Shelving readout using even- and odd-parity electromagnetic eigenmodes of a dimon

High-fidelity and fast quantum non-demolition (QND) measurement is important for realizing quantum error correction. Most superconducting qubits use dispersive readout with a resonator, which is only approximately QND and can cause state leakage. As an alternative, we implement shelving readout using a dimon that naturally provides a large cross-Kerr interaction without relying on dispersive coupling. A dimon supports two orthogonal electromagnetic modes, the even- and odd-parity modes, which we use as the readout and qubit modes, respectively. The strong cross-Kerr coupling (~370 MHz) makes the excitation of the readout mode conditional on the qubit state: a transition occurs only when the qubit is in the ground state. By detecting the phase of the reflected signal, the qubit state can be discriminated in a shelving manner. The external coupling port is located along the symmetry axis of the dimon. Since the odd and even modes are orthogonal, the former has a voltage node and the latter a voltage antinode at the port. This geometry isolates the qubit mode from the environment while allowing the readout mode to couple strongly (~18 MHz) to the external circuit, achieving a coupling selectivity ratio of over 2000. Thanks to the large cross-Kerr interaction, the external coupling can be made stronger, enabling broadband measurement. We will discuss the experimental characterization, quantum efficiency, and future prospects of this shelving-type readout architecture.