Ultra--low noise differential AC-coupled photodetector for sensitive pulse detection applications

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.