The detectors are based on the Perkin-Elmer SPCM-AQ4C single photon counting array with 4 single photon avalanche diodes (SPADs). The dark count rate of <500 counts/second is perfectly suited for entanglement based system (see Appendix II). Each detector system is housed in a separate box, so the detectors can be used for other experiments as well. Additionally, some features (e.g. a common enabling input) are realized. Depending on the purpose of the setup, one or two separate detectors are required:

[DetENT] Entanglement detection system with one detector module:

• At minimum, two single-photon detectors are needed to measure both pair-photons. The pricing for the two typical Perkin-Elmer single-photon modules is roughly the same as for one detector array; therefore we use only these quad-modules. More input channels as necessary can be used to detect photons, which have been analyzed by a polarizer (included in the setup) or went through customer built extensions (e.g. interferometers).

[DetQKD] BB84-single-photon analyzers and two detector modules:

• Two measurement bases are needed, if the BBM92-protocol is implemented in entanglement based QKD. This can be realized by active switching or by passive choice of the basis by beamsplitters or mirrors. In both cases two quart-detectors are needed, but the former method would need additional switching optics. Therefore we implemented the bases-choice by passive elements and use 4 detectors for each side corresponding to the four different polarizations H, V, +45 and -45.
• At this option, two very compact BB84-analyzers with one single-mode input and 4 outputs corresponding to measured values of H, V, +45° and -45° in multimode fibres matched to the detector array, are also included. Specially selected optical elements guarantee a low loss and small error of only <0.3% due to remaining imperfections of optical elements.
• One electronic polarization rotator compensates the drift of polarization in the fibres, also included in this option. The electronic feedback loop compensates slow drifts for typical temperature changes within fibres.

[No] Detectors not needed:

• It is possible to use own detectors, but the system will be optimized to the Perkin-Elmer quart-modules. Thereby it is necessary to specify parameters (e.g. dead times, …) in the software parts of the system, because some effects (e.g. after-pulsing) can only be reduced by blanking, if all the parameters of the overall systems are known and well balanced.

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