Ultra-low-noise microwave to optics conversion in gallium phosphide

Article https://doi.org/10.1038/s41467-022-34338-x Ultra-low-noise microwave to optics conversion in gallium phosphide Received: 13 April 2022 Accepted: 21 October 2022 1234567890():,; 1234567890():,; Check for updates Robert Stockill1,2,6, Moritz Forsch1,6, Frederick Hijazi1,2, Grégoire Beaudoin3, Konstantinos Pantzas 3, Isabelle Sagnes 3, Rémy Braive 3,4,5 & Simon Gröblacher 1,2 Mechanical resonators can act as excellent intermediaries to interface single photons in the microwave and optical domains due to their high quality factors. Nevertheless, the optical pump required to overcome the large energy difference between the frequencies can add significant noise to the transduced signal. Here we exploit the remarkable properties of thin-film gallium phosphide to demonstrate bi-directional on-chip conversion between microwave and optical frequencies, realized by piezoelectric actuation of a Gigahertzfrequency optomechanical resonator. The large optomechanical coupling and the suppression of two-photon absorption in the material allows us to operate the device at optomechanical cooperativities greatly exceeding one. Alternatively, when using a pulsed upconversion pump, we demonstrate that we induce less than one thermal noise phonon. We include a high-impedance onchip matching resonator to mediate the mechanical load with the 50-Ω source. Our results establish gallium phosphide as a versatile platform for ultra-lownoise conversion of photons between microwave and optical frequencies. The physical carrier of quantum information plays a crucial role in how the information is processed, communicated and measured. Encoding quantum information in microwave-frequency photons has allowed for the development of circuit1 and spin-based2,3 quantum information processing. At the same time, optical photons at telecom wavelength are a natural carrier for quantum information4, benefitting from particularly low loss rates in optical fibers5, an effectively noise-free environment at room temperature and near-unity efficiency singlephoton detection technology6. Bridging the five-orders-of-magnitude frequency difference between the microwave and optical domains will allow for flexible manipulation and communication of quantum information7–9, enabling both networked quantum computation and complex processing of long-range entangled states. Steady progress in various experimental realizations of such microwave-to-optics converters has been achieved in the past few years10, with significant challenges remaining. These include low-noise operation, efficient transduction, and the construction of scalable platforms. In particular, the requirement of an optical-frequency pump to compensate for the large frequency mismatch introduces the possibility of absorptionbased noise which can overwhelm the ultra-low power signal and corrupt the quantum information. Owing to weak electro-optic coupling rates, the large optical pump powers required for efficient operation typically prohibit direct conversion without the addition of significant absorption-induced noise11. An alternative is to find intermediaries which can reduce the required optical pump size, while at the same time providing highcooperativity interfaces to both electrical and optical domains, a role for which mechanical resonators are an attractive system12–18. To this end, the interaction between a mechanical oscillator and a co-localized optical mode has enabled the creation and measurement of nonclassical states of motion19–21. At the same time, by using the 1 Kavli Institute of Nanoscience, Department of Quantum Nanoscience, Delft University of Technology, 2628CJ Delft, The Netherlands. 2QphoX B.V., 2628XG Delft, The Netherlands. 3Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, C2N, 91767 Palaiseau, France. 4Université Paris-Cité, 75006 Paris, France. 5Institut Universitaire de France (IUF), Paris, France. 6These authors contributed equally: Robert Stockill, Moritz Forsch. e-mail: Nature Communications | (2022)13:6583 1 Article piezoelectric interaction or mechanical modification of resonator capacitances mechanical modes have been successfully interfaced with excitations in superconducting qubits22,23. A recent demonstration has shown the generation of optical photons from a superconducting qubit embedded within a transducer24, however the close proximity of the transducer to the qubit resulted in the production of quasi-particles in the superconductor, disturbing the qubit coherence. The realization of a stand-alone quantum transducer, which could in turn be connected to a shielded quantum system, remains an ongoing challenge. Gallium phosphide (GaP) combines a high refractive index (nGaP = 3.1) and a large band gap (2.3 eV) with a non-centro-symmetric crystal structure enabling applications for low-loss integrated photonics in the telecom band25. Thanks to the suppressed two-photon absorption of telecom-wavelength light and the intrinsic piezoelectric coupling in GaP, the material has shown promise in a demonstration of nonclassical optomechanics26 as a suitable host for a full microwave-tooptical transducer. Here we build on the impressive properties of GaP (for more details see Methods) and realize microwave-to-optics conversion using this material. We enhance our previously demonstrated optics-tomechanical interface with an electrical-to-mechanical interface, whereby we directly actuate a breathing mode of the optomechanical resonator. We include an impedance matching network to mediate the high-impedance load presented by the mechanical resonance with a 50-Ω source27. The ultra-low absorption of telecom photons in the material enables operate of the device with optomechanical cooperativities Com ≫ 1 under continuous driving. For pulsed operation, we demonstrate total conversion efficiencies of 6.8 × 10−8, featuring a mechanical-to-optical efficiency of 1.9 × 10−2 (see Methods), while maintaining a thermal occupation in the nanobeam mechanical mode of only nth = 0.55 ± 0.05 phonons. Results Our converter consists of an optomechanical and an electromechanical interface (Fig. 1a). The former is realized by a nanobeam optomechanical crystal (OMC)28, with an optical resonance around 1550 nm and a mechanical breathing mode (Fig. 1d) with a resonance frequency of ωm = 2π × 2.81 GHz. We fabricate our devices out of a 230-nm-thick layer of suspended gallium phosphide29. The electromechanical interface is realized by a piezoelectric resonator with a breathing mode of the same symmetry and polarisation as the nanobeam mode (Fig. 1d). These two interfaces are mechanically connected and are brought into resonance by careful design of the width of the piezoelectric resonator. In order to facilitate the mechanical coupling between the two parts of the converter, we design the OMC such that the mechanical mode can leak out of the resonator30. We attach the converter to its surrounding through an acoustic shield2 (...truncated)