Entangling transmons with low-frequency protected superconducting qubits

TL;DR

This paper proposes a scheme to entangle low-frequency protected superconducting qubits with tunable transmons, enabling high-fidelity quantum gates while leveraging noise-protected qubit advantages.

Contribution

It introduces a method for entangling a transmon with a low-frequency protected qubit using non-computational states, facilitating hybrid quantum gate operations.

Findings

Non-computational states mediate entangling gates.

Gate preserves Cooper-pair parity.

Standard calibration protocols can be adapted.

Abstract

Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits. However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging. In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island. By tuning the external flux on the transmon, we show that non-computational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence. Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices. Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating hybrid qubit devices.