A Boundary Condition Perspective on Circuit QED Dispersive Readout

TL;DR

This paper presents a boundary condition perspective on circuit QED dispersive readout, deriving the phenomena from first principles and revealing how boundary conditions encode qubit states and influence measurement outcomes.

Contribution

It introduces a first-principles derivation of dispersive readout in circuit QED using boundary conditions and spectral theory, providing new insights into the underlying physics.

Findings

Dispersive shift derived as $rac{2Lg^2 ext{ω}_q^2}{v^4}$

Vacuum Rabi splitting explained via transcendental eigenvalue equation

Parity-dependent degeneracies identified for two qubits

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: $\delta_{ge} = 2Lg^2\omega_q^2/v^4$, 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.