Engineering a controlled-phase gate between two superconducting cavities (Part 2)

Bosonic cQED systems are a promising platform for hardware-efficient and error-correctable quantum information processing due to the infinite Hilbert space of superconducting cavities. However, to achieve universal control, these harmonic oscillators are usually coupled to one or more auxiliary qubits, whose discrete transitions are used to perform single- and multiple-cavity gates. This ultimately limits the quality of gates performed on the bosonic mode, due to the shorter qubit coherence times.

Here, we present an experimental demonstration of a controlled-phase gate for several rotationally-symmetric codes that relies only on the non-linearity provided by the Josephson junction, without actively populating the excited energy levels of the qubit. Such a gate can also be applied to detect for photon loss in a fault-tolerant manner.

In Part 2, we present a tailored device, designed to minimize gate time, while ensuring minimal participation of the ancillary qubit excitations.