A direct controlled-phase gate between microwave photons

TL;DR

This paper demonstrates a direct controlled-phase gate between microwave photons in superconducting cavities using a Raman-assisted cross-Kerr interaction, enabling entanglement and improved quantum memory lifetime.

Contribution

It introduces a novel method to implement a direct nonlinear coupling between microwave photons without exciting the nonlinear element, reducing decoherence and leakage.

Findings

Successfully implemented a controlled-phase gate between two microwave photons.

Demonstrated a photon-number parity check with enhanced storage lifetime.

Engineered a decoherence-suppressing bosonic interaction for quantum processing.

Abstract

The rich dynamics and large Hilbert space of quantum harmonic oscillators make them natural candidates for hardware-efficient and error-correctable quantum information processing. However, implementing direct entangling operations between oscillators remains an outstanding challenge. Existing strategies typically rely on parametrically activating interactions that populate the excited states of a nonlinear element, which introduces additional dissipation channels and potential leakage from the encoded manifold. Here, we engineer a Raman-assisted cross-Kerr interaction between microwave photons hosted in two superconducting cavities. Crucially, this dynamics does not excite the mediating nonlinear coupler, thereby suppressing coupler induced decoherence and leakage out of the bosonic code space. We use this direct nonlinear coupling to implement a controlled-phase gate within the single- and two-photon subspaces of two oscillators, deterministically generating entanglement between them. Finally, we use these engineered dynamics to implement a photon-number parity check on a storage cavity via purely bosonic interactions with an ancillary cavity, demonstrating an enhancement in the storage lifetime. Our work provides a promising pathway toward engineering robust operations that act entirely within a protected bosonic code space and realizing fault-tolerant quantum information processing with bosonic elements.