Emergence of irreversible decoherence from unitary dynamics

The decoherence of superpositions of classically distinguishable states (cat states) is crucial for understanding quantum-to-classical transitions and quantum measurements. So far, decoherence processes of mesoscopic cat states have been demonstrated in several experiments. However, the issue of how the unitary system-reservoir dynamics can lead to irreversible system decoherence remains largely unexplored in experiments. Here we experimentally explore this fundamental issue with a circuit quantum electrodynamics device, where a bus microwave resonator storing a photonic cat state is connected to many nonlinear electronic oscillators. Each of these oscillators that are effectively coupled to the bus resonator serves as one degree of freedom of the reservoir. By gradually increasing the number of the reservoir's degrees of freedom, we find that the revivable quantum coherence progressively decays, owing to the growth in the amount of inerasable information about the system's state encoded in the reservoir. Our results illustrate that irreversible decoherence of a quantum system is an emergent phenomenon, arising from the unitary dynamics involving the system and many of the reservoir's degrees of freedom, which is crucial for the reconciliation of quantum mechanics and classical physics.