  Quantum entanglement is of central importance to quantum computing, quantum
metrology, quantum information as well as the nature of quantum physics.
Quantum theory does not prevent entanglement from being created and observed in
macroscopic physical systems, in reality however, the accessible scale of
entanglement is still very limited due to decoherence effects. Recently,
entanglement has been observed among atoms from thousands to millions level in
extremely low-temperature and well-isolated systems. Here, we create
multipartite entanglement of billions of motional atoms in a quantum memory at
room temperature, and certify the genuine entanglement via $M$-separability
witness associated with photon statistics. The information contained in a
single photon is found strongly correlated with the excitation shared by the
motional atoms, which intrinsically address the large system and therefore
stimulate the multipartite entanglement. Remarkably, our heralded and quantum
memory built-in entanglement generation allows us to directly observe the
dynamic evolution of entanglement depth and further to reveal the effects of
decoherence. Our results verify the existence of genuine multipartite
entanglement among billions of motional atoms at ambient condition,
significantly extending the boundary of the accessible scale of entanglement.
Besides probing the quantum-to-classical transition in an entirely new realm,
the developed abilities of manipulating such a large-scale entanglement may
enhance a wide spectrum of applications for emerging quantum technologies.
