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Photon Interactions: Durham Physicists discover a whole new system for manipulating light
(8 March 2013)
A team within the Department of Physics has achieved some exciting results recently by demonstrating controlled strong interactions between individual photons.
Led by Prof Charles Adams, Daniel Maxwell and his team generated a small ultracold atomic gas cloud in which strong interactions between neighbouring photons can be switched on and off using microwaves. The team believes that the technique could be used to create optical quantum logic gates in which single photons could be processed one at a time.
Optical photons make very good "flying" quantum bits (qubits) because they can travel hundreds of kilometres through fibres without losing their quantum information. However, it is very difficult to get such photons to interact either with each other or with "stationary" qubits.
To make them interact, these photons (the "signal") can be stored in the cloud of atoms with the help of a bright blue "control" laser beam. Together, they excite some atoms into a high energy level known as a Rydberg state, named after one of the pioneers of atomic structure. A few photons are trapped in Rydberg atoms when the control light is turned off. It is these Rydberg states that interact so strongly, both with each other and with microwaves beamed into the cloud. The states then turn back into photons when the control beam is switched back on.
Importantly, the released photon has the same direction as the signal laser because of constructive interference. In this system, the Rydberg state acts like a cavity to store the light, with the control beam effectively "opening the cavity" to release a photon.
Interactions between Rydberg-stored photons have been demonstrated by other groups, as has microwave control of qubits. But the combination of the two is unique, and provides a whole new system for manipulating light. It reveals unexpectedly rich behaviour that raises many questions for future study. The next steps will be to measure the interactions in different ways, and harness it in the specific manner required for quantum logic gates.
See the full Physics Viewpoint article for more information: