QKD Survives 18-Kilometer Air-to-Fiber Quantum Link

Here’s the seemingly impossible feat: a single photon carrying an encryption key traveled 18 kilometers through the turbulent, temperature-varying air over northern Italy. Despite losing approximately 99.9% of its counterparts along the way, the surviving photons landed cleanly enough within a standard telecom fiber to establish a functional secret key. This wasn’t achieved in a laboratory, but over an actual link connecting a hilltop to a university.

These findings were published in a new, peer-reviewed paper in npj Quantum Information by a team from the University of Padua, collaborating with the Italian spin-off company ThinkQuantum and the National Research Council. The trials took place in April 2025, which confirms that these results are based on measured hardware, not projections or simulations. This involved genuine photons, turbulence, and detectors.

Allow me to explain the significance of a photon changing its transmission medium.

The merge lane challenge

Future quantum networks will be a composite of various elements: fiber cables beneath cities, open-air links between buildings or mountaintops, and satellite beams from orbit. Each transmission channel affects light differently, and the delicate quantum states used for encoding are not robust to transitions between these mediums. Losing the quantum state compromises security.

Quantum key distribution (QKD) relies on these photons. It enables two parties to generate a shared secret key using the principles of quantum physics. Crucially, any eavesdropper attempting to intercept the photons will leave detectable traces in the error rate. If an interception occurs, the data becomes noisy, which signals a breach. This is the core mechanism. QKD doesn’t transmit your data; it transmits the key that you then use with conventional encryption methods.

Quantum key distribution over fiber is relatively well-established. QKD through open air has also been demonstrated. However, seamlessly integrating the two into a continuous channel, without the receiving equipment malfunctioning, presents a “merge lane” problem. This experiment directly addresses that challenge.

Mitigating atmospheric turbulence

The primary antagonist here is atmospheric turbulence. You’ve likely observed it as the shimmering distortion over a hot road in summer. Pockets of air at varying temperatures bend and warp light passing through them. Though merely an annoyance for creating a mirage, this phenomenon is catastrophic when attempting to funnel a light beam into the tiny core of a single-mode fiber, which is a strand approximately 50x thinner than a human hair and highly sensitive to the precise arrival of light.

The team’s solution employs adaptive optics, a technique adapted from astronomy. A 41-centimeter telescope at the Padua ground station captured the incoming signal. A wavefront sensor precisely measured the real-time distortions of the arriving light. Subsequently, a deformable mirror, equipped with 64 tiny actuators, adjusted its surface to correct these distortions before feeding the rectified beam into the fiber.

It’s like a mirror that flexes to counteract the atmospheric warping. Essentially, it reverses the atmospheric effects.

This didn’t entirely eliminate the issue. After correction, only about 12% of the available light entered the fiber during a representative run, falling short of the team’s predicted 19%. They attributed this discrepancy to manual fiber alignment and residual distortions that the mirror couldn’t fully correct. Nevertheless, it was sufficient. The total channel loss was approximately 30 decibels, meaning around one in a thousand photons survived the journey. This performance is close to the operational limits of commercial fiber QKD equipment.

The room-temperature detector revelation

Here’s the detail that particularly caught my attention: the experiment was conducted using two different setups.

One setup utilized superconducting nanowire detectors, which are 80% efficient and considered the gold standard for photon detection. These achieved a secret key rate of nearly 1,000 bits per second with a quantum bit error rate under 1%, which are excellent figures.

The other setup employed less expensive avalanche diodes operating at room temperature, with only 15% efficiency. These managed about 200 bits per second at approximately 2% error. Though lower, this was expected given their reduced photon capture and increased noise.

However, superconducting detectors require cryogenic cooling, entailing refrigerators, power, complexity, and cost. The room-temperature version eliminates these requirements. Achieving any secure key at all without a cryostat suggests the possibility of smaller, more affordable ground terminals. Though not fast enough for streaming Netflix, it’s perfectly adequate for the actual task of periodically refreshing an encryption key.

Andrea Stanco, one of the researchers on the Padua team, clearly articulated the benefit, stating that the commercial QKD platform operated across the combined air-and-fiber channel with no functional modifications, as the adaptive optics handled all the complex adjustments before the signal ever reached the fiber.

What this represents

Now for a dose of realism, as the actual scope is narrower than the headline might suggest.

This experiment involved a fixed horizontal link between two ground stations. It didn’t include a moving satellite, daytime operation, or severe weather conditions. The turbulence encountered during the trials was categorized as weak to moderate, which is a mild atmospheric state. Strong daytime turbulence changes faster than the current system can compensate for.

This speed limitation is the primary obstacle. The adaptive optics operated at an effective correction bandwidth of approximately 10 hertz, falling below its theoretical maximum of 24 hertz due to sensor noise and the use of a general-purpose computer for control. Dedicated real-time controllers could potentially push this beyond 100 hertz. Until such advancements occur, challenging atmospheric conditions remain off-limits. The manual fiber alignment is another area for improvement; motorizing it should enhance coupling efficiency.

What does this indicate for the future? The architecture is protocol-agnostic, which means the same air-to-fiber technique could later be used to transmit entangled states for quantum networking. It could also benefit satellite programs like ESA’s Eagle-1 and SAGA, which face similar challenges in collecting distorted light and coupling it into fiber. Furthermore, the ground station acted as a transparent relay, correcting and passing the signal without ever measuring the key. This allows a single station to discreetly serve multiple users without becoming a single point of trust.

What is certain: a commercial QKD system successfully operated through a genuine hybrid channel, at moderate data rates, under relatively benign atmospheric conditions. The aspiration, however, is to overcome challenges much greater than this, which encompasses most of the sky.