Gradually opening Schr\"odinger's box reveals a cascade of sharp dynamical transitions

TL;DR

This paper experimentally investigates how increasing measurement strength on a superconducting qubit causes three distinct dynamical transitions, revealing sharp changes in quantum behavior and the quantum Zeno effect.

Contribution

It uncovers that the transition from quantum to classical dynamics occurs through three abrupt stages, challenging the view of a smooth crossover and highlighting the role of decoherence.

Findings

Identification of three distinct dynamical transitions

Observation of the quantum Zeno effect at strong measurement

Decoherence reorganizes transition signatures

Abstract

Quantum mechanics predicts that unobserved systems may exist in a superposition of states, yet measurement produces definite outcomes, a tension at the heart of the quantum-to-classical boundary. How the transformation between these opposing regimes unfolds as observation strength increases has remained experimentally unexplored. Here, by continuously tuning the measurement strength on a superconducting qubit, we reveal that measurement-dominated dynamics emerge not gradually but through three distinct transitions: coherent oscillations abruptly halt; the system then freezes near a stable quantum state; and finally enters the quantum Zeno regime, where stronger observation paradoxically slows relaxation. Decoherence, rather than washing out this structure, reorganizes it, inverting the order in which transitions appear and decoupling signatures that coincide in idealized models. These results establish that the route from quantum dynamics to measurement-dominated behavior unfolds in sharp transitions governed by the interplay between observation and environment.