Optimising the interconnection of free space to fibre quantum networks

Future quantum communication networks will be composed of both fibre and free-space links. In many cases, interconnection between free space and fibre channels will be desired and thus, optimising this interface will be of... [ view full abstract ]

Future quantum communication networks will be composed of both fibre and free-space links. In many cases, interconnection between free space and fibre channels will be desired and thus, optimising this interface will be of utmost importance to avoid signal losses. It is therefore required to couple the free space-transmitted beams into optical fibres efficiently. The best fibre selection will depend on several parameters, such as the turbulence regime, the emitter and receiver apertures, the solar background level, etc.

For discrete-QKD systems, in order to keep the receiver field of view (FOV) narrow and avoid the increase of QBER due to solar background noise, active optics techniques are necessary to compensate for fast atmospheric effects. To this end, a correcting strategy using two fast steering mirrors (FSM) and two sensitive positions sensors was used (see Figure 1). The coupling efficiency into different fibres was evaluated (see Figure 2) for a link of 100 m in open air and a 9.5 µm-core fibre (standard telecommunication fibre, SMF), 25 µm-core fibre, and 50 µm-core fibre at a turbulent regime of C_n² = 2 · 10^-12m^-2/3 before and after activating the correction. No improvement was found for the 50-µm core fibre since the core area accommodated most of the beam fluctuations at this distance. However, a factor of 4 and 2 improvement in the coupling efficiency was measured for the SMF and 25 µm-core fibre, respectively. This corresponded to a 75% and 45% estimated reduction of the QBER (caused by solar background) for the SMF and 25 µm-core fibre, respectively. The secret key rate (SKR) as a function of distance was also estimated (see Figure 3) for C_n² = 10^-12m^-2/3 and C_n² = 10^-13 m^-2/3(the most common regimes observed in daylight). This increase is promising to enable fast free space QKD in daylight in strong turbulent regimes, one of the main challenges to make this technology worldwide deployable in realistic conditions.