Measurements are funny things sometimes. It is, on the whole, pretty easy to detect light. It is even pretty easy to detect single photons. But a single photon has very little energy in it, so a single photon detector can't easily distinguish between one photon or two photons. Instead, the experimenter needs to arrange things such that only a single photon is around at any one time. Likewise, in a large light field, it is very difficult to selectively remove just a single photon with any certainty.
Riding to the rescue comes the weirdness of quantum mechanics. A group of researchers has shown that by carefully coupling lasers together, an atomic gas can be set up to absorb just a single photon. And that absorption can be efficiently detected with a third laser.
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