It might not look like much, but just stop and think about it for a second: this fuzzy grey image is the first time we've seen the particle interaction that underpins the strange science of quantum mechanics and forms the basis of quantum computing. Quantum entanglement occurs when two particles become inextricably linked, and whatever happens.
Quantum entanglement is a physical resource, like energy, associatedwith the peculiar nonclassical correlations that are possible betweenseparated quantum systems. Entanglement can be measured, transformed,and purified. A pair of quantum systems in an entangled state can beused as a quantum information channel to perform computational andcryptographic tasks that are impossible for classical systems. Thegeneral study of the information-processing capabilities of quantumsystems is the subject of quantum information theory.
A quantum internet that connects remote quantum processors, should enable a number of revolutionary applications such as distributed quantum computing. Its realization will rely on entanglement of remote quantum memories over long distances. Despite enormous progress, at present the maximal physical separation achieved between two nodes is 1.3 kilometres, and challenges for longer distances remain. Here we demonstrate entanglement of two atomic ensembles in one laboratory via photon transmission through city-scale optical fibres. The atomic ensembles function as quantum memories that store quantum states. We use cavity enhancement to efficiently create atom–photon entanglement, and we use quantum frequency conversion to shift the atomic wavelength to telecommunications wavelengths.
We realize entanglement over 22 kilometres of field-deployed fibres via two-photon interference, and entanglement over 50 kilometres of coiled fibres via single-photon interference. Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.