Nonlinear Quantum Evolution of a Dissipative Superconducting Qubit

Unitary and dissipative models of quantum dynamics are linear maps on the space of states. This linearity underlies the superposition principle, a key feature of quantum theory. However, this principle can break down under effective dynamics of postselected quantum evolution. We experimentally investigate this breakdown in a dissipative superconducting transmon circuit by comparing the trajectories arising from initial superposition states to superpositions of the trajectories arising from initial basis states. We quantify deviations from linearity across the PT-symmetry breaking transition. Our results provide direct evidence of effective nonlinearity in quantum evolution, challenging a core assumption of standard quantum mechanics and highlighting the unique quantum features that emerge from postselected dynamics.