Decoherence via coupling to a finite quantum heat bath

Decoherence of a quantum state coupled to an exterior environment is at the foundation of our understanding of the emergence of classical behavior from the quantum world, but how does it emerge in a finite closed quantum system? Here this is studied by modeling an isolated quantum system of a handful of ultracold atoms confined to a double well potential and coupled to a single atom of a different type. The ultracold atoms thermalize and serve as an environmental bath for the single atom. We observe accelerated decoherence of the single atom when ultracold atoms have thermalized. This is explained by the emergence of chaotic eigenstates in the thermalized system.