A Simple Impedance Formula for the Exchange Coupling Rates between Low Anharmonicity Qubit Modes in Superconducting Quantum Processors*

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” at the terminals of the Josephson junctions and “drive ports” where transmission lines carrying drive signals launch the chip and we obtain a 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 a simple function of the off-diagonal entry connecting the qubit ports evaluated at the qubit frequencies. 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 is present in the distributed electromagnetic structure.