Understanding the mechanism governing the quantum-to-classical transition in finite-dimensional systems from the viewpoint of phase-space quasiprobability remains challenging. In this Letter, we establish an operational connection between discrete rounds of generalized measurements and continuous-time decoherence, with an explicit correspondence between the number of measurement rounds and the evolution time. Operationally, we show that a single round of such a generalized measurement eliminates quasiprobability negativity in finite-dimensional systems. From the decoherence perspective, this loss of negativity occurs abruptly at a critical time. In particular, this critical time can be shorter than the conventional decoherence time, indicating that the latter does not always faithfully track the disappearance of nonclassicality. Our results provide insight into the quantum-to-classical transition in finite-dimensional systems from the viewpoint of phase-space quasiprobability and suggest feasible experimental quantum-circuit tests.

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• Open quantum systems & decoherence
• Quantum correlations, foundations & formalism
• Quantum measurements
• Quantum-to-classical transition
• Phase space dynamics