  Most polarization-based BB84 quantum key distribution (QKD) systems utilize
multiple lasers to generate one of four polarization quantum states randomly.
However, random bit generation with multiple lasers can potentially open
critical side channels, which significantly endangers the security of QKD
systems. In this paper, we show unnoticed side channels of temporal disparity
and intensity fluctuation, which possibly exist in the operation of multiple
semiconductor laser diodes. Experimental results show that the side channels
can enormously degrade security performance of QKD systems. An important system
issue for the improvement of quantum bit error rate (QBER) related with laser
driving condition is furtherly addressed with experimental results.
