardness the way classical schemes like RSA or ECC work. The biggest weakness of terrestrial fiber-based QKD is loss: single photons are absorbed and scattered exponentially with distance, which in practice limits fiber QKD to a few hundred kilometers.

Satellites solve this in a completely different way. Instead of sending photons through hundreds of kilometers of fiber, the signal travels through the near-vacuum of space, where loss is very low, and only experiences meaningful attenuation over a short atmospheric segment near the ground. This is why satellite QKD experiments, China’s Micius mission among them, have already demonstrated key distribution over thousands of kilometers, a range fiber alone simply cannot reach.

What degrades the signal over a free-space channel

Satellite-to-ground QKD links typically rely on Free-Space Optics, sending photons through the air via a narrow, collimated laser beam. Three sources of loss and noise matter most. The first is atmospheric loss, from absorption and scattering by air molecules, aerosols, and especially clouds; thick cloud cover alone can introduce more than 20 to 30 dB of loss, enough to fail a session outright. The second is atmospheric turbulence, refractive-index fluctuations driven by temperature gradients that produce scintillation and beam wander, usually modeled with a Gamma-Gamma distribution. The third is background noise, sunlight or ground light entering the receiver, which directly raises the Quantum Bit Error Rate, the metric that ultimately decides whether a session stays secure enough to extract a key.

A problem specific to tropical ASEAN

Most existing satellite QKD research, whether from China or Europe, has been conducted or simulated in temperate climates, where clear-sky ratios are relatively high and stable across seasons. Tropical ASEAN, Vietnam included, looks quite different. Cloud cover stays high nearly year-round, especially through the rainy season, a challenge similar to what optical remote sensing runs into, something I wrote about in my post on . The gap between wet and dry seasons is sharp, directly shrinking the available window for a satellite session. And tropical humidity and aerosols make atmospheric loss hard to estimate using models calibrated for temperate climates in the first place.

This is precisely the research gap I am working on with the team at PTIT’s Quantum Technology & Applications Lab.

A three-state weather model

Rather than simulating the atmosphere continuously, which is computationally expensive and hard to calibrate against real data, we formalize the satellite FSO channel’s weather into three discrete states. A clear state carries low atmospheric loss and mild turbulence, producing a high and stable SKR. A cloud state introduces moderate to high loss that varies with cloud thickness, dropping SKR sharply, sometimes close to zero. A rain state is the harshest, combining the highest loss with increased turbulence, pushing SKR near zero.

For each state we estimate how often it occurs, using regional climate data, along with its corresponding atmospheric loss, and from there compute an effective Secret Key Rate, the time-averaged secure key rate rather than the instantaneous SKR under ideal conditions that much of the literature still assumes.

From model to a working SIKD system

This weather model feeds into a larger pipeline: a Simultaneous Information and Key Distribution system, transmitting classical data and quantum keys at the same time over the same satellite optical hardware. The pipeline has three stages: channel modeling based on loss per weather state combined with Gamma-Gamma turbulence, QBER estimation from the signal-to-noise ratio and receiver characteristics, and finally QKD performance analysis, computing SKR per scenario along with the probability that a session fails outright.

Early results show a clear gap between dry-season and wet-season performance across the ASEAN region. That points to a practical conclusion: real satellite QKD deployments in the tropics need weather-adaptive routing, choosing ground stations or time windows with a higher chance of clear conditions, rather than assuming a fixed channel the way many existing models still do.

I am now extending this to compare several candidate ground stations across Vietnam and the region, and looking at whether real satellite weather data, rather than historical statistics alone, could enable short-term forecasting of usable communication windows. It is a fairly interesting intersection of the two fields I work across in parallel: Earth observation and quantum communication.

Seminar: Quantum Internet in the Sky (Vision, Challenges, Solutions)

Fri, 15 May 2026 00:00:00 +0000

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 .

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 .

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 .

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 .

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 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 . 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.