Quantum Non-Linear Optics With Cold Rydberg Atoms

Quantum networks, analogous to the classical ones, allow the transport of quantum information between spatially separated quantum nodes. Quantum nodes are in charge of processing and storing the quantum information, which is transported from one node to another through a quantum channel. Photons are very good carriers of quantum information because they almost do not interact with the environment or with themselves. However, we would want to manipulate photons arrived at quantum nodes by using quantum gates, and this processing would be carried out in the single-photon level, so it requires strong photon-photon interactions. As mentioned before, interactions between two photons are completely negligible in free space, but fortunately, they are possible to achieve by mapping the information in a highly nonlinear system, specifically, with non-linearities at the single-photon level. Such strong non-linearity has been demonstrated in a medium of Rydberg atoms, which are atoms excited to states with a high principal quantum number.

In our experiment, we excite atoms into Rydberg states in a coherent way, so the quantum state of light is preserved. In order to do that, we use a two-photon excitation technique called electromagnetically-induced transparency. This technique converts an initially opaque medium into transparent to photons over a narrow frequency window. When photons travel inside the medium, their group velocity depends on an excitation field parameter, so it can be strongly and even completely, reduced. Therefore, we can store the light inside the medium and retrieve it after some time. The storage process, besides of increasing the non-linearities [1], preserves quantum correlations and quantum statistics of single photons [2], which is crucial for quantum information applications.

Our previous experiments were done in an atomic cloud with non-linearities at the level of hundred photons. Therefore, currently, we are implementing improvements in the way of loading our atomic cloud in order to increase the non-linearities of our medium closer to the single-photon level.

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