Periodic Reporting for period 2 - uNIQUE (NanophononIcs for QUantum information procEssing)
Reporting period: 2022-03-01 to 2023-08-31
Over the past thirty years, the remarkable technological advances in micro- and nano-fabrication processes have thrust mechanical vibrations into the quantum realm. Mechanical motion can indeed couple coherently to a variety of physical degrees of freedom, therefore holding promises of scalable hybrid quantum platforms. But mechanical vibrations are also powerful conveyors of physical information. They are ubiquitously used in wireless communication systems, where bulk and surface acoustic wave (BAW and SAW) devices are prevalent. Their high achievable quality factors and frequencies, as well as their low propagation speed, are indeed appropriate ingredients for information processing: they are synonymous of storage and delay. Recent works have shown that SAW could be operated in the single-phonon regime, potentially behaving as a quantum bus between solid-state qubits. The proposed approaches, however, do not yet take advantage of wave propagation management at the substrate surface itself.
The uNIQUE project aims at the development of an all-electro-acousto-mechanical quantum information platform exploiting the potential offered by guided acoustic waves in the single-phonon regime. It adopts a yet unexplored approach at the crossing of phononics, nanomechanics and quantum acoustics to yield an integrated device that could be used at the interface with other solid-state or photon qubits or as an independent quantum signal processing system. It will exploit the substrate surface to prepare and transfer non-classical states of motion of surface-coupled phononic resonators with the utmost ambition to encode the state information in a travelling single-phonon, allowing remote entanglement. This platform will allow manipulating quantum states in exceedingly compact systems driven by a radio-frequency signal. It will also contribute to the emergence of hybrid quantum platforms by providing a means to provide signal transduction between otherwise incompatible systems.
The uNIQUE project aims at the development of an all-electro-acousto-mechanical quantum information platform exploiting the potential offered by guided acoustic waves in the single-phonon regime. It adopts a yet unexplored approach at the crossing of phononics, nanomechanics and quantum acoustics to yield an integrated device that could be used at the interface with other solid-state or photon qubits or as an independent quantum signal processing system. It will exploit the substrate surface to prepare and transfer non-classical states of motion of surface-coupled phononic resonators with the utmost ambition to encode the state information in a travelling single-phonon, allowing remote entanglement. This platform will allow manipulating quantum states in exceedingly compact systems driven by a radio-frequency signal. It will also contribute to the emergence of hybrid quantum platforms by providing a means to provide signal transduction between otherwise incompatible systems.
This firs part of the project has been dedicated to two main actions.
The first one pertains to the acquisition and installation of a dilution refrigerator. Preliminary measurements of surface acoustic wave (SAW) devices (quartz and lithium niobate one and two-port resonators operating at various frequencies) have been performed and allowed us to get a clearer view of the first instrumental challenges to address. Investigation of the supraconductivity of the materials used for the fabrication of the SAW devices have also been conducted.
In parallel, we have been working on different strategies for surface acoustic wave propagation control, first at room temperature and normal pressure, to start identifying the most relevant phononic devices to be pushed and used in the remaining of the project. We worked in particular on the design, fabrication and characterisation of phononic structures for SAW waveguiding and for sub-wavelength elastic energy confinement. In particular, we demonstrated the feasibility of a SAW wave guiding and splitting in a phononic waveguide constructed by managing line defects in a square-lattice phononic crystal made pillars deposited atop a piezoelectric substrate. We showed that the guiding effect could be achieved through the optimisation of pillar geometry, mode selection, and channel design. We also showed the possibility to confine elastic energy by exploiting interactions between coupled pillar resonators and surface acoustic waves. Sub-wavelength scale confinement, accompanied by a six-fold field enhancement were hence obtained.
Eventually, we also demonstrated the existence of particular elastic nonlinearities triggered by the interaction of surface acoustic waves with micron-scale phononic resonators. These nonlinearities are characterised by a frequency softening of the pillar pair response for short gap distances, while isolated resonators retain a linear behavior. This points at a mechanism lying outside the usual geometric Duffing nonlinearities linked to the resonator’s geometry. A preliminary analytical model was therefore developed and shown to describe the experiment well by introducing a nonlinear coupling term in the Duffing equation. The nonlinear coupling strength is conditioned by the SAW amplitude, but also by the SAW wave vector, that sets the resonator mode symmetries. The oscillation of the resonator itself was shown to lead to a mechanical back-action towards the substrate surface, that in turns allowed for the occurrence of circular-polarisation states,hence constituting promising results towards the implementation of polarisation-based signal processing functionalities. An experimental method aimed at characterising the nature of these nonlinearities is currently being developed. We also observed nonlinear effects in a surface phononic crystal resonator made of periodical high-aspect-ratio nanostrips fabricated atop a lithium niobate substrate and applied these effects to demonstrate the possibility to implement tunable phononic resonators in the GHz freiqncy range.
