Stimulated Brillouin Scattering based RF to Optical Signal Transduction and Amplification

Project Information

SBS3-5

Grant agreement ID: 758843

DOI

10.3030/758843

Project closed

EC signature date 15 September 2017

Start date 1 January 2018

End date 30 June 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 712 581,00

EU contribution

€ 1 712 581,00

Coordinated by

UNIVERSITY OF BRISTOL
United Kingdom

Periodic Reporting for period 5 - SBS3-5 (Stimulated Brillouin Scattering based RF to Optical Signal Transduction and Amplification)

Reporting period: 2024-01-01 to 2024-06-30

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main results from the project (and their estimated impact):

1) Solving the phonon injection efficiency problem via mode hybridization: As discussed above, the injecting GHz acoustic fields into and out of nanoscale cavities with near-unity efficiency is an open problem. For quantum tranducers, one way to circumvent this bottleneck is using mode hybridisation, which reduces this to a coupled pendulum problem. All leading state of the art devices use some variant of this approach.
2) Re-thinking microwave signal processing using acoustic fields. While acoustic wave devices are common in all modern smart phones and underpin modern wireless communication, all modern acoustic devices manipulate quasi plane waves. By moving to strongly confined geometries (in analogy with integrated photonics), we demonstrated new capabilities (such as record low loss resonators and delay lines approaching 5 us on-chip delays) and the prospect of re-designing the RF signal processing architecture from the ground-up.
3) Shaping surface magnetic fields using sound waves for sensitive spin readout: Piezoelectric devices are traditionally analyzed in the quasistatic regime with the magnetic fields ignored. We showed via scaling arguments and a proof-of-principle experiment that the magnetic field strengths can be significant at GHz frequencies and strongly confined geometries, and provides a natural interface to nanoscale spin systems with exquisite spin detection sensitivity. In principle, single spin readout at cryogenic temperatures is within reach using this approach.
4) Pushing MEMS + photonic ideas to the limit: A key technical advance we made was to demonstrate that high performance cm-scale suspended waveguide devices with sub-um mode confinement can be reliably built in material plaftorms like GaAs where one doesn't have easy access to a high contrast substrate, unlike say silicon on insulator. While these ideas were originally developed with transduction in mind, they have led us to realize state of the art active devices like electro-optic modulators with SWaP metrics that were previously inconceivable.
5) New methods for mapping shapes at the nanoscale (using resistance measurements to work out geometry) and methods to control surface absorption and loss (using XPS to map surface states and combine with atomic layer deposited films to reduce surface loss and preserve surface quality).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This is closely linked to what was listed above, but recapping with metrics in mind:

1) Improvement in phonon injection efficiency using mode hybridization: we demonstrated a 25x improvement experimentally. Variations of this approach have been used in all state of the art transducers.
2) Low loss phononic integrated circuits: we demonstrated acoustic microring resonators with losses at the material limit and spiral delay lines with on-chip delays approaching 5 us. More importantly, this results underpin a way to re-think microwave systems from the ground up and underlie the successful follow-up ERC consolidator grant application.
3) We demonstrated that piezoelectric devices can be used to shape magentic fields at the nanoscale and in theory, provide a route towards single spin readout. These ideas can also be used to improve the spin detection sensitivity of modern electron spin resonance spectrometers.
4) MEMS + photonics: we have demonstrated the largest (as far as we are aware) suspended active PIC devices with the cm-scale GaAs electro-optic modulators.
5) New metrology methods for shape determination at the nanoscale and bringing together techniques to map and limit surface losses.
6) Offshoots: using robots for generating vectorial magnetic fields around spin systems and using system timing constraints to quantify the overheads in photonic quantum processors.

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