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Extremely Coherent Mechanical Oscillators and circuit Cavity Electro-Optics

Periodic Reporting for period 2 - ExCOM-cCEO (Extremely Coherent Mechanical Oscillators and circuit Cavity Electro-Optics)

Reporting period: 2021-04-01 to 2022-09-30

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 many 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. 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 oscillators 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 ultracoherent mechanical resonators on chip, with the aim of augmenting the traditional architecture of superconducting computing based on microwave circuits. Specifically, mechanical resonators can act as a coherent quantum memory as they exhibit much longer coherence times than current state of the art superconducting qubits. While superconducting quantum circuits are very successful as quantum processors, quantum information cannot be transmitted over long distances in the microwave domain. We will therefore develop technologies for translating microwave signals from dilution refrigerators to the optical domain, where quantum information can propagate over long distances via optical fibers. This technology can also overcome the technical 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 room temperature quantum optomechanics, we realized a highly sideband-resolved silicon optomechanical crystal to demonstrate laser sideband cooling to a mean thermal phonon occupancy of 0.092 quanta, which corresponds to 92% ground state probability. Within the scope of the project’s first and second objectives, we have realized ultracoherent hierarchical and polygon resonators and strained silicon nanomechanics. We theoretically proposed and experimentally demonstrated an alternative approach to low dissipation mechanical resonators, 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. Our recent work on polygon resonators has further improved the performance, reaching a record quality factor of 3.6 billion at room temperature with devices that are more compact than the state of the art. To explore the possibilities of low mechanical dissipation in a different material, 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.

To advance the state of the art in superconducting electromechanical devices, we developed a novel nanofabrication process that allows us to fabricate 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 two times higher quality factors than in 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. The consistent and reproducible fabrication process we developed allowed us to make multimode circuits of electromechanical systems where we can explore topological physics.

To pursue our goal of transducing quantum information from the microwave domain to the optical domain, we developed a new method to optically readout superconducting circuits via light. The cryogenic electro-optical interconnect exploited a commercial optical phase modulator to transduce microwave signals to light at 800 milliKelvin temperatures. Operation at optical frequencies allows long range transmission of information in optical fibers. To improve the performance of our transducer, 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 (quantum) coherent microwave-optical conversion. We also developed an ultra-low loss hybrid lithium niobate and silicon nitride integrated photonic platform. We used wafer bonding to combine the low-loss waveguiding of our silicon nitride PICs with electro-optic properties of lithium niobate.
We have integrated our newly developed ultracoherent mechanical resonators in optomechanical systems. In particular, the polygon resonators are much more compact than past state of the art mechanical resonators which is highly advantageous for monolithic, chip-scale optomechanical systems. The low dissipation of these mechanical resonators will allow us to achieve quantum control of a mechanical resonator at room temperature – a long standing challenge in the field. In this system, ponderomotive squeezing of light can be generated and schemes for enhanced sensing can be explored. Room temperature operation will simplify experiments and obviate the cryogenic techniques used in the field.

Our progress in electro-optomechanics has allowed us to realize orders of magnitude longer coherence times and we can now study the time domain dynamics of mechanical systems in the quantum regime. This system combines the sensitivity of high-Q mechanics with the power of quantum electrodynamics techniques in superconducting circuits. We are now working towards hybrid systems which couple superconducting qubits to mechanical resonators. The long coherence times of our mechanical system may allow it to serve as a long-lived quantum memory.

The current quantum computing paradigm with superconducting qubits supports only a limited number of qubits due to the constraints of current dilution refrigerator technology. There is therefore a need to transmit quantum information from these circuits. However, quantum information cannot be encoded in the microwave domain at room temperature. One way to circumvent this constraint is to encode the information in the optical domain, which we are aiming to do with our new platforms for microwave-to-optical conversion. Achieving this goal would also enable coupling of superconducting qubits to other quantum systems, which could bring forth a new generation of quantum computers that combine the advantages of competing quantum processing methods.

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 a new platform 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. 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