Our main goal in this project was to engineer efficient interactions between light and sound in chipscale platforms with a primary objective of enabling efficient bi-directional microwave to optical signal frequency conversion down to the single photon level. The acoustic wave here serves as an intermediary which helps bridge the disparity between the microwave (cm) and optical wavelengths (um) and enables efficient interactions. Such microwave to optical transducers are critical for interconnecting remote superconducting qubit based processors using fiber optic networks. It is envisioned that any large scale useful error-corrected quantum computer will be a networked entity and therefore, such quantum transducers are critical to future quantum computing and networking efforts. During the timeframe of this project, the importance of such devices has only grown to the point where they now appear on company product roadmaps (cf. IBM). While we have not achieved the original goal of quantum transduction as envisioned, working on this problem has led us to some important conclusions:
1) The real challenge with building efficient microwave to optical transducers is with phonon injection efficiency, i.e. can you design structures that can efficiently focus GHz acoustic fields into and out of nanoscale cavities with near-unity efficiency? Our proposal of using mechanical mode hybridisation (supermodes) is one approach to solve this problem, but the questions remains open.
2) Efficient transduction usually relies on a hybrid material platform wherein you have a material with strong piezoelectric coefficient (like lithium niobate and aluminum nitride) for microwave to acoustics conversion and a material with low loss and high acousto-optic interaction strength (like silicon and gallium arsenide). Sticking with a single material (like GaAs) results in an upper bound on conversion efficiency but a hybrid material platform significantly increases the fabrication complexity. Building transducers at scale will need a better understanding of the efficiency = complexity tradeoff.
3) Nanoscale metrology is critical for progress: as we push the interaction strengths higher in nanoscale devices, surfaces losses and shape induced imperfections in resonant structures become critical. Methods to quantify surfaces (beyond the tools available from microelectronics) and 3D shape mapping are integral to progress. We have taken the first steps in this direction in this project, but more work needs to be done here.
4) While the quantum transduction problem is what initially drove this project and is still of long-term interest, working on manipulating acoustic fields in waveguide geometries made us realize that we could rethink standard RF architectures from the ground-up with potential near term impact. This comes because ultimately one is manipulating microwave fields in the chip with the acoustics working primarily as an intermediary.
5) Merging ideas between MEMS and photonics can be very fruitful in the long run and have an impact outside of optomechanics. One of the key realizations we had in the project was that outside of silicon on insulator, getting access to x on insulator substrates where x is any interesting material of interest (III-Vs mainly for us) is prohibitively expensive and the quality is not assured. On the other hand, by properly incorporating MEMS based ideas and working with suspended waveguides at scale (cm), devices with arbitrary complexity and high performance can be built. In principle, this idea is not surprising. What we count as our real achievement was to demonstrate them working in practice.
