The use of quantum key distribution (QKD) to improve security may impact the quality of service (Qos) of high rate classical data links. The key generation process with an empty buffer can be modeled as a queuing system, and data throughput limits can be determined using queueing theory to assess the impact on the QoS. The proposed key generation model is based on an on-demand paradigm. Based on the results of this study, a discrete variable (DV)-QKD-secured link could be designed using the data rate bounds to guarantee a given QoS. Furthermore, we validate the derived bounds by comparing the theoretical derivations with simulations. We show that in a one-time-pad system, the achievable throughput approaches 0.59 times the secret key rate as the key length increases. In the case of a key scheme with key reuse, the throughput is asymptotically linear with the number of key reuses, and the slope of the curve corresponds to the secret key rate multiplied by a function of the protocol and channel characteristics. The analysis is carried out on optical fiber and free-space links.

In this paper, we report on the demonstration of a high-rate free-space optical communication downlink from a fast airborne platform to a ground station. The flight platform used was a Panavia Tornado with a laser communication terminal installed in an attached avionic demonstrator pod. A transportable optical ground station equipped with a free-space receiver front end was used as the receiver station. Downlink wavelength for communication and uplink wavelength for beacon laser were chosen to be compatible with the C-band DWDM grid. New optomechanical tracking systems were developed and applied on both sides for link acquisition and stabilization. The flight tests were carried out at the end of November 2013 near the Airbus Defence & Space location in Manching, Germany. The campaign successfully demonstrated the maturity and readiness of laser communication for aircraft downlinks at a data rate of 1.25 Gbit/s. We outline the experiment design based on link budget assessments, the developed optomechanical terminal technology, and the results of the flight campaign. The experiment itself focused on the tracking performance of the airborne terminal and the ground station. Performance could be measured at aircraft speeds up to Mach 0.7, and video data from an onboard camera was transmitted. Tracking accuracy of up to 20 μrad rms for the airborne terminal and the ground station were achieved at instantaneous tracking errors below 60 and 40 μrad, respectively. The tracking link worked up to a horizontal distance of 79 km, and data transmission was possible up to 50 km.