An entangled quantum state of two or more particles or objects exhibits someof the most peculiar features of quantum mechanics. Entangled systems cannot bedescribed independently of each other even though they may have an arbitrarilylarge spatial separation. Reconciling this property with the inherentuncertainty in quantum states is at the heart of some of the most famousdebates in the development of quantum theory. Nonetheless, entanglementnowadays has a solid theoretical and experimental foundation, and it is thecrucial resource behind many emerging quantum technologies. Entanglement hasbeen demonstrated for microscopic systems, such as with photons, ions, andelectron spins, and more recently in microwave and electromechanical devices.For macroscopic objects, however, entanglement becomes exceedingly fragiletowards environmental disturbances. A major outstanding goal has been to createand verify the entanglement between the motional states of slowly-movingmassive objects. Here, we carry out such an experimental demonstration, withthe moving bodies realized as two micromechanical oscillators coupled to amicrowave-frequency electromagnetic cavity that is used to create and stabilisethe entanglement of the centre-of-mass motion of the oscillators. We infer theexistence of entanglement in the steady state by combining measurement ofcorrelated mechanical fluctuations with an analysis of the microwaves emittedfrom the cavity. Our work qualitatively extends the range of entangled physicalsystems, with implications in quantum information processing, precisionmeasurement, and tests of the limits of quantum mechanics.
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