  We report on the performance of ultra low noise differential photodetectors
especially designed for probing of atomic ensembles with weak light pulses. The
working principle of the detectors is described together with the analysis
procedures employed to extract the photon shot noise of light pulses with
$\sim1 \mu$s duration. As opposed to frequency response peaked detectors, our
approach allows for broadband quantum noise measurements. The equivalent noise
charge (ENC) for two different hardware approaches is evaluated to 280 and 340
electrons per pulse, respectively which corresponds to a dark noise equivalent
photon number of $n_\mathrm{3dB}=0.8\cdot 10^5$ and $n_\mathrm{3dB}=1.2\cdot
10^5$ in the two approaches. Finally, we discuss the possibility of removing
classical correlations in the output signal caused by detector imperfection by
using double--correlated sampling methods.
