The article covers a pioneering experiment by researchers from the Beijing Academy of Quantum Information Sciences and their collaborators. They pulled off phase-encoded quantum communication across an 11.4 km mixed link, combining a 1.4 km urban free-space path with 10 km of optical fiber.
This work tackles doubts about using phase encoding in free space. The team achieved stable interference for successive picosecond pulses, even with atmospheric turbulence.
They also managed seamless free-space–fiber interoperability, using real-time turbulence compensation. It kind of hints at practical space–ground quantum networks and architectures that can build on existing fiber infrastructure.
Overview of the demonstration
The team built and tested a phase-encoded quantum communication system over a mixed free-space and fiber link. It performed robustly, even when the environment threw curveballs.
The experiment shows that phase encoding can work in a real-world, heterogeneous channel. That opens doors for hybrid networks connecting ground stations to satellites and other free-space nodes.
Experimental setup and link architecture
They used a 1.25 GHz weak-coherent source, sending 50 ps pulses at 1549.32 nm. The system produced signal, decoy, and vacuum states with intensities μ = 0.71, ν1 = 0.28, ν2 = 0, in a 30:2:1 ratio.
Two cascaded phase modulators generated random four-phase encoding, making phase-encoded quantum communication possible across the link. Photons traveled a 1.4 km urban lake path in Hefei, got collected by a passive telescope, and then coupled through a triplet fiber-optic collimator into a 10 km fiber. That gave the setup a seamless free-space–fiber interface.
Real-time, active compensation corrected turbulence-induced phase drifts on timescales of 0.01–0.1 s, which is much longer than the 400 ps pulse separation. Single-mode spatial filtering helped out here, too.
Stability in free-space against atmospheric turbulence
Atmospheric conditions usually mess with phase relations needed for high-visibility interference. But the experiment’s real-time correction kept phase coherence across the link.
This allowed for long-duration, stable operation. Active feedback and spatial filtering let the system handle environmental fluctuations that would normally wreck key-generation performance in a free-space channel.
Interference visibility and QBER during operation
- During almost an hour of continuous operation, interference visibility hit 99.07%, with a mean quantum bit error rate (QBER) of 2.38%.
- Over two days, visibility averaged 98.38%, with QBERs of 3.61% and 2.38% depending on the conditions.
Security protocol and data rates
The team used a one-way quasi-quantum secure direct communication protocol (STIKE). It delivered practical security performance over the hybrid link.
The reported metrics show robust operation in a real-world environment. The protocol seems pretty resilient when paired with phase-encoded free-space transmission.
Key rates and recycling efficiencies
- Secure communication rate: 4.22 kbps (and 3.90 kbps when things calmed down).
- Key generation rate: up to 30.42 kbps in calm conditions, showing high throughput potential when the weather cooperates.
- Key recycling efficiencies: nearly 99.9–99.97% in measured runs, so there’s excellent reuse of secure material.
Detector technology and noise management
The detector system used dual-channel InGaAs/InP single-photon detectors running at 1.25 GHz. They had 20% efficiency, a dark count rate of 1×10−6, and a 1% afterpulse probability.
These specs helped keep the QBER low and the quantum states’ fidelity high as they made their way through the turbulent urban free-space path and into the fiber link.
Scalability and future implications
Numerical simulations and a cascaded-link model suggest the architecture can scale to free-space distances over 30 km. That hints at real potential for satellite-to-ground integration and smoother compatibility with current fiber networks.
The researchers saw an eightfold increase in free-space secure communication rate and a 140-fold distance improvement compared to earlier demonstrations. If you ask me, that’s a big step toward practical space–ground quantum networks and multifunctional quantum systems that take advantage of hybrid channels.
Roadmap toward space–ground quantum networks
- Hybrid free-space–fiber approaches can bridge terrestrial networks and satellite links.
- Robust phase encoding works well in realistic atmospheric and urban environments.
- Integrating with existing telecom-grade fiber infrastructure could speed up quantum-secure communications deployment.
Honestly, as someone who’s spent years in quantum communications, this work feels like a real leap toward practical, scalable quantum networks. When you combine advanced phase-encoding with real-time turbulence fixes and high-performance detectors, phase-encoded QKD finally starts looking less like a science experiment and more like something we might actually roll out for the space–ground era.
Here is the source article for this story: Quantum Communication Breaks 11km Barrier Using Free Space And Fibre Optics