Ordinarily, rubidium atoms are opaque. But after being hit with the second laser beam, they became transparent. In that state, photons were able to pass through the atoms, albeit slowly.
The photon didn't just slow down, either. Because the the rubidium atoms were in such an excited state, only one photon at a time was able to pass through the atom. When another photon entered the cloud of atoms, the rubidium becames opaque and the photon was unable to pass through.
Turning off the control beam also rendered the rubidium atom opaque, blocking any photons from passing through. That means the apparatus is an optically controlled switch, one that works on single photons.
The effect also implies that one could build a kind of quantum transistor. A rubidium chamber emitting single photons could be placed next to another just like it. The single photons would go to the second chamber. The first ones to get there could be let through while later ones would be stopped. Turn off the first chamber for a moment and the second one "opens" again.
That is essentially how transistors in conventional computers work, only those are controlled by electric current.
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The ability to send single photons also means it's possible to build a true quantum communications system. Quantum communications can't be eavesdropped upon, because doing so alters the state of the bits being transmitted. That's a dead giveaway to the person receiving the transmission.
This isn't the only group that managed to control single photons. At Georgia Tech, Alex Kuzmich and Yaroslav Dudin used a similar technique to produce single-photon emissions.
Walker noted these experiments also open the way to a lot of fundamental physics research. Ordinarily when a quantum state is measured, the photon is destroyed. The ability to control the transmission of one photon at a time also means one can measure the quantum state of the incoming photon without actually "touching" it.
The MIT research appeared in the journal Nature on July 25.
Image: MIT / Ofer Firstenberg and Yoav Sterman.