The first one pertains to the acquisition and installation of a dilution refrigerator. Preliminary measurements of surface acoustic wave (SAW) devices (quartz and lithium niobate one and two-port resonators operating at various frequencies) have been performed and allowed us to get a clearer view of the first instrumental challenges to address. Investigation of the supraconductivity of the materials used for the fabrication of the SAW devices have also been conducted.
In parallel, we have been working on different strategies for surface acoustic wave propagation control, first at room temperature and normal pressure, to start identifying the most relevant phononic devices to be pushed and used in the remaining of the project. We worked in particular on the design, fabrication and characterisation of phononic structures for SAW waveguiding and for sub-wavelength elastic energy confinement. In particular, we demonstrated the feasibility of a SAW wave guiding and splitting in a phononic waveguide constructed by managing line defects in a square-lattice phononic crystal made pillars deposited atop a piezoelectric substrate. We showed that the guiding effect could be achieved through the optimisation of pillar geometry, mode selection, and channel design. We also showed the possibility to confine elastic energy by exploiting interactions between coupled pillar resonators and surface acoustic waves. Sub-wavelength scale confinement, accompanied by a six-fold field enhancement were hence obtained.
Eventually, we also demonstrated the existence of particular elastic nonlinearities triggered by the interaction of surface acoustic waves with micron-scale phononic resonators. These nonlinearities are characterised by a frequency softening of the pillar pair response for short gap distances, while isolated resonators retain a linear behavior. This points at a mechanism lying outside the usual geometric Duffing nonlinearities linked to the resonator’s geometry. A preliminary analytical model was therefore developed and shown to describe the experiment well by introducing a nonlinear coupling term in the Duffing equation. The nonlinear coupling strength is conditioned by the SAW amplitude, but also by the SAW wave vector, that sets the resonator mode symmetries. The oscillation of the resonator itself was shown to lead to a mechanical back-action towards the substrate surface, that in turns allowed for the occurrence of circular-polarisation states,hence constituting promising results towards the implementation of polarisation-based signal processing functionalities. An experimental method aimed at characterising the nature of these nonlinearities is currently being developed. We also observed nonlinear effects in a surface phononic crystal resonator made of periodical high-aspect-ratio nanostrips fabricated atop a lithium niobate substrate and applied these effects to demonstrate the possibility to implement tunable phononic resonators in the GHz freiqncy range.
The demonstration of SAW beamsplitting and sub-wavelength confinement as well as the report of acousto-mechanical nonlinearities in scalable, integrated devices operating in the 100-MHz range constitute first steps towards the obtaining of high-frequency multi-functional linear and nonlinear electromechanical devices. The observation of complex polarisation states that vary upon SAW excitation also constitute encouraging results in this perspective. We will then aim at implementing models for designing linear and modal interactions to implement signal processing functionalities (parametric interactions, frequency combs, logic gates... ). We will implement hybrid architectures based on low-number or collective arrangements of resonators to increase mechanical vibration control at the micron-scale. The first experiments will be performed at room temperature and normal pressure.
If we indeed intend to transpose these devices to ultra-low temperature operations, we will first focus on implementing more conventional, yet carefully-designed,SAW device architectures and operating them below 100 mK.The characterisation of these SAW devices will provide information on the acoustic loss mechanisms involved as well as providing sources for mechanical resonator excitation and control. We will also aim at the development of strategies for integrated phonon detection. The extreme level of integration of such devices, associated with the intrinsically low propagation speed (five order of magnitude lower than the speed of electromagnetic waves) and high coherence of phonons opens unexplored perspectives for the development of quantum information processing schemes or for the investigation of quantum effects at a chip-scale.
If we indeed intend to transpose these devices to ultra-low temperature operations, we will first focus on implementing more conventional, yet carefully-designed,SAW device architectures and operating them below 100 mK.The characterisation of these SAW devices will provide information on the acoustic loss mechanisms involved as well as providing sources for mechanical resonator excitation and control. We will also aim at the development of strategies for integrated phonon detection. The extreme level of integration of such devices, associated with the intrinsically low propagation speed (five order of magnitude lower than the speed of electromagnetic waves) and high coherence of phonons opens unexplored perspectives for the development of quantum information processing schemes or for the investigation of quantum effects at a chip-scale.