Scientists propose practical, efficient scheme for metropolitan free-space quantum networks

Experimenting with quantum key distribution in a 1.7 km realistic scenario

This new invention provides a heterogeneous network consisting of special-purpose sub-networks with different links and interconnections; the concept of a quantum key distribution network drives this development, paving the way for other distributed quantum information processing methods and thus setting a benchmark for the overall technological maturity of quantum networks.

Specifically, in this work, Kržič and colleagues describe a metropolitan free-space network architecture for securing communications for summits, conferences, and other events, with the additional capability of complementing existing network infrastructure in the absence of end-to-end fiber-optic connectivity. Quantum physicists built the architecture around a central entanglement server that transmits streams of entangled photons to network users.

They built the key building blocks of this architecture, including a portable, highly visible source of entangled photon pairs for metropolitan applications, with specialized filters for daytime functions. Ultimately, the physicists performed quantum key distribution experiments in a 1.7-kilometer real-world scenario, demonstrating the ability to achieve record kilobyte-per-second rates at night and in daylight.

Entanglement-based free-space QKD systems

The researchers developed a network architecture in which entangled photon sources help servers transmit entangled streams into a metropolitan-scale network built from free-space links: the scientists chose to place the entangled servers in a central high-rise building.

Each user has a subsystem of quantum receivers for detecting the dissected structure of quantum states. The team easily scaled the network by introducing multiple entanglement servers, interconnected by a centralized trust model, to distribute entanglement to users. The physicists hope the entanglement servers will serve as a convenient interface to connect a metropolitan free-space network to the fiber-optic backbone of a larger intercity network.

Kržič and his team have developed a quantum communication system specifically for metropolitan applications that consists of the key building blocks necessary to implement a free-space quantum network in a new location in less than a day. The platform does not require any infrastructure other than a power supply. The system is similar to the Alice-Bob segment of the network, in which an entanglement server generates a pair of polarized entangled photons at 810 nm: one is sent to Alice over a single-mode fiber, while Bob receives the other over a free-space link.

The closed-loop stabilization system contributes to long-term functionality, with both Bob and Alice possessing a quantum receiving subsystem. Bob's version has specially designed spectral and spatial filtering modules to work in daylight, while Alice's version does not require specialized filtering of daylight noise.

Ultimately, the team implemented a classical channel between Alice and Bob's quantum receiving subsystem using a commercial radio antenna.

Up to 5.7 KBPS secure transfer rate

Quantum engineers used the system to establish a quantum link between a site in Jena, Germany, and a makeshift shipping container on top of a public service building 1.7 kilometers away. They conducted two experiments, at night and during the day when background noise was relatively high, to benchmark the system's performance against quantum bit error rates and achievable secure key rates. The scientists divided the count rate detected by Bob into signal and background noise components, which included solar radiation measured at a weather station.

In this way, Andrei Krzyzewski and team developed an entanglement-based free-space quantum network architecture as a viable solution on a metropolitan scale. They achieved secure transmission rates of up to 5.7 kbps under different conditions, demonstrating its ability to operate 24/7 to facilitate full urban coverage.

Further, they explored high-dimensional entanglement, to improve the information capacity, security and robustness to noise per photon. The distribution of entanglement is central to a variety of applications beyond quantum key distribution, including quantum clock synchronization and quantum cryptography.