Fast high fidelity qubit readout of a transmon molecule using longitudinal coupling*

The most common technique of qubit readout in cQED relies on the transverse dispersive coupling between a qubit and a microwave cavity. However, despite important progresses, implementing fast high fidelity readout remains a major challenge. Indeed, inferring the qubit state is limited by the trade-off between speed and accuracy due to Purcell effect and unwanted transitions induced by readout photons in the cavity. To overcome this, we introduce a transmon molecule based on two transmons coupled by a large inductance, which is inserted inside a 3D-cavity. The full system presents one transmon –used as qubit– with a large direct cross-Kerr(longitudinal) coupling to a non-linear readout resonator, called polaron mode. This polaron mode results from the hybridization between the microwave cavity and the second mode of the transmon molecule circuit. The direct cross-Kerr coupling is a key point of our readout scheme since it protects the qubit from Purcell effect. We will present qubit readout performance with fidelity as high as 95.7% in 120ns and discuss the quantum non-demolition properties of this novel readout.