[Image: Toshiba Digital Solutions Corp.]
On 19 March 2024, Toshiba Digital Solutions Corp. and SoftBank Corp., Japan, announced that they had demonstrated an integration of secure quantum communications with short-run optical wireless communication. In a joint press release, the firms said the demonstrated setup—in which a Toshiba quantum key distribution (QKD) system was deployed in a SoftBank optical wireless test environment—provided “a new, cost-effective and timely method for deploying QKD secure networks in urban environments.”
Building QKD in urban settings
Toshiba and SoftBank have collaborated for some time on bringing optically based QKD to a variety of corporate contexts. Last September, for example, the two firms announced that they had engineered a field trial demonstrating inter-site, long-distance QKD communication between a data center and SoftBank headquarters across optical fiber, in a setup that integrated Toshiba QKD equipment with a QKD-compatible virtual private network (VPN) router custom built for SoftBank. The tie-in with familiar technology such as a VPN, SoftBank said at the time, might “accelerate social implementation” of quantum-encrypted data communication.
The new test, in contrast, demonstrates a use case for building out QKD links in short-haul urban settings where laying new optical fiber is too costly to be practical, such as “between buildings in urban areas or across public roads.” Instead, the test seeks to validate a solution in which QKD in optical fiber seamlessly links up with optical wireless technology, where encoded IR, UV or visible light carries signals across short distances in free space while still keeping the quantum encryption intact.
Wired-wireless-wired chain
Schematic of the hybrid optical-fiber/optical-wireless setup tested by Toshiba and SoftBank. [Image: Toshiba Digital Solutions Corp.]
The demo was set up by tying Toshiba QKD transmitter and receiver units to an optical wireless test environment already deployed at SoftBank. In subsequent tests, the two companies confirmed that stable quantum keys could be generated and passed across the system, even when optical wireless transmission was included as part of the chain.
The data collected in the demo—including information on optical attenuation across the entire setup—should, according to the companies, “enable the delineation of requirements for wireless devices and the design of network connections necessary for seamless operation of a combined optical wireless QKD system.”
Such hybrid optical-fiber/optical-wireless systems, the firms believe, could prove essential to practical rollout of QKD. “By installing optical wireless devices on building rooftops, etc.,” the firms said, “it will be possible to deploy QKD without the need for dedicated or new optical fiber installations, enabling rapid deployment.”