"The requirements for the basic building blocks of Quantum Information Processing (QIP) are sometimes contradictory: one must seek components that are immune to interactions to preserve their fragile quantum properties, and, at the same time, to have strong, controllable interactions to perform operations on the quantum information they carry.
Different implementations using atomic, solid state or photonic systems have different strengths and weaknesses, so there has been a growing interest in the study of hybrid systems, that combine different technologies to implement those basic building blocks. Photons are the best system to transfer quantum information, because they travel very fast (as fast as it is possible) and are virtually immune to losing their quantum properties; however, photon-photon interactions are very weak, and most materials only show nonlinearities at high photon numbers.
Recently, it has been shown that Rydberg atoms can mediate strong, controllable interactions between individual photons, thus being an ideal system for QIP tasks. ""Exploring Photon-photon Interactions using Cold Rydberg Atoms"" (EPICA) is a project aimed at exploring the effective photon-photon interactions mediated by Rydberg atoms. The aims of this project were three:
– To combine Rydberg atoms with externally generated single-photons.
– To describe in a fundamental way dipole-dipole interactions in the context of multiple stored photons in a highly interacting medium (Rydberg atoms).
– To apply this fundamental knowledge to implement basic building blocks for Quantum Information Processing, such as two-photon gates, that could eventually work with single-photons that can be entangled; or others such as a Fock state discriminator or a Fock state emitter.
As a result of our work on EPICA, we have obtained two key results in line with the objectives of the proposal:
- We have shown that the nonlinear properties of this cold cloud of Rydberg atoms are enhanced when the state of the photons are stored in the cloud, compared to the case when the photons just propagate through it. By using this mechanism, we were able to show nonlinearities at the level of tens of photons in a simple magneto-optical trap.
- We have coupled a single photon from a correlated pair inside this highly-nonlinear medium, and we have checked that the correlations in the initial pair still persist. The combination of the photon pair source and the highly-nonlinear Rydberg medium (when the nonlinearity reached the single-photon level) would implement a building block for quantum information processing; with this combination, one could for example construct two-photon gates -universal for quantum computation-, or implement deterministic Bell-state measurements.
The work on this hybrid system continues in the group of Hugues de Riedmatten at ICFO and elsewhere, but we have provided some key stones towards the use of Rydberg atoms for photonic quantum information processing and quantum communication.
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