The Laser

At the starting point of these experiments is the production of a pair of photons. They are produced by the process of spontaneous parametric down conversion. This is a well known nonlinear effect where two photons are produced from one “pump” photon. Since these photons are produced simultaneously (to within the length of the pump photon's wave packet), they are correlated in time. Coincidence detection of the two photons can be used to convince us of the quantum nature of the source, although more stringent conditions can be set up. The efficiency of this detection is the product of the efficiencies of the two detectors. Avalanche photodiodes (APD) have the highest efficiencies for single-photon detection. Current efficiencies of single-photon APD’s peak around 80% for 700 nm, dropping off rather quickly: above 1000 nm they are below 10%.¹ This imposes severe restrictions on the pump source. The clear alternative today is the GaN diode laser, lasing at 405 nm. APD’s have an efficiency of about 60% at twice the wavelength of the blue laser, at 810 nm.

Today there are anumber of vendors of these "blue" diode laser. We bought this laser (Power Technologies) to do the experiments about 15 years ago. Today's there are much more compact and inexpensive. There are even laser pointers with such diode lasers.  Beware of powers above 50 mW. See if they offer a power-control ooption. If not, you should consider safety precautions (attenuators and laser goggles). Some vendors sell free-running diode lasers as laser pointers, for as low as $25. They drift a little, but certainly make down-converted pairs.

Fig. 1. Our 18 mW blue diode laser.

Fig. 2 Our simpler laser diode module (current control only).

Fig. 3 A blue laser pointer

¹EG&G product notes.

E.J. Galvez/Colgate U.

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