Nonlinear quantum evolution of a dissipative superconducting qubit

• URL: http://arxiv.org/abs/2510.25836v1
• Date: Wed, 29 Oct 2025 18:00:02 GMT
• Title: Nonlinear quantum evolution of a dissipative superconducting qubit
• Authors: Orion Lee, Qian Cao, Yogesh N. Joglekar, Kater Murch,
• Abstract summary: Superposition principle can break down in effective non-Hermitian dynamics.<n>We experimentally investigate this breakdown in a dissipative superconducting transmon circuit.<n>Results provide direct evidence of nonlinearity in non-Hermitian quantum evolution.
• Score: 1.0924851843152197
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Unitary and dissipative models of quantum dynamics are linear maps on the space of states or density matrices. This linearity encodes the superposition principle, a key feature of quantum theory. However, this principle can break down in effective non-Hermitian dynamics arising from postselected quantum evolution. We theoretically characterize and experimentally investigate this breakdown in a dissipative superconducting transmon circuit. Within the circuit's three-level manifold, no-jump postselection generates an effective non-Hermitian Hamiltonian governing the excited two-level subspace and an anti-Hermitian nonlinearity. We prepare different initial states and use quantum state tomography to track their evolution under this effective, nonlinear Hamiltonian. By comparing the evolution of a superposition-state to a superposition of individually-evolved basis states, we test linearity and observe clear violations which we quantify across the exceptional-point (EP) degeneracy of the non-Hermitian Hamiltonian. We extend the analysis to density matrices, revealing a breakdown in linearity for the two-level subspace while demonstrating that linearity is preserved in the full three-level system. These results provide direct evidence of nonlinearity in non-Hermitian quantum evolution, highlighting unique features that are absent in classical non-Hermitian systems.

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