Physicists in the UK have come up with a new way of storing a handful of photons in an ultracold atomic gas, 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 logic gates in which single photons could be processed one at a time. The method could also prove useful for connecting quantum-computing devices based on different technologies.
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 such as those based on trapped ions or tiny pieces of superconductor. Exchanging quantum information between such devices can therefore be tricky.
What Charles Adams and colleagues at Durham University have now done is come up with a way of storing individual optical photons in highly excited states of an atomic gas. Once stored, the photons can be made to interact strongly, before being released again. An important feature of the technique is that it uses microwaves, which are also used to control some types of stationary qubit.
The Durham experiment involves holding up to 100 rubidium atoms in an optical trap created at the focus of a laser beam, before two pulses of light are fired at the trapped atoms. One pulse is "signal" light that is to be stored and the other is "control" light. The control light allows 10 or so neighbouring rubidium atoms to absorb a signal photon, creating a collective state called a "Rydberg polariton". Such a state is similar to that of a Rydberg atom, which has an electron in a highly excited state – in this case, with a principal quantum number of 60.
Physics World: Stored photons interact with atom cloud