The Detection System

The photon detectors need to have high efficiency because coincidence detection depends on the product of the efficiencies on the two detectors. Photomultipliers are not good enough because their efficiencies are low in the near infrared. For this reason everybody uses avalanche photodiodes. Currently there is only one vendor of these modules (Perkin Elmer/EG&G Canada/Pacer-UK/Excelitas).¹ The modules, though expensive, take care off the electronics, producing convenient TTL pulses. Figure 1 below shows our two detectors. Alpha now sells low-cost photon detectors from Excelitas. See this link.

Fig. 1 Single photon detectors (fiber coupled).

Since the down converted light is broad band we need to filter it for doing interference experiments. We bought band-pass filters from Andover Corp., Newlight Photonics or Thorlabs with bandwidths of  1, 10, 30 and 40 nm. 

The next step is to detect the pulses produced by the photon pairs in coincidence. One basic possibility that we used in the past is NIM coincidence modules. However, the advent of FPGA circuits has made this much easier, cheaper and compact. Currently we recommend the Altera board, shown below. See Mark Beck's website for more information.

¹Perkin Elmer model SPCM-AQR-13.

E.J. Galvez/Colgate U.

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