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Cryogenic thermal noise behavior in release-free optomechanical crystals

Johan Kolvik

Published Sep 4, 2026• 0 citations
Publisher

Abstract

Noise is an ever-present reality of the world we live in. From the non-deterministic movement of matter to the fluctuations of the physical fields surrounding us, noise is the ultimate adversary of all things pure. Noise limits the fidelity of information transfer, particularly for quantum signals relying on coherence. Yet at sufficiently low temperatures, ambient thermal noise is suppressed to a level where even quantum signals can maintain coherence, making them available for technological applications. Modern cryogenic technologies enable access to environmental temperatures of 10 mK, allowing quantum-coherent transfer and processing of signals with carrier frequencies in the GHz range.A technology that operates in this regime is the optomechanical crystal: a nanofabricated device that supports high-quality resonances for both optical and acoustic fields and enables coherent coupling between the two. Through the optomechanical interaction, the acoustic mode can serve as a universal bus for the optical field, coherently linking it to other nonlinear systems, such as superconducting qubits – thereby enabling quantum microwave-to-optical transduction. As with many quantum systems, however, device performance is constrained by excess noise. In optomechanical crystals, this noise primarily stems from optical absorption combined with insufficient thermal anchoring. Achieving robust quantum-level operation in future devices therefore requires a detailed understanding and careful mitigation of these noise mechanisms.This thesis makes progress toward overcoming optical-absorption-induced noise in optomechanical crystals by (I) introducing a new class of optomechanical devices with superior thermal anchoring: the release-free optomechanical crystal; and (II) carefully investigating its noise performance and potential for microwave-to-optical transduction. The results presented in this thesis contribute to the understanding of noise limits in silicon optomechanical crystals and showcase the release-free platform as a viable alternative for on-chip, low-noise optomechanics.

