Circuit optimization to improve transmon qubit readout via cross-Kerr coupling*

As it has been demonstrated recently, transmon based on non-perturbative cross-Kerr coupling has shown a promising fast high-fidelity quantum non-demolition readout [1]. First generation of V-shape qubits had presented 4us relaxation time which limited fidelity and QND-ness of the readout. Moreover, unwanted qubit transitions were induced by readout photons in the cavity. In order to improve the qubit and readout performance, we have optimized different parameters: the circuit geometry has been adjusted to reduce the amplitude of the microwave electrical field at the interfaces; the electric circuit parameters were optimized to reach large detuning between the readout and the transmon frequencies; unwanted dipolar coupling between the transmon qubit and the cavity was reduced to avoid the Purcell effect; design and fabrication process were modified in order to optimize electron beam doses and angle of evaporation. We will present the readout performance based on the cross-Kerr coupling of the improved transmon circuit.