Photon time-stamping

Sending photons via open air requires identifying QKD photons from others such as coming from the sun. In the NIST system QKD photons are time stamped and looked for only when they are expected to arrive. 

A different approach is event , time stamp", or time tag" recording. The technique writes the time of the individual detection events into memory. The general principle is shown in Fig. 2.11. A fast counter counts the clock periods from the moment when a pulse arrives at the trigger input. When a photon is detected the state of the counter is read and written into the next location of the memory. The memory is usually configured as FIFO (first in first out), i.e. the bytes at the output are read in the same order as they were written into the input. 

A variation of the TCSPC technique does not build up photon distributions but stores information about each individual photon. This is called time tag , time stamp , list , or FIFO mode. The memory is configured as a FIFO buffer. For each photon, this method stores the time in the signal period ( micro time ), the time from the start of the experiment ( macro time ), and the data word at the channel input. During the measurement, the FIFO is continuously read, and the photon data are transferred into the main memory or to the hard disc of a computer. Advanced TCSPC devices often can be configured either to build up a multidimensional photon distribution or to store the individual photons. The FIFO mode is described in Sect. 3.6, page 43. 

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