Still waiting for optical fiber at your home? For private households, this is an inconvenience. For critical infrastructure and cybersecure networks, it’s a make-or-break resource. Free-space optical QKD enables connectivity at sites and between platforms where fiber deployment is impractical or impossible—like connectivity with mobile users of critical infrastructures, across campuses, harbors, industrial areas, and temporary ad hoc networks.

The critical part is to take free-space QKD out of the laboratory. Our latest demonstration shows how it can be integrated into a practical wireless communication stack, with encryption, broadband data transport, and monitoring working together. 

Why QKD? Classical public-key cryptography relies on mathematical problems that are difficult for today’s computers. Powerful quantum computers threaten widely used cryptographic methods. QKD takes another path by distributing keys using quantum states of light, so eavesdropping changes the physical system and can be detected. Properly implemented QKD provides security grounded in physics rather than computational assumptions. It doesn’t replace every security tool, but rather belongs in a hybrid future with post-quantum cryptography, classical symmetric encryption, and authentication to allow future cybersecurity certification. The goal is defense in depth for data that must remain confidential for many years.

QKD on optical free-space channels

KEEQuant adapted its commercial continuous-variable (CV) QKD devices for operation on optical free-space channels. This includes the optical coupling of a telecom-band CV-QKD signal into a free-space link, development of hardware and software interfaces for integration with key management, encryption applications, and network-management functions. This is relevant for security-critical environments because the result isn’t an isolated QKD experiment but rather a route toward deployable key delivery over line-of-sight links.

This achievement was demonstrated during a recent German research project called QuINSiDa, which involved six partners from academia and industry. The consortium set out to answer a practical question: What would happen if quantum-secure communications left its fiber environment and stepped out into the open air? The answer is a one-of-a-kind optical wireless secured communication stack that combines free-space QKD, LiFi, pointing/acquisition/tracking (PAT), key management, encryption, and network monitoring.

The stack combines several optical systems. Fraunhofer IPMS contributed LiFi and free-space terminal technology, including PAT, to acquire and maintain beam alignment. Fraunhofer IOF contributed a discrete-variable (DV) QKD system. TELCO TECH integrated encryption. Infosim brought monitoring and network management workflows. BESCom contributed use case, transfer, and dissemination expertise. Together, these pieces made the central point visible: Wireless quantum-secured communications are ripe for real-world applications.

The optical neighborhood was dense. Our demonstrator combined CV-QKD at 1550 nm within the middle of the C-band, DV-QKD at 810 nm, and LiFi within the 850- to 940-nm range. Wavelength separation as well as optical and spectral filtering allow the channels to coexist.

From our perspective, the telecom-band aspect is particularly important. CV-QKD builds on the component universe of coherent optical communications: Narrow-linewidth lasers, modulators, efficient photodiodes, narrow-band coherent receivers, digital signal processing, and packaging. This is the path to cost reduction, miniaturization, and scalability. We’ve made a major step in this direction with our recent achievement of the first QKD system on photonic integrated chips, and it’s now commercially available. These miniature and low-cost QKD chips are well suited for integration on mobile devices with limited payloads, which typically communicate wirelessly.

CV-QKD is ideal for optical free-space links because beyond the spatial filtering associated with coupling to a single-mode fiber, the local oscillator acts like an extremely narrow spectral filter and makes the receiver selective against stray photons from sunlight, sky background, and reflections. In free space, where background light and alignment hinder performance, these advantages are decisive.

From the perspective of the end user, it’s interesting to have an integrated system that you can buy, deploy, and maintain conveniently. A modular and interoperable architecture is valuable to maintain, update, and upgrade the system over time. The design of the QuINSiDa project demonstrates this: Different modules and technologies—CV-QKD and DV-QKD, LiFi, key management, and telemetry—from different vendors can work together via standard interfaces and protocols.

CV-QKD works over free-space line-of-sight links, so it can operate inside real-world cybersecurity machinery. This ability turns “no fiber available” from a roadblock into an optical design problem. Wherever line-of-sight links are feasible, quantum-secured channels are deployable. It’s a powerful solution to the last-meter-connectivity problem.

Our chip-scale CV-QKD platform is designed to provide a route to smaller and more cost-efficient systems that are suitable for integration into devices with strict size and weight constraints. This miniaturized approach also supports ruggedized and tamper-resistant QKD terminals for demanding field operations. We’ve also shown that CV-QKD can coexist with C- and O-band telecom multi-terabit traffic, which supports its integration into existing backbone, metro, and access infrastructure—and it makes an affordable quantum-safe future available for a plethora of applications.