Fast high fidelity quantum non-demolition superconducting qubit readout*

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 and QND 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 propose and experimentally demonstrate a new readout scheme based on a transmon molecule inserted inside a 3D-cavity_[1,2]. The full system presents a transmon qubit mode coupled to a readout mode through an original non-pertubative cross-Kerr coupling. The readout mode, called polariton mode, results from the hybridization between the microwave cavity and 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. This first implementation, though perfectible, already enables a very efficient single-shot QND readout of the qubit in only 50ns, with a QND-ness of 99% and a fidelity of 97.4%.
[1] I. Diniz et al, Phys. Rev. A 87 033837 (2013).
[2] R. Dassonneville et al, Phys. Rev. X 10, 011045 (2020).