 As bizarre as the physical implications of quantum mechanics may be, real technologies based on quantum principles are beginning to emerge. One of the most promising is optical quantum computing. Unlike the solid state processors found in our laptops, an optical quantum processor would be able to encode information in packets of light known as photons using light itself, giving rise to unprecedented computational power. To realize this capability, however, single photons must be coaxed to interact strongly, a decidedly difficult task, but not an unsurmountable one. In this study, scientists exploited a special optical phenomenon to induce a strong optical response in a collection of atoms and thereby quantify the effect of single photons on a light beam. To measure one photon's effect on a beam of light, the quantum equivalent of determining how a raindrop would alter the stream from a fire hose, the researchers first had to create a beam that could elicit a sufficiently strong, measurable response. This was accomplished by designing an optical system that could support a phenomenon called electromagnetic induced transparency or EIT. EIT occurs when two lasers, a coupling laser and a probe laser, are specially tuned so that individually, either one can excite atoms in a vacuum chamber to a precise energy level. If either laser alone were shown on the atoms, we would observe this excitation as the absorption of light by the atoms, but when both are turned on simultaneously, the quantum interference effect allows both beams to pass through, effectively making the absorbing material transparent. In this study, researchers induced EIT in cold rubidium atoms and then used pulses of a third laser, called the signal laser, to alter the probe laser beam. The effect on the probe beam was registered as a change in its refractive index, or a cross-phase shift, as it passed through the rubidium atoms. As such, the researchers could modulate the optical transmission of light from one laser using light from a different laser, a function desirable for creating a quantum logic gate. To isolate the effect of single photons, the researchers measured the refractive index change induced by signal pulses carrying zero or one photon at a time. Over several experiments, their measurement system could differentiate the effect of one photon from that of no photons, thus demonstrating that the shift caused by a single photon, albeit weak, could in fact be measured. The researchers' findings represent the first time the cross-phase shift due to single photons has been measured, holding important implications for future studies in nonlinear quantum optics and applications in quantum computing. They note, however, that stronger beam-focusing, higher optical density, and tighter conditions under which EIT occurs, are needed to push the detection level to that required for generating functional quantum logic gates.