  A typical implementation of BB84 protocol for quantum communication uses four
laser diodes for transmitting weak coherent pulses, which may not have the same
characteristics. We have characterized these lasers for mismatch in various
parameters such as spectral width, pulse width, spatial mode, peak wavelength,
polarization and their arrival times at the receiver. This information is
utilized to calculate possible information leakage through side channel attacks
by evaluating mutual information between source and eavesdropper. Based on our
experimental observations of cross correlation between parameter values for
different laser diodes, we suggest ways to reduce information leakage to Eve.
