Single photons are a key ingredient in quantum communication systems, but producing them reliably is difficult. A laser or other conventional light source can be sufficiently dimmed to produce pulses with an average of one photon each. But some of those pulses will contain no photons, and some will contain two or more, in accordance with a Poisson distribution. The multiphoton pulses are of particular concern when the security of quantum information is at stake: If two identical photons are produced instead of one, one photon can be intercepted by an eavesdropper and the other could continue to its intended recipient, who would be none the wiser about the security breach.
Researchers therefore use various tricks to reduce the number of two-photon events a source produces. The standard measure of their success is g(2)(0), called the second-order intensity correlation function at zero time delay. For a conventional source,...
