Instituto de Tecnologías Físicas
Contact: Verónica Fernández Mármol
Free-space quantum communication networks will be composed of many different aerial platforms such as airborne vehicles such as UAVs, drones, etc., and stationary platforms on ground, which in many cases will also be connected to the fibre infrastructure providing hybrid nodes to connecting air to fibre links. These platforms will need to interchange information securely, such as for example, ephemeral or session keys to share information securely among all the users of the network. Therefore, these systems will have to be relatively fast and capable to operate under atmospheric turbulence. The developed QKD system with potential technology transfer achieves high quantum key transmission through a polarization-based protocol at GHz clock frequencies at the transmitter and uses an 850nm-wavalegnth for the quantum signal and 1550nm-wavelength for the synchronization timing pulse. On the other hand, it implements fast automatic beam tracking in the receiver to reduce atmospheric turbulent effects, capable of reducing the quantum bit error rate due to solar background photons up to an order of magnitude, which improves their usability in realistic conditions.
Due to the limitation in distance caused by absorption in optical fibre, free-space quantum communication links — whether they are satellite-, ground-based or among stationary or mobile platforms — are necessary to provide the required flexibility in communication infrastructures to face all types of possible scenarios in current infrastructures. Moreover, these links must satisfy the demands of communication networks, namely, high speed, operation under daylight conditions and high turbulence regimes.
High-speed operation need of a careful design of the whole QKD system; including a high-speed transmitter, capable of operating at high clock frequencies, an efficient and precise optical synchronization, low timing jitter optoelectronic components in both transmitter and receiver, and methods for compensating the degrading of physical properties encrypting the quantum signal (polarization, phase, etc.). When single photon detectors are used, which are most free-space QKD systems, the filtering of solar background remains a considerable challenge, and often, classical filtering mechanisms, such as spatial, spectral and software filtering are not enough to obtain a good signal to noise ratio of the received signal. In these cases, which correspond to the majority of daylight conditions, additional measures are required. These measures include reducing the field of view of the receiver to minimise the amount of solar noise coupled in the system. However, keeping the quantum signal in a reduced field of view requires beam stabilization techniques to compensate for the effects of atmospheric turbulence, which tend to deviate the beam outside the field of view of the rediscover causing losses of quantum signal.
Figure 1. Setup of the transmitter and receiver configuration for a beam tracking system using two actuators (FSMs). (PSD: lateral-effect position sensitive detector; QD: quadrant detector; BS: beamsplitter; FSM: fast steering mirror; Lc: collimating lens; Ld: detector lens; T: Schmidt-Cassegrain telescope; F: single mode or multimode fibre).
Compensation (also referred to as tracking) techniques use some sort of actuator (a fast steering mirror, FSM) to redirect the optical to a predetermined position in a position sensitive detector (PSD) through a feedback loop and thus reduce the atmospheric deviations. Several types of correcting techniques are available, depending on the used configuration: close or open loop, the number of actuators used, etc. In order to evaluate the correction achieved, beam deviations caused by turbulence at the focal plane of the receiver (referred to as the spot focal wander, ra) were measured using a position sensitive detector in the quantum channel (see Figure 1, left). In the tracking channel, two quadrant PSDs were used to fix the beam in two different spatial points of the receiver through two feedback loops each connected to a FSM. Using the measured value of ra, the refractive index structure parameter Cn2 can be calculated providing a baseline of the turbulent regime in each instant. The correction achieved enables the reduction of beam fluctuations in the receiver’s field of view of the QKD system of up to an order of magnitude under very strong turbulent regimes (Cn2 ~10-12 m-2/3), which is equal to the reduction of QBER caused by background. This reduction is critical to enable fast quantum communications in daylight.
Figure 2. Refractive index structure parameter, Cn2, and reduction of the QBER due to background estimated from the measured beam fluctuations, as a function of the hour of the day.