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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Q-CEOM (Quantum Cavity Electro- and Opto-Mechanics)

Teaser

Summary

\"Next-generation electronic and optical systems can be operated in a quantum regime, in which effects such as quantum correlations and entanglement are employed to realise higher measurement precision, or novel information processing schemes. Such capabilities can be further extended if subsystems can be linked in a coherent way, that is, without perturbing the quantum mechanical properties of the electronic and optical signals. Examples include a quantum \"\"modem\"\", that translates electronic quantum signals into optical signals that can be propagated through optical fibre. This would enable the implementation of quantum sensor and computing networks across laboratories, cities or even continents.

Realising such links necessitates physical systems that are coherent, and couple to optical and electronic signals alike. In this project, we develop nanomechanical resonators coupled to microwaves and light through capacitive and radiation pressure forces. They are designed to be highly coherent in spite of the inevitable coupling to a thermal environment. In this manner, they will be able to transfer signals, and mediate correlations between different domains of the electromagnetic spectrum as required.
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Work performed

\"Addressing the first challenge we have, as an important enabling step, realized a fabrication process that reproducibly achieves cryogenic mechanical Q-factors above 10^7. It is based on a phononic crystal shield in the silicon frame, which isolates the modes of interest, yet provides strong thermal anchoring to the cryostat cold finger for proper thermalisation. Importantly, the high quality factors are achieved for many modes simultaneously, enabling mechanical multimode experiments. As anticipated, we have thus realized cooperativities C>10^5, about 5-times larger than the thermal occupation. In combination with the purchased low-noise laser systems, this has, for the first time, enabled the demonstration of multimode optomechanical physics in the quantum regime. In particular, we have observed ponderomotive squeezing of light in a membrane-in-the-middle system. A large number of mechanical modes contributes to the generation of such a nonclassical state of light, which arises as a consequence of quantum correlations between the light\'s amplitude and phase, and mechanical motion. Interestingly, in the process, optical forces hybridise degenerate mechanical modes into a pair of dark and bright modes. We have realised a vibration-imaging (\"\"Chladni\"\") microscope as well as an imaging polariscope, which have proven to be invaluable tools in further development of ultrahigh-Q mechanical resonators. The latter are a crucial component for the successful implementation of the action: the quality factor Q, together with the temperature T, determine the mechanical quantum coherence time tau=hbar Q/k_B T, and thereby the ability to suppress thermal noise in all envisioned protocols. Indeed employing novel fabrication methods, we we could develop soft-clamped membranes with quality factors up to Q~1e9 at cryogenic temperature. This relaxes requirements for quantum-coherent opto- and electro-mechanical coupling, and may even lead to new applications in force-sensing an nano-imaging. Addressing the second challenge, we have realised superconducting 3d resonators with quality factors up to 1e6, and fabricated chips with NbTiN antennae to couple capacitively to soft-clamped membranes. Finally, we have performed theoretical studies to determine relevant figures of merit for quantum transducers. An important outcome is a differentiated analysis of the importance of transfer efficiency and added noise, depending on the task of the transducer.\"

Final results

The action has already advanced the state of the art significantly. This includes, in particular, the development of highly coherent mechanical resonators through phononic shielding. The latter strongly suppresses thermomechanical noise, and thus allows the application of mechanical systems in quantum-enabled technologies. As a consequence, action work could already demonstrate the generation of quantum correlations in a multimode optomechanical system in the presence of a hot mechanical reservoir. Theoretical work has further elucidated the relevant characteristics of quantum transducers. Both are important steps in the envisioned merging of electronic, optical and mechanical platforms to a multifunctional hybrid quantum technology.