# Microwave-to-optics conversion using a mechanical oscillator in its quantum ground state

## Abstract

Conversion between signals in the microwave and optical domains is of great interest both for classical telecommunication and for connecting future superconducting quantum computers into a global quantum network. For quantum applications, the conversion has to be efficient, as well as operate in a regime of minimal added classical noise. While efficient conversion has been demonstrated using mechanical transducers, they have so far all operated with a substantial thermal noise background. Here, we overcome this limitation and demonstrate coherent conversion between gigahertz microwave signals and the optical telecom band with a thermal background of less than one phonon. We use an integrated, on-chip electro-optomechanical device that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal featuring a 2.7 GHz mechanical mode. We initialize the mechanical mode in its quantum ground state, which allows us to perform the transduction process with minimal added thermal noise, while maintaining an optomechanical cooperativity >1, so that microwave photons mapped into the mechanical resonator are effectively upconverted to the optical domain. We further verify the preservation of the coherence of the microwave signal throughout the transduction process.

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## Data availability

The data represented in the figures are available as Supplementary information files. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank V. Anant, J. Davis, M. Jenkins and C. Schäfermeier for valuable discussions and support. We also acknowledge assistance from the Kavli Nanolab Delft, in particular from M. Zuiddam and C. de Boer. The sample growth was realized in the NanoLab@TU/e cleanroom facility. This project was supported by Foundation for Fundamental Research on Matter (FOM) Projectruimte grants (15PR3210, 16PR1054), the European Research Council (ERC StG Strong-Q, 676842) and by the Netherlands Organisation for Scientific Research (NWO/OCW), as part of the Frontiers of Nanoscience programme, as well as through a Vidi grant (680-47-541/994), the Gravitation programme Research Center for Integrated Nanophotonics and the ARO/LPS CQTS programme.

## Author information

M.F., R.S., K.S. and S.G. planned the experiment and performed the device design. M.F., R.S., C.G. and R.A.N. fabricated the sample and F.v.O. and A.F. supplied the material. M.F., R.S., A.W. and I.M. performed the measurements, while M.F., R.S., K.S. and S.G. analysed the data and wrote the manuscript with input from all authors. S.G. supervised the project.

Correspondence to Simon Gröblacher.

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### Competing interests

M.F., R.S. and S.G. declare that there is a pending patent application related to this research.

Peer review information Nature Physics thanks David Vitali and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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