1. Institute of Physics and Astronomy, University of Aarhus, 8000 Aarhus, Denmark

Correspondence to: Eugene S. Polzik Correspondence and requests for materials should be addressed to E.P. (e-mail: Email: polzik@ifa.au.dk).

Entanglement is considered to be one of the most profound features of quantum mechanics^{1, 2}. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems^{3, 4, 5, 6}. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 10¹² atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples⁷. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation^{8, 9, 10} of quantum states of matter and quantum memory.