Fig. 2: Quantum bit error rate (QBER) and coincident counts over time with a coincidence window of tCC = 114 ps.
From: Continuous entanglement distribution over a transnational 248 km fiber link
When polarization drifts along the overland fiber link increase the QBER above 9%, the piezo-based polarization controller (PPC) starts an iterative alignment procedure to reduce the QBER below 7%. The limits were chosen such that alignment started well before the 11% limit of no key. Alignment takes between 8 min 10 sec and 1 h 22 min, and 57 min on average. We assume that the longer alignment phases are caused by polarization drift during alignment. The longest uninterrupted stable operation time amounted to 16 h 36 min. The coincidence counts stay at a constant value of ~7.7 s−1 over the full 110 h for tCC = 114 ps.
