High-fidelity QND readout of Heavy-Fluxonium Qubit*

Heavy fluxonium qubits with their rich anharmonic spectrum, high coherence,and high-fidelity two-qubit gates are emerging as a potential alternative to transmon qubits for building superconducting quantum processors. In addition,it is also necessary to have a high-fidelity, quantum non-demolition (QND) readout to realize a quantum processor. In principle, the fidelity of the circuit-QED based readout for the fluxonium qubit can be increased by using larger microwave power. But in some earlier work, it was observed that the qubit lifetime decreases significantly with increasing readout power [1] which resulted in lower fidelity.This effect also takes place in the transmon qubit readout but was observed to be stronger in fluxonium qubits. Recently,  high fidelity readout was demonstrated in fluxonium qubit with a super-inductance made using granular Aluminium [2]. Here, we describe a series of experiments to characterize and optimize measurement fidelity in heavy-fluxoium qubits which use the conventional Josephson-junction array for the super-inductance. We will discuss the relaxation induced by cavity photons, signal-to-noise ratio, and the QND aspect of the readout in the presence of a high readout photon number.

[1] V. Manucharyan, Superinductance, Ph.D. thesis, Yale University (2007).

[2] Daria Gusenkova, et.al, Phys. Rev. Applied 15, 064030 (2021).