Engineering Strong Beamsplitter Interaction between Bosonic Modes via Quantum Optimal Control Theory

TL;DR

This paper demonstrates how quantum optimal control theory can systematically enhance the strength and speed of beamsplitter interactions between bosonic modes in superconducting cavities, improving quantum information processing capabilities.

Contribution

The authors develop a systematic OCT-based approach to optimize multi-tone protocols for stronger, faster beamsplitter interactions in bosonic modes, surpassing previous two-tone methods.

Findings

Three-tone protocol accelerates effective beamsplitter rate

Gradient-based optimization further improves drive parameters

Optimized protocols are experimentally feasible

Abstract

In continuous-variable quantum computing with qubits encoded in the infinite-dimensional Hilbert space of bosonic modes, it is a difficult task to realize strong and on-demand interactions between the qubits. One option is to engineer a beamsplitter interaction for photons in two superconducting cavities by driving an intermediate superconducting circuit with two continuous-wave drives, as demonstrated in a recent experiment. Here, we show how quantum optimal control theory (OCT) can be used in a systematic way to improve the beamsplitter interaction between the two cavities. We find that replacing the two-tone protocol by a three-tone protocol accelerates the effective beamsplitter rate between the two cavities. The third tone's amplitude and frequency are determined by gradient-free optimization and make use of cavity-transmon sideband couplings. We show how to further improve the three-tone protocol via gradient-based optimization while keeping the optimized drives experimentally feasible. Our work exemplifies how to use OCT to systematically improve practical protocols in quantum information applications.