Every detected photon (or dark count) manifests itself as an electronic pulse from the detector. If only the rates are needed, counting the pulses is the appropriate method. Contrarily, if the measurement results of individual photons must be processed further as in QKD, only time correlations between detection events reveals photons generated in pairs. Thereby the rising edge of the detector event gives accurate timing information.

[LDLC] Local detection logic + counters:

• A simple electronic AND-gate can be used to generate a coincidence signal, only if both pair-photons have been successfully detected within a fixed, but short time frame of few nanoseconds. This option is an easy solution in hardware electronics, but only practical to the simplest setup with two attached detector channels.
• Both single detection count rates S1,2 and the coincidence rate C will be displayed on three counters also included in this option. This cheap solution is ideal for projects and laboratories educating students. A change of the rotatable polarisers is immediately displayed. No computer or further electronics is needed.

[FPGA] FPGA-board:

• The programmable FPGA-board has eight input channels and uses internal counters to display the corresponding count rates at a Linux-PC. Thereby full information of the whole system of two BB84-modules (Detector option: two detectors) can be attached. This system is ideal as a local lab-system to analyze the source in both bases at the same time.
• All the needed information is displayed at the graphical user interface including the expected QBER. Clearly no key can be distilled, because the different events are only counted and not processed further on. To distil a secret key is only possible with time-tagging modules.

[OneTT8] One time-tagging module TT8:

• Accurate time stamps with a resolution of 160ps are far more accurate than the typical timing jitter in the order of 500ps caused by the single-photon detectors. Time stamps of up to eight channels are delivered to a computer via an Ethernet connection. Simple Software tools are included to analyze the data.

[TwoTT8] Two time-tagging modules TT8:

• For QKD with separated stations, each detector module needs its own TT8 module, so two TT8 modules are needed.

[No] No coincidence logic needed:

• This option is for customers already equipped with appropriate electronic data processing.

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