Bidirectional microwave-optical conversion with an integrated soft-ferroelectric barium titanate transducer

TL;DR

This paper presents an integrated, bidirectional microwave-optical transducer using BaTiO3, achieving efficient quantum signal conversion with low noise, suitable for quantum communication networks.

Contribution

The authors develop a novel on-chip transducer with monolithically integrated BaTiO3 and superconducting resonators, enabling efficient bidirectional conversion without excess microwave loss.

Findings

Achieved total off-chip efficiency of 10^-6 with pulsed pumping.

Demonstrated in-situ ferroelectric poling without added microwave loss.

Revealed fast thermalization and quasiparticle resilience in the microwave resonator.

Abstract

Efficient, low-noise, and high-bandwidth transduction between optical and microwave photons is key to long-range quantum communication between distant superconducting quantum processors. Recent demonstrations of microwave-optical transduction using the broadband direct electro-optic (Pockels) effect in optical thin films made of AlN or LiNbO$_3$ have shown promise. To improve efficiency and added noise, materials with larger Pockels coefficients, such as the soft ferroelectrics BaTiO$_3$ or SrTiO$_3$, are required. However, these materials require adapted designs and fabrication approaches due to their nonlinear and, in some cases, hysteretic electro-optic response. Here, we engineer an on-chip, triply resonant transducer comprising low-loss BaTiO$_3$-on-SiO$_2$ waveguides monolithically integrated with a superconducting microwave resonator made of Nb. We demonstrate bidirectional microwave-optical transduction and reach total off-chip efficiencies of $1\times10^{-6}$ using pulsed pumping. Our novel device concept permits in-situ poling of the ferroelectric material without introducing excess microwave loss, using a fully subtractive fabrication process with superconducting air bridges. In addition, we investigate optically induced heating, revealing fast thermalization and quasiparticle resilience of the microwave resonator. Our transducer concept and fabrication process are applicable to other materials with a large bias-induced Pockels effect and pave the way for efficient, low-power quantum interconnects.