Simple Impedance Response Formulas for the Dispersive Interaction Rates in the Effective Hamiltonians of Low Anharmonicity Superconducting Qubits

TL;DR

This paper presents a comprehensive microwave-based method to derive simple impedance formulas for dispersive interaction rates in low anharmonicity superconducting qubits, enhancing circuit modeling accuracy.

Contribution

It introduces a complete impedance-based framework relating qubit interactions to multiport impedance responses, accounting for all electromagnetic modes in superconducting circuits.

Findings

Derived simple formulas linking impedance matrix entries to qubit coupling rates.

Demonstrated the method's efficiency in modeling complex distributed microwave structures.

Provided a systematic approach for analyzing qubit-environment interactions.

Abstract

For superconducting quantum processors consisting of low anharmonicity qubits such as transmons we give a complete microwave description of the system in the qubit subspace. We assume that the qubits are dispersively coupled to a distributed microwave structure such that the detunings of the qubits from the internal modes of the microwave structure are stronger than their couplings. We define qubit ports across the terminals of the Josephson junctions and drive ports where transmission lines carrying drive signals reach the chip and we obtain the multiport impedance response of the linear passive part of the system between the ports. We then relate interaction parameters in between qubits and between the qubits and the environment to the entries of this multiport impedance function: in particular we show that the exchange coupling rate J between qubits is related in a simple way to the off-diagonal entry connecting the qubit ports. Similarly we relate couplings of the qubits to voltage drives and lossy environment to the entries connecting the qubits and the drive ports. Our treatment takes into account all the modes (possibly infinite) that might be present in the distributed electromagnetic structure and provides an efficient method for the modeling and analysis of the circuits.