The Quantromon – a qubit-cavity system with orthogonal qubit and readout modes*

The multimodal circuit nicknamed Quantromon has two modes: a transmon qubit and a linear LC oscillator coupled to each other via cross-Kerr coupling. An integrated qubit-cavity system is realized using these modes with the linear LC oscillator playing the role of the readout cavity. We previously demonstrated a high measurement fidelity of 97.63% without using the Josephson parametric amplifier due to the possibility of using higher photon numbers in the Quantromon. Another unique feature is the inherent orthogonality of the qubit and readout mode which allows strong qubit-readout coupling without being limited by Purcell decay. This also enables the qubit and readout frequencies to be independent of each other, unlike the standard dipolar coupling scenario. We study this effect by designing a Quantromon device with multiple SQUID loops to tune the qubit frequency over a large range with only small changes to the readout frequency. We also discuss the possibility of using such designs for enabling improved readout multiplexing capabilities in a multi-qubit architecture.