🔬 optics

End-to-End Quantum Key Distribution Across Hybrid Fiber and Free-Space Links with All-Optical Encoding Conversion

This paper demonstrates a secure, end-to-end quantum key distribution system that seamlessly bridges fiber and free-space links by performing all-optical time-bin to polarization encoding conversion, enabling continuous key generation across varying atmospheric conditions while maintaining the security assumptions of the BB84 protocol.

Original authors: Khen Cohen, Tomer Nahum, Michael Tzukran, Paz Or, Yehuda Pilnyak, Nitzan Livneh, Hagai Eisenberg, Yaron Oz, Haim Suchowski

Published 2026-07-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Khen Cohen, Tomer Nahum, Michael Tzukran, Paz Or, Yehuda Pilnyak, Nitzan Livneh, Hagai Eisenberg, Yaron Oz, Haim Suchowski

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to send a secret message using a super-secure language that only the laws of physics can protect. This is called Quantum Key Distribution (QKD). It's like sending a letter where if anyone tries to peek inside, the paper instantly turns into confetti, alerting you that your secret is compromised.

For a long time, scientists have had two different ways to send these secret letters, and they speak different "dialects":

The Fiber-Optic Cable: This is the underground tunnel used by the internet. It's great for cities, but it prefers to write its messages using time. Think of it like a train schedule: "The message arrives at 1:00 PM" means one thing, and "The message arrives at 1:01 PM" means another.
The Open Sky (Free-Space): This is for sending messages through the air, like to a satellite or across a park. It prefers to write using colors (polarization). Think of it like holding a flashlight: "The light is vertical" means one thing, and "The light is horizontal" means another.

The big problem? You can't just plug a fiber-optic cable into a telescope and expect them to understand each other. The "time" train doesn't know how to talk to the "color" flashlight. Usually, to connect them, you'd have to stop the message, read it, and rewrite it in the new language. But in the quantum world, reading the message destroys it. If you stop to translate, you break the security.

The Big Breakthrough: The Magic Translator

In this new study, a team of researchers in Tel Aviv built a magic translator that speaks both languages without ever stopping to read the message.

They created a system that connects a fiber-optic cable to an open-air path, but instead of stopping to translate, they use a special optical trick to instantly change the message from "Time" to "Color" and back again. It's like having a magical conveyor belt that turns a train ticket into a colored badge and back again while the ticket is still moving at full speed. The message is never looked at, never copied, and never stopped. It just flows through.

The Real-World Test

The team didn't just do this in a quiet lab; they took it to a rooftop in Tel Aviv to see if it could handle the messy real world. They set up a link that went:

Through a fiber-optic cable.
Out into the open air for 90 meters (about the length of a football field).
Bounced off a mirror (a retroreflector) on a nearby building.
Came back through the air.
Went back into the fiber-optic cable.

They tested this during the Day, at Sunset, and at Night. The air conditions changed wildly:

Daytime: The sun heated the ground, creating hot air bubbles (turbulence) that made the air wobble like a mirage. This is the hardest time to send a clear signal.
Night: The air was calm and still.

What They Found

The system worked! Even when the air was wobbly and noisy during the day, the secret messages got through.

They successfully generated a secure key over the 90-meter link.
They even tested a longer 750-meter (about half a mile) link to prove the idea could scale up, though the signal was weaker there.
The "error rate" (how many mistakes the message made) stayed between 5.6% and 6.8%. This is a very good score because the security rules say you can handle up to 11% mistakes before the system becomes unsafe. They were safely below the danger line.
They managed to create a total of 2.88 Mbit of secure secret keys over about 2.5 hours of testing.

Why This Matters (And What It Isn't)

This is a huge step because it proves you can mix fiber cables and open-air links without breaking the security rules. The "magic translator" (the device that changes time to color) is part of the untrusted path, meaning even if a hacker tried to tamper with the translator, they couldn't steal the key without getting caught.

However, the paper is careful to say this isn't a solved problem for every distance yet.

The 90-meter test was a success, but the 750-meter test showed that as you get further away, the air turbulence gets much worse. At that longer distance, the signal dropped too low to keep a continuous conversation going with their current setup.
The team suggests that for longer distances (like kilometers), they would need better tools to fix the wobbly air, like advanced mirrors that can steer the beam perfectly.
They also noted that the "translator" itself loses some light (about half the signal gets lost in the conversion process), which is a hurdle they need to improve in the future.

The Bottom Line

The researchers showed that you can build a bridge between underground fiber cables and open-air sky links using a device that translates the message without ever reading it. They proved it works in the real world, from hot sunny days to cool nights, creating a secure path for future quantum networks. It's not a perfect, infinite-distance solution yet, but it's a working prototype that shows the path forward is real.

