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Extremely Coherent Mechanical Oscillators and circuit Cavity Electro-Optics

Periodic Reporting for period 1 - ExCOM-cCEO (Extremely Coherent Mechanical Oscillators and circuit Cavity Electro-Optics)

Reporting period: 2019-10-01 to 2021-03-31

Mechanical resonators are found everywhere in our modern society. Accelerometers in cars and phones, motion sensors in automotive industries, and filters in our cell phones are a few of the common examples of such systems. A key element in a mechanical resonator that determines its sensitivity and performance is its quality factor (Q). Higher Q resonators are more desirable for different applications owing to their enhanced sensitivity and higher isolation from the noisy environment. In the context of this ERC, we develop mechanical resonators with extremely high Q factors suited for different applications. We aim to develop ultrahigh Q factors at room temperature in tensile resonators made of both amorphous and in particular crystalline materials, the latter enabling extremely coherent resonances at low temperatures, higher than any mechanical oscillator demonstrated to date. We will equally explore this direction for superconducting electromechanical systems, and study novel ways to couple superconducting qubits to compact mechanical vibrations on chip – with the aim of replacing the traditional architecture of superconducting computing that is based on microwave readout resonators. Moreover, in the scope of this project we plan to use our expertise and develop technologies for translating microwave signals from dilution refrigerators, as used in superconducting quantum computing, to optical signals that can propagate at room temperature. This technology can in particular overcome the bottleneck associated with excessive heat loads when interrogating thousands of superconducting qubits. Our project will advance mechanical based sensing approaches, and equally lay the foundation for novel ways in which microwave and optical signals are interconverted.
As underlying work towards objective 1, we realized a highly sideband-resolved silicon optomechanical crystal to demonstrate laser sideband cooling to a mean thermal phonon occupancy of 0.92 quanta, which corresponds to 92% ground state probability. Within the scope of the project’s first and second objectives, we have worked towards room temperature quantum optomechanics using ultra coherent hierarchical resonators and strained silicon nanomechanics. We theoretically proposed and experimentally demonstrated an alternative approach, which unprecedentedly allows one to create mechanical resonators with soft-clamped fundamental modes by exploiting hierarchical branching of string demonstrating 780 million quality factor for 107 kHz resonance frequency at room temperature. To explore the possibilities of low mechanical dissipation in different material and temperatures, we have demonstrated strain engineering and soft clamping in strained silicon on insulator (sSOI) platform and have shown experimentally 13 billion quality factor for 1 MHz resonators at 6 K temperatures. This result aligns with out first objective and will open new avenues in nanomechanical sensing.

Towards our efforts in objectives 3 and 4 of developing strain-engineered superconducting mechanical resonators, we developed a novel nano-fabrication process to explore ultracoherent mechanical resonators for superconducting circuits. Within the first year of the project, we purchased and installed a new cryogenic dilution fridge funded by ERC project. This fridge provides environment temperature of 8 mK that results in twice higher mechanical quality factors compared to our old system. We characterized the new devices using this fridge and measured the record of 18 million mechanical quality factor for a superconducting electromechanical system. We successfully performed quantum ground state cooling of the mechanical motion and measured 0.1 quanta occupation of mechanical motion. We established a time-domain experimental setup to conduct pulse sequential measurements on this device and currently are improving the efficiency of this approach.

We developed a new method to optically readout superconducting circuits via light. The cryogenic electro-optical interconnect we purposed was exploiting a commercial optical phase modulator to transduce microwave signals to light at 800 milli-Kelvin temperatures.

Shifting the window of operation to optical frequencies provides the advantages of a wider operations frequency range, low loss and compact transmission in optical fibers. We have analysed and proposed a new kind of device harnessing the strong piezoelectric coupling of microwave signals to a mechanical excitation, a high overtone bulk acoustic resonance (HBAR), parametrically interacting with optical supermodes of optically coupled ring cavities to realize a (quantum) coherent microwave-optical conversion. We also developed an ultra-low loss hybrid lithium niobate - silicon nitride integrated photonic platform. We used wafer bonding to combine low-loss waveguiding of our silicon nitride PICs with electro-optic properties of lithium niobate.
With the progress made so far in the scope of this project, we aim to explore integration of our newly developed ultracoherent mechanical resonators in opto-electromechanical systems. Such high level of coherence will allow us to enter the quantum regime of optomechanics at room temperature – a long standing challenge in the field. This system will allow us to gain quantum control over the mechanical degrees of freedom at room temperature and lead to generation of non-classical light with pondermotive optomechanical squeezing. These outcomes, beyond the state of the art, will lead to enhanced sensing and simplified experimental apparatus to enter the quantum regime of mechanical resonators. Obtaining this goal will simplify experiments and eliminate the excessive cryogenic techniques used in the field and will allow a quantum transducer to work in the ambient environmental conditions (high temperature and pressure). Expanding the ultracoherent mechanics to crystalline materials will unlock one of the highest ever achieved vibrational lifetimes that could lead to extremely high levels of nanomechanical-based force sensitivity. Our electromechanical progress moves towards enhancement of mechanical coherence and will lead to mechanical squeezing and studying the time domain dynamics of mechanical systems in the quantum regime. This will combine the sensitivity benefits of high Q mechanics with the power quantum electrodynamics techniques in superconducting circuits, leading to new physics.

All data and experimental details of our publications are available on public repositories and libraries such as ZENODO (https://zenodo.org/) and arXiv (https://arxiv.org/). In addition, our group has developed and launched a new platform as part of and open science initiative at EPFL for collecting and sharing the details of nanofabrication processes (https://nanofab-net.org/) from tacit knowledge in the field. This comprises a collection of notes that will significantly reduce the time and resources spent in nanofabrication facilities by all researchers world-wide. This initiative upholds a culture of Open Science leading to higher transparency between the researchers and public society as well as increasing the reliability and reproducibility of our research.
Ultra-high Q electro-mechanics