A Boundary Condition Perspective on Circuit QED Dispersive Readout

• URL: http://arxiv.org/abs/2512.24466v1
• Date: Tue, 30 Dec 2025 21:10:19 GMT
• Title: A Boundary Condition Perspective on Circuit QED Dispersive Readout
• Authors: Mustafa Bakr,
• Abstract summary: Boundary conditions in confined geometries and measurement interactions in quantum mechanics share a common structural role.<n>This paper develops this perspective for circuit QED dispersive readout through a first-principles derivation.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Boundary conditions in confined geometries and measurement interactions in quantum mechanics share a common structural role: both select a preferred basis by determining which states are compatible with the imposed constraint. This paper develops this perspective for circuit QED dispersive readout through a first-principles derivation starting from the circuit Lagrangian. The transmon qubit terminating a transmission line resonator provides a frequency-dependent boundary condition whose pole structure encodes the qubit's transition frequencies; different qubit states yield different resonator frequencies. Two approximations, linear response and a pole-dominated expansion valid near resonance, reduce the boundary function to a rational form in the Sturm-Liouville eigenparameter. The extended Hilbert space of the Fulton-Walter spectral theory then provides a framework for the dressed-mode eigenvalue problem conditional on the qubit state. The dispersive shift and vacuum Rabi splitting emerge from the transcendental eigenvalue equation, with the residues determined by matching to the splitting: $δ_{ge} = 2Lg^2ω_q^2/v^4$, where $g$ is the vacuum Rabi coupling. A level repulsion theorem guarantees that no dressed mode frequency coincides with a transmon transition. For two qubits with matched dispersive shifts, odd-parity states become frequency-degenerate; true parity-only measurement requires engineered suppression of linear dispersive terms.

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