Technical Summary: End-to-End Quantum Key Distribution Across Hybrid Fiber and Free-Space Links with All-Optical Encoding Conversion

Problem Statement
Future quantum communication networks must integrate terrestrial fiber-optic infrastructure with long-range free-space links to achieve global connectivity. However, these two media inherently favor different photonic encoding schemes: fiber systems typically utilize time-bin or phase encoding to avoid polarization drift and mode dispersion, while free-space channels favor polarization encoding due to its resilience to atmospheric turbulence and ease of state preparation. Bridging these heterogeneous networks requires an interface that transfers quantum information between encodings without measuring or reconstructing the quantum state, as such actions would compromise the information-theoretic security of the Quantum Key Distribution (QKD) protocol. Prior to this work, an end-to-end demonstration of QKD across a hybrid fiber and free-space link with in-line, all-optical encoding conversion had not been reported.

Methodology
The authors demonstrated a complete hybrid QKD link operating at 1550 nm, connecting a fiber-based time-bin QKD transmitter (Alice) and receiver (Bob) via a free-space channel. The system employs a decoy-state BB84 protocol and relies on two key architectural components:

All-Optical Encoding Conversion: The link utilizes Time-bin-to-Polarization (T2P) and Polarization-to-Time-bin (P2T) converters. These devices are fiber-based unbalanced Mach-Zehnder interferometers that map time-bin states (early/late) to polarization states (vertical/horizontal) and vice versa. Crucially, this conversion occurs entirely in the optical domain without measurement, state reconstruction, or the introduction of trusted intermediate nodes. The converters remain part of the untrusted quantum channel, preserving the security assumptions of the BB84 protocol.
Hybrid Link Architecture: The experiment was deployed on an urban rooftop at Tel Aviv University. The free-space segment uses a folded path architecture where Alice and Bob share a single fiber-to-free-space port via an optical circulator. The signal travels to a passive, gold-coated retroreflector mounted on a remote building and returns. This design eliminates the need for actively aligned terminals at both ends.
Experimental Conditions: The system was tested over two distances: a 90 m link evaluated across three sessions (Day, Sunset, Night) covering a wide range of atmospheric turbulence (refractive-index structure parameter $C_n^2$ spanning two orders of magnitude), and a 750 m round-trip link to validate scalability. Environmental parameters, including turbulence strength, background noise, and alignment drift, were continuously monitored.

Key Contributions

First End-to-End Hybrid Demonstration: The paper reports the first successful end-to-end QKD across a hybrid fiber and free-space link where encoding conversion is performed in-line and entirely optically.
Security Preservation: By avoiding measurement at the conversion interface, the system maintains the security of the distilled key. The converters are treated as part of the untrusted channel, ensuring that the security analysis remains valid without requiring physical security of the intermediate nodes.
Robustness to Turbulence: The system demonstrated stable operation across atmospheric conditions ranging from strong daytime turbulence to quiescent nighttime conditions, maintaining a Quantum Bit Error Rate (QBER) well below the security threshold.

Results

Secure Key Generation: Over approximately 2.5 hours of cumulative operation on the 90 m link, the system generated 2.88 Mbit (352 KiB) of secure key material.
QBER Performance: The session-mean QBER ranged from 5.6% (Night) to 6.8% (Day), consistently remaining below the 7.5% operational threshold and the 11% theoretical BB84 security limit. The QBER was decomposed to show that intrinsic optical errors and background noise were the primary contributors, with turbulence-induced errors being negligible at the 90 m distance.
Key Rates: Sifted key rates averaged between 3.2 kbps and 4.6 kbps. The secure key rate peaked at 0.93 kbps during the Sunset session. The Day session exhibited the lowest secure key rate due to higher background noise and detector hold-off requirements, despite only a modest increase in QBER.
Scalability: A 750 m extension was successfully tested, confirming photon-level operation. While the transmission penalty increased by 3.8 dB compared to the 90 m link (primarily due to alignment and beam expansion imperfections rather than the encoding conversion itself), the system maintained operation, indicating that the architecture is scalable toward kilometer-scale links, though turbulence will become a limiting factor at longer distances.

Significance
The authors claim that this work establishes a practical interface between terrestrial fiber and long-range free-space quantum communication platforms. By decoupling the choice of photonic encoding from the transmission medium, the demonstrated architecture allows fiber and free-space channels to operate in their respective optimal encoding schemes within a single, secure network. The system proves that all-optical encoding conversion can be integrated into a QKD link without compromising security or introducing trusted nodes. This serves as a foundational building block for future heterogeneous quantum networks, enabling secure connectivity across local, metropolitan, and global scales, and potentially extending to airborne and satellite deployments. The paper emphasizes that the current limitations are engineering challenges (e.g., conversion losses, alignment) rather than fundamental physics constraints.