DOI
10.63959/chalmers.dt/5935

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•Oct 29, 2018•0 citations
Noise is an ever-present reality of the world we live in. From the non-deterministic movement of matter to the fluctuations of the physical fields surrounding us, noise is the ultimate adversary of all things pure. Noise limits the fidelity of information transfer, particularly for quantum signals relying on coherence. Yet at sufficiently low temperatures, ambient thermal noise is suppressed to a level where even quantum signals can maintain coherence, making them available for technological applications. Modern cryogenic technologies enable access to environmental temperatures of 10 mK, allowing quantum-coherent transfer and processing of signals with carrier frequencies in the GHz range.A technology that operates in this regime is the optomechanical crystal: a nanofabricated device that supports high-quality resonances for both optical and acoustic fields and enables coherent coupling between the two. Through the optomechanical interaction, the acoustic mode can serve as a universal bus for the optical field, coherently linking it to other nonlinear systems, such as superconducting qubits – thereby enabling quantum microwave-to-optical transduction. As with many quantum systems, however, device performance is constrained by excess noise. In optomechanical crystals, this noise primarily stems from optical absorption combined with insufficient thermal anchoring. Achieving robust quantum-level operation in future devices therefore requires a detailed understanding and careful mitigation of these noise mechanisms.This thesis makes progress toward overcoming optical-absorption-induced noise in optomechanical crystals by (I) introducing a new class of optomechanical devices with superior thermal anchoring: the release-free optomechanical crystal; and (II) carefully investigating its noise performance and potential for microwave-to-optical transduction. The results presented in this thesis contribute to the understanding of noise limits in silicon optomechanical crystals and showcase the release-free platform as a viable alternative for on-chip, low-noise optomechanics.
Optomechanical crystals (OMCs) enable coherent interactions between optical photons and microwave acoustic phonons, and represent a platform for implementing quantum transduction between microwave and optical signals. Optical-absorption-induced thermal noise at cryogenic (millikelvin) temperatures is one of the primary limitations of performance for OMC-based quantum transducers. Here, we address this challenge with a two-dimensional silicon OMC resonator that is side-coupled to a mechanically detached optical waveguide, realizing a six-fold reduction in the heating rate of the acoustic resonator compared to prior state-of-the-art, while operating in a regime of high optomechanical-backaction and millikelvin base temperature. This reduced heating translates into a demonstrated phonon-to-photon conversion efficiency of 93.1±0.8% at an added noise of 0.25±0.01 quanta, representing a significant advance toward quantum-limited microwave-optical frequency conversion and optically controlled quantum acoustic memories.
Electro-optic modulation is central to classical optical communications and emerging quantum technologies. High-confinement optomechanical crystal modulators enable microwave-optical transduction through strong optomechanical interactions and offer a promising interface between superconducting qubits and optical fibers. However, their performance is limited by thermal noise from optical absorption. Release-free optomechanical crystals provide improved thermal anchoring but have not yet been integrated into a microwave-optical transducer. Here, we demonstrate a release-free electro-optomechanical transducer combining strong optomechanical interactions in silicon with the efficient piezoelectricity of lithium niobate via micro-transfer printing. We observe electro- and optomechanical coupling rates compatible with quantum-level operation when co-integrated with a superconducting microwave circuit. This advance moves release-free electro-optomechanical devices toward practical microwave-optical interfaces.
Interactions between light and mechanics provide a powerful interface between optical and microwave-frequency signals, with applications spanning classical signal processing and quantum technologies. High-performance optomechanical devices require both strong photon-phonon coupling and tolerance to parasitic laser heating. Release-free optomechanical crystals provide improved thermal anchoring compared to suspended nanobeams, but have so far exhibited weaker vacuum optomechanical coupling rates, leaving a trade-off between coupling strength and thermal robustness. Here, we largely close this gap: we design and experimentally demonstrate a release-free silicon optomechanical crystal with a record vacuum optomechanical coupling rate of about gOM/(2π)=800g_\text{OM} / (2 π) = 800gOM​/(2π)=800 kHz, comparable to suspended state-of-the-art devices. The resulting optomechanical scattering rate ΓOM/(2π)=1.1Γ_\text{OM}/(2 π)= 1.1ΓOM​/(2π)=1.1 kHz is nearly twice that of previous release-free implementations. This performance is achieved by combining physics-guided human intuition with a multiphysics inverse-design algorithm introduced here for resonant optomechanical structures. Beyond the specific device demonstrated, the inverse-design framework is applicable to co-optimizing optical and mechanical resonances and eigenmodes more broadly. These results strengthen release-free optomechanical crystals as a platform for fast, low-noise classical and quantum optomechanics.
Optical quantum networks provide a natural route for connecting distant superconducting quantum processors, enabling distributed quantum computation, sensing, and communication. Piezo-optomechanical transducers are among the leading candidates for scalable microwave-to-optical quantum interfaces. However, prior one-dimensional piezo-optomechanical transducers remain limited by optical-absorption-induced heating and the resulting thermal noise. Two-dimensional optomechanical crystals offer substantially improved thermalization from better thermal anchoring, but their structural complexity has so far hindered the realization of a fully integrated two-dimensional transducer. Here, we overcome this challenge with a new design strategy based on band structure engineering. We fabricate the devices and experimentally characterize the response, measuring an electromechanical damping rate of 4.6 kHz at room temperature and electromechanical coupling rate of 0.17 MHz by wire-bonding to a multi-mode microwave resonator at 10 mK. Bidirectional transduction is performed with a calibrated internal efficiency of 0.85\% in the continuous-wave operation, alongside pulsed photon-phonon pair generation near its quantum ground state. Our results represent a significant step toward high-efficiency and low-noise transducers for entangling remote superconducting qubits.
Quantum transduction between microwave and optical photons offers the potential to merge the long-range connectivity of optical photons with the deterministic quantum operations of superconducting microwave qubits. A promising approach to achieving this uses an intermediary mechanical mode along with piezo-optomechanical interactions. Traditionally, these transducers are suspended to confine mechanical fields, but this complicates manufacturing and comes with the major challenge of poor thermal anchoring and a trade-off between noise and efficiency. To overcome these issues, we introduce the—to our knowledge—first design of a release-free electro-optomechanical quantum transducer. Our release-free, i.e., non-suspended, design leverages a silicon-on-sapphire platform. It combines release-free lithium niobate electromechanical crystals with silicon optomechanical crystals on a sapphire substrate, optimizing thermal anchoring and microwave and mechanical coherence. Despite departing from the traditional suspended transducer paradigm, our release-free design achieves coupling rates sufficient for quantum-level interactions between microwave photons, phonons, and optical photons. Unconventionally, it utilizes high-wavevector mechanical modes tightly confined to the chip surface. Beyond quantum science and engineering, this platform and its design principles could also propel low-power acousto-optic systems in integrated photonics.
Microscale and nanoscale mechanical resonators have been used in advanced technological applications, from high precision time keeping and mass sensing, to processing high frequency signals in mobile communications. In the last few decades, they have been an important part of progress in the field of quantum information and metrology and have been proposed as quantum memories or transducers for measuring or connecting different types of quantum systems. The field of cavity optomechanics and electromechanics is concerned with coupling the electromagnetic field of a resonant optical cavity or electrical circuit to mechanical motion. These systems provide potential means to control and engineer the state of a mechanical object at the quantum level. This thesis contains the description of mechanical systems in megahertz to a few hundred megahertz frequency range formed by nano-fabricating photonic, phononic, and electrical circuits on a chip. These structures are designed to provide a large radiation pressure coupling between mechanical motion and electromagnetic fields to address and manipulate motional degrees of freedom. Qualitatively novel quantum effects are expected when one takes a step beyond linear coupling and exploits higher order interactions. To that end, we integrate electrical, mechanical and photonic structures in a multimode photonic crystal structure to observe "x2-coupling", where the optical cavity frequency is coupled to the square of the mechanical displacement. Moreover, we have developed two integrated on-chip platforms based on Si3N4 and Si nanomembranes capable of interfacing superconducting qubits and optical photons and realizing reversible microwave-to-optical conversion. We employ radiation pressure to cool these mechanical resonators to their quantum ground state. Finally, we demonstrate a form of electromechanical crystal for coupling microwave photons and hypersonic phonons of frequency ωm/2π = 0.425 GHz by capacitively coupling a phononic crystal acoustic cavity to a superconducting microwave resonator. Moving to higher frequency acoustic cavities not only facilitates the integration of electromechanical circuits and nanophotonic systems capable of operation in the resolved sideband limit of optomechanics for noise-free quantum signal conversion, but it opens up the possibility of using phonons as information carriers via phononic circuits. Utilizing a two-photon resonance condition for efficient microwave pumping and phononic bandgap shield to eliminate acoustic radiation, we achieve large cooperative electromechanical coupling (C ≈ 30) and intrinsic decay time of 2.3 ms. Moreover, electrical read-out of the phonon occupancy shows that the acoustic mode thermalizes close to its quantum ground-state of motion (phonon occupancy nm=1.5) at a fridge temperature of Tf = 10 mK. We conclude by considering several designs and fabrication improvements to the hypersonic electromechanical crystals that would enable them to perform quantum conversion between the electrical and acoustic domain.