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Bidirectional Microwave-Optical Conversion with an Integrated Soft-Ferroelectric Barium Titanate Transducer

Charles Möhl*, Annina Riedhauser*, Max Glantschnig†, Daniele Caimi, Ute Drechsler, Antonis Olziersky, Deividas Sabonis, David I. Indolese, Thomas M. Karg‡ et al.

Paul Seidler§

  • *These authors contributed equally to this work.
  • †Present address: Infineon Technologies Austria AG, Siemensstraße 2, 9500 Villach, Austria.
  • ‡Contact author: thomas.karg@ibm.com
  • §Contact author: pfs@zurich.ibm.com
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Phys. Rev. X 15, 041044 – Published 8 December, 2025

DOI: https://doi.org/10.1103/1gvf-w6lx

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 LiNbO3 have shown promise. To improve efficiency and added noise, materials with larger Pockels coefficients, such as the soft ferroelectrics BaTiO3 or SrTiO3, are needed. 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 BaTiO3-on-SiO2 waveguides monolithically integrated with a superconducting microwave resonator made of Nb. We demonstrate bidirectional microwave-optical transduction and reach total off-chip efficiencies of 1×10−6 using pulsed pumping. The device design permits in situ poling of the ferroelectric material without introducing excess microwave loss, exploiting superconducting air bridges fabricated using only subtractive patterning steps. 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 like SrTiO3 and pave the way for efficient, low-power quantum interconnects.

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Popular Summary

Superconducting quantum computers promise breakthroughs in areas like drug discovery, materials science, and artificial intelligence, but connecting them over long distances is difficult because their processors must operate near absolute zero. One solution is to convert the microwave quantum signals in the superconducting electrical circuits to optical photons that can travel through fiber networks at room temperature. The problem is that conversion efficiency is limited by the weak nonlinear properties of the materials used to date. To address this, we build the first microwave-to-optical converter using a soft ferroelectric, a class of materials offering the potential for much stronger microwave-optical interaction.

In our design, we integrate an optical resonator made of the soft ferroelectric barium titanate (BaTiO3) with a superconducting microwave resonator to create an on-chip device capable of bidirectional, coherent conversion between microwave and optical signals. A key feature of our approach is a microwave circuit engineered to pole the ferroelectric and maximize electro-optic coupling, while maintaining high-quality microwave resonances. We also measure heating effects caused by absorption of optical photons, separating the contributions from the dielectric and the superconductor. Remarkably, we find that the heating does not reduce conversion efficiency.

These results show that soft-ferroelectric materials are attractive candidates for building efficient microwave-to-optical quantum interconnects. To outperform existing platforms, optimization of the material properties of BaTiO3, such as strain engineering, will be necessary. Nevertheless, the concepts developed are applicable to other soft ferroelectrics such as SrTiO3, and our work provides insight into the design of electro-optic transducers in general.

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Phys. Rev. X 15, 041044– Published 8 December, 2025
  • Received 26 January 2025
  • Revised 27 May 2025
  • Accepted 8 August 2025
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