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Macroscopic Entanglement in Crystals

Final Report Summary - MEC (Macroscopic Entanglement in Crystals)

This project looked at questions that are both fundamentally fascinating and a source or increasing curiosity for the general public - Macroscopic Quantum Effects and Measurements. We have a good understanding of what we see – what we measure – in the classical world, as well as a good understanding of measurements in the quantum realm. Where this project focused was on what happens when these quantum systems increase in size, in distance, or in the number of particles. What happens when we get to macroscopic systems that we could in principle see with our own eyes.
This work covered both theoretical and experimental activities. On the theory side, we looked at how macroscopic quantum measurements can probe the quantum-to-classical transition and even describe our everyday experiences, including the measurements we constantly perform by merely looking around us, despite seeming incompatible with standard quantum mechanics. We also looked at how this could be exploited, for example in metrology connecting fundamental concepts like entanglement, Bell nonlocality, Quantum Fisher Information to provide a metrological advantage for large quantum states. More fundamentally, several publications and theoretical work, arose from early publications addressing questions such as “How Difficult Is It to Prove the Quantumness of Macroscropic States”.

Combining progress in theory and experiment, we were able to demonstrate entanglement of millions of atoms. These atoms were “frozen” in solid state crystal that are also used as quantum memories – systems capable of storing and re-emitting quantum states. The performance of quantum memories and our control over their behaviour allowed us to extend this storage time beyond the initial targets of tens of microseconds to truly long-lived milliseconds regime. We were also able to exploit these in a teleportation experiment between a photon and a quantum memory; a key step towards distributing entanglement and performing quantum communication over large distances.

During this project, many of the ideas, concepts and experiments have started to bear fruit, which is witnessed by the large number of publications. In particular, a highlight was a review article on “Macroscopic quantum states: Measures, fragility, and implementations”, published in Reviews of Modern Physics along with over 30 other publications, clearly demonstrating both the productivity of the project but also the interest in the subject. Along with this, these results were discussed and disseminated at many scientific conferences and, befitting such a fascinating subject, numerous public lectures.