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Arrays of entangled atoms

Final Report Summary - ARENA (Arrays of entangled atoms)

The goal of this project is to prepare non-classical states of a few tens of individual atoms. These states exhibit quantum correlations, also called entanglement, that cannot be described by classical probabilities. Beside providing us with a better understanding of the foundations of quantum physics, these states are useful for quantum information and quantum metrology. They are also central in condensed matter where the ground states of ensembles of interacting particles are usually entangled. Despite being so important, entangled states are very hard to prepare in the laboratory, particularly when the number of particles involved increases, as the quantum states decohere due to the coupling to the environment. This project proposes to exploit a controllable interaction between cold atoms excited to Rydberg states to produce entanglement between a few tens of particles. This interaction between Rydberg atoms is due to the large electric dipole that atoms develop when they are excited to this state. It is also long-range in nature: two atoms separated by a distance as large as a few micrometers can interact.
During the course of the project we first demonstrated the trapping of laser-cooled individual atoms in arrays of microscopic dipole traps arranged in planar, arbitrary geometries, such as circles, squares, triangles… The arrays of traps are produced by a single laser beam focused by a large-numerical aperture lens, and is spatially shaped by a computer-controlled reconfigurable spatial-light-modulator. We produced patterns of arbitrary geometry consisting of more than 100 traps, filled randomly with individual atoms.
We then demonstrated excellent control over the interaction between up to 9 individual atoms held in small arrays (lines, triangles). In particular, we measured for the first time the van der Waals 1/R6 interaction between two individual Rydberg atoms as a function of their relative distance R. We used this van der Waals interaction to demonstrate the impossibility to excite more than one atom in the ensemble, a phenomenon called Rydberg blockade. We also observed evidence of entanglement in the ensemble by looking at the scaling of the frequency of the collective excitation with the number of atoms. We then tuned the van der Waals interaction to a resonant regime with a 1/R3 dependence using an electric field and showed an enhanced interaction energy. Finally we demonstrated the dipole-induced energy exchange between 2 Rydberg atoms prepared in two different Rydberg states. By extending this study to a small string of three atoms we benchmarked the quality of the elementary quantum simulator of spin interaction, useful to understand e.g. quantum magnetism or energy transport in biological systems.
The results of this project establish individual Rydberg atoms as a new promising platform for quantum state engineering and quantum simulation.