Scientists Send Unhackable Quantum Keys Across 120 Kilometers of Optical Fiber

Level 3 — Intermediate

A joint German-Chinese collaboration has demonstrated remarkably stable quantum key distribution across 120 kilometers of standard telecom-grade optical fiber, generating an average secure key rate of about fifteen bits per second and operating continuously for more than six hours without any manual realignment. The work, published in Nature Photonics, edges quantum cryptography substantially closer to deployment on existing telecommunications infrastructure.

At the heart of the system is a single-photon source built from a telecom C-band quantum dot — a tiny semiconductor nanostructure capable of emitting individual particles of light on demand. The team used the source to encode three time-bin qubit states at a clock rate of 76 megahertz, while at the receiving end an actively stabilized Sagnac interferometer decoded the signal and corrected for thermal and mechanical drift in the fiber.

Throughout the six-hour run the average quantum bit error rate remained below eleven percent — comfortably under the threshold at which secret-key extraction becomes impossible. Under realistic finite-key assumptions, the achieved key rate is sufficient for encrypted text messaging applications today and, with foreseeable engineering improvements, for low-bandwidth voice traffic in the near future.

Most importantly, the experiment used standard SMF-28 fiber, the workhorse of today's telecom networks. That eliminates the need for purpose-built quantum fiber, which has historically been one of the biggest obstacles to commercial QKD. The path now opens for incremental integration of quantum keys into the public internet, beginning with high-value financial and governmental traffic.

telecom-grade: Built to the quality standards used in commercial telephone and internet networks.

single-photon source: A device that emits one photon — a single particle of light — at a time, on demand.

qubit: A unit of quantum information, the quantum equivalent of a bit.

time-bin encoding: A method of representing a qubit by which of two time windows a photon occupies.

interferometer: An instrument that combines light waves to measure tiny differences.

drift: A slow, gradual change in a measured quantity over time.

bit error rate: The fraction of bits transmitted that are received incorrectly.

incremental: Happening in small steps rather than all at once.

Level 4 — Advanced

A German-Chinese consortium led by groups in Würzburg and Hefei has reported a striking advance in practical quantum cryptography, demonstrating uninterrupted quantum key distribution across 120 kilometers of standard single-mode SMF-28 fiber for more than six hours. Their result, published in Nature Photonics, marries a deterministic semiconductor single-photon source with a robust time-bin protocol and self-stabilizing detection, yielding an asymptotic secure-key rate of roughly fifteen bits per second under realistic finite-key analysis.

Most prior demonstrations at comparable distances have relied on weak-coherent-state decoy protocols, in which a heavily attenuated laser approximates single-photon emission statistically. The Würzburg-Hefei team instead drives a telecom C-band InAs/GaAs quantum dot embedded in a circular Bragg grating microcavity, producing nearly on-demand single-photon emission at a 76-megahertz repetition rate with measured purity well above the values required for security against photon-number-splitting attacks. The encoded states are three time-bin qubits — early, late, and superposition — sufficient to implement a three-state BB84 protocol with one decoy intensity level.

On the receiving end, the apparatus uses an actively phase-stabilized Sagnac interferometer to decode time-bin states with high visibility despite slow polarization and phase drift in the deployed fiber. Two superconducting nanowire single-photon detectors register the outcomes, while an FPGA-controlled feedback loop continuously corrects bias to maintain quantum bit error rates below the secret-key threshold. Throughout the six-hour test run, mean QBER hovered just under eleven percent, with no operator intervention required.

Scientifically, the experiment marks the first time a true on-demand quantum-dot single-photon source has delivered a positive secure-key rate at metropolitan-scale fiber distances under continuous operation. Practically, it carries an equally important message: quantum key distribution can now ride the SMF-28 fiber already buried beneath every major financial centre, rather than requiring dedicated dark-fiber loops or low-loss specialty cabling. The remaining bottlenecks — detector cost, integration density, and trusted-node geography — are increasingly engineering rather than fundamental, suggesting that a regional quantum internet for high-value traffic may be only a few years away.

single-mode fiber: An optical fiber in which only one mode of light propagates, suited for long-distance telecom.

asymptotic: Approaching a particular value as some quantity becomes very large.

weak-coherent-state: A heavily attenuated laser pulse that mimics single-photon statistics.

decoy protocol: A QKD method using varied intensity levels to detect eavesdropping attempts.

Bragg grating: A periodic structure in a waveguide that reflects specific wavelengths of light.

BB84: The first quantum-key-distribution protocol, proposed by Bennett and Brassard in 1984.

FPGA: Field-programmable gate array — a reconfigurable digital logic chip.

dark-fiber loop: An unused optical fiber dedicated for private or experimental use.

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Level 4 — Advanced

Scientists Send Unhackable Quantum Keys Across 120 Kilometers of Optical Fiber

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