In polarization-encoding free-space Quantum Key Distribution (QKD), unwanted variations in the states of polarization may cause errors, leading to an increase in the quantum bit error rate (QBER), and a reduction of the Secure Key Rate (SKR). Optical elements that reflect or transmit the quantum signal at the transmitter and/or receiver are the main cause of polarization variations in the quantum states. Although most of these optical elements affect polarization in a constant manner, requiring relatively simple compensation techniques, mobile elements such as steering mirrors, cause time-varying polarization variations that are more complex to characterize and correct. In this work, we simulate the effect of steering mirrors’ typical values of angular rotation on the QBER. In particular, we show that, for most cases, an active correction of the polarization reference frame and a passive correction of the phase shift between the electric field components is sufficient to obtain QBER values due to the steering mirrors below 0.2%. This avoids the need of a fully active correction strategy, which increases the complexity, weight, and cost of the implementation. Furthermore, we present different design considerations to further reduce the steering mirrors’ contribution to the QBER (less than 0.05%).
Funding: This work had the support of the Spanish National Research Council (CSIC), project 202050E232, by the Spanish State Research Agency (AEI), project PID2020-118178RB-C22/AEI / 10.13039/501100011033, and by the Community of Madrid (Spain) under the CYNAMON project (P2018/TCS-4566), co-financed with European Social Fund and EU FEDER funds. We also acknowledge the support of CSIC’s PTI Platform in Quantum Technologies QTEP PTI-001.
