In a significant step toward quantum computing, scientists at MIT and Harvard University have identified a method that converts laser beams into streams of single photons in a controlled way. The discovery could lead to new quantum devices such as quantum gates, where a single photon switches another photon’s direction of travel or polarization. It is difficult to control photons because the interaction between two photons is very weak at best, said Vladan Vuletic, the Lester Wolfe professor of physics at MIT. Encouraging such interactions requires atoms that interact strongly with photons – as well as with other atoms that, in turn, can affect other photons. For example, a single photon traveling through a cloud of such atoms might pass through easily, but also might change the state of the atoms so that a second photon is blocked when it tries to pass through. An artist’s conception shows how any number of incoming photons (top) can be absorbed by a cloud of ultracold atoms (center), tuned so that only a single photon can pass through at a time. Being able to produce a controlled beam of single photons has been a goal of research toward creating quantum devices. In the new MIT-Harvard system, no matter how many photons are sent into the cloud, only one emerges from it at a time. The cloud acts as a kind of turnstile, forcing the jumbled photons into an orderly succession of individuals. The technique is based on electromagnetically induced transparency (EIT), a technique used previously to slow a beam of light. The scientists focused a laser beam through a dense cloud of ultracold rubidium atoms at 40 µK to produce the EIT state, exciting the cloud of atoms, which are normally opaque to light, while letting photons pass through at a slow speed. The atoms, which are in a Rydberg state, do not allow a second photon to pass through if the first one has yet to emerge from the cloud. If a single photon enters, it passes through the temporarily transparent medium; if two or more enter, the gas becomes opaque again, blocking all but the first photon. “If you send in one photon, it just passes through, but if you send in two or three, forcing them to squeeze through the tight focus of the laser beam, just one passes,” said Ofer Firstenberg, a Harvard postdoc. “It’s like a lot of sand going into an hourglass, but only one grain at a time can pass through.” The technique could be used to alter the state of atoms according to the number of photons striking them, with a second laser beam detecting those changed states. This could help scientists measure a photon without altering it. The system could form the basis of a single-photon switch and also could be used to develop quantum logic gates, an essential component of an all-optical quantum information-processing system. Such systems, in principle, could be immune from eavesdropping when used for communication, and could allow for more efficient processing of some kinds of computational tasks. The work appeared in Nature (doi: 10.1038/nature11361).