Balanced photodetection
= Balanced photodetection
Subtracting two <photocurrents> can suppress a shared classical intensity fluctuation when the optical responses and electronic gains are matched. Independent <shot noise> contributions do not cancel: their <variances> add in the difference. With equal charge $q$ and mean currents $I_1,I_2$, the independent-arrival one-sided current <power spectral density> is $2q(I_1+I_2)$. Correlated optical inputs require their <covariance> as well.