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Peer-reviewedCC BY 4.0Physical Review Research

Nonlinear quantum evolution of a dissipative superconducting qubit

  • Orion Lee1
  • Qian Cao1
  • Yogesh N. Joglekar2,3,4
  • Kater Murch1,5
5 affiliations
  1. 1Washington University, St. Louis
  2. 2Indiana University Indianapolis
  3. 3Portland State University
  4. 4Oregon Health and Science University
  5. 5University of California, Berkeley
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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 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 an operational demonstration of postselection-induced nonlinearity in a controlled superconducting platform. The observed nonlinearity, organized by the exceptional-point degeneracy of the non-Hermitian Hamiltonian, is absent in classical non-Hermitian systems, where norm change reflects real exchange with an environment rather than renormalization.

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Cite

BibTeXRIS
BibTeX
@article{lee2026nonlinear,
  title = {{Nonlinear quantum evolution of a dissipative superconducting qubit}},
  author = {Orion Lee and Qian Cao and Yogesh N. Joglekar and Kater Murch},
  journal = {Physical Review Research},
  year = {2026},
  month = oct,
  doi = {10.1103/7w9t-c5xf},
  url = {http://link.aps.org/doi/10.1103/7w9t-c5xf},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Nonlinear quantum evolution of a dissipative superconducting qubit
AU  - Orion Lee
AU  - Qian Cao
AU  - Yogesh N. Joglekar
AU  - Kater Murch
PY  - 2026
DA  - 2026/10/23
JO  - Physical Review Research
DO  - 10.1103/7w9t-c5xf
UR  - http://link.aps.org/doi/10.1103/7w9t-c5xf
AB  - 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 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 an operational demonstration of postselection-induced nonlinearity in a controlled superconducting platform. The observed nonlinearity, organized by the exceptional-point degeneracy of the non-Hermitian Hamiltonian, is absent in classical non-Hermitian systems, where norm change reflects real exchange with an environment rather than renormalization.
SN  - 2643-1564
ER  -