The decoherence of superpositions of classically distinguishable states (cat states) is crucial for understanding quantum-to-classical transitions and quantum measurements. So far, irreversible decoherence processes of mesoscopic cat states have been demonstrated in several experiments. We here report on an experimental demonstration of the dynamics of such states in a reservoir with controllable degrees of freedom, which allows observation of the transition from reversible to irreversible mesoscopic decoherence. The experiment is performed 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 the oscillators that are effectively coupled to the bus resonator serves as one degree of freedom of the reservoir. We characterize the which-path information about the resonator’s state with the distinguishability. The experimental results reveal that the amount of spontaneously erasable which-path information progressively decreases with the increase in the number of the reservoir’s degrees of freedom. When the reservoir contains sufficiently many oscillators, the which-path information acquired by the reservoir cannot be erased by the system-reservoir interaction at later times, which makes the decoherence irreversible. We further characterize the non-Markovianity of such a controllable open mesoscopic system.

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• Quantum circuits
• Quantum coherence & coherence measures
• Quantum foundations
• Quantum-to-classical transition