Glen I. Harris

Despite extensive experimental efforts over the past two decades, topological superconductivity and Majorana fermions have remained elusive. By shifting focus from superconductors—whose topological nature is often debated—to the well-established p -wave superfluid He 3 , we propose an experimental approach designed to identify quantum Hall nature of the chiral superfluid He 3 − A phase. The method exploits the interaction between surface waves and the induced current transverse to varying density of the superfluid. It allows one to quantitatively measure this Hall current and thus to directly evaluate whether the Hall current is quantized or not. As the quantum Hall effect is a hallmark of topological condensed matter, such an experiment would offer direct evidence of nontrivial topology of superfluid He 3 . With the advent of microscopic superfluid resonators coupled to optical cavities, this proposal is both realistic and promising, potentially opening the door to future experiments ranging from topological acoustics to fault-tolerant quantum computing.

Optomechanical sensors enable exquisitely sensitive force measurements, with emerging applications across quantum technologies, standards, fundamental science, and engineering. Magnetometry is among the most promising applications, where chip-scale optomechanical sensors offer high sensitivity without the cryogenics or magnetic shielding required by competing technologies. However, a lack of compatibility with integrated photonics and electronics has posed a major barrier. Here, we introduce silicon-on-insulator optomechanical magnetometers to address this barrier. A new post-release lithography process enables high-quality metallization of released mechanical structures, overcoming the incompatibility between silicon-on-insulator fabrication and functional magnetic films. This allows us to employ photonic-crystal cavities that enhance motion-to-optical signal transduction by over an order of magnitude. The resulting devices achieve magnetic field sensitivity of 800pT/Hz, three orders of magnitude beyond previous waveguide-integrated designs. The advance report we provide, leads to a path toward high-performance, room temperature, and chip-integrated magnetometers for applications ranging from biomedical imaging and navigation to resource exploration.

Phononic circuits constructed from high tensile stress membranes offer a range of desirable features such as high acoustic confinement, controllable nonlinearities, low mass, compact footprint, and ease of fabrication. This Tutorial presents a systematic approach to modelling and designing phononic integrated circuits on this platform, beginning with acoustic confinement, wave propagation and dispersion, mechanical and actuation nonlinearities, as well as resonator dynamics. By adapting coupled mode theory from optoelectronics to suspended membranes and validating this theory with several numerical techniques (finite element modelling, finite difference time domain simulations, and the transfer matrix method), we provide a comprehensive framework to engineer a broad variety of phononic circuit building blocks. As illustrative examples, we describe the implementation of several acoustic circuit elements including resonant and non-resonant variable-ratio power splitters, mode converters, mode (de)multiplexers, and in-line Fabry–Pérot cavities based on evanescent tunnel barriers. These building blocks lay the foundation for phononic integrated circuits with applications in sensing, acoustic signal processing, and power-efficient and radiation-hard computing.

Shallow-water waves are a notable example of nonlinear hydrodynamics, giving rise to phenomena such as tsunamis and undular waves. These dynamics are typically studied in hundreds-of-meters-long wave flumes. In this work, we demonstrate a chip-scale wave flume, which exploits nanometer-thick superfluid helium films and optomechanical interactions to achieve nonlinearities surpassing those of extreme terrestrial flows. Measurements reveal wave steepening, shock fronts, and solitary wave fission—nonlinear behaviors predicted in superfluid helium but never directly observed. Our approach enables lithography-defined wave flume geometries, optomechanical control of hydrodynamic properties, and orders-of-magnitude faster measurements than terrestrial flumes. This approach combining quantum fluids and nanophotonics provides a platform to explore complex wave dynamics at the microscale.

Many applications, including industrial processes, sonar, and navigation, rely on the detection of acoustic waves. Photonic hydrophones demonstrate comparable sensitivity to piezoelectric-based hydrophones but with significantly reduced size, weight, and power requirements. In this paper, we demonstrate a micron-sized free-standing silicon photonic hydrophone. We demonstrate sensitivity on the order of ?mPa/Hz from 10-200 kHz, with a minimum detectable pressure of 145 Pa/Hz at 22 kHz. We also deployed our hydrophone in a wave flume to evaluate its suitability for underwater measurement and communication. Our hydrophone matches the sensitivity of commercial hydrophones but is many orders of magnitude smaller in volume, which could enable high spatial resolution imaging of micron-sized acoustic features (i.e., living cell vibrations). Our hydrophone could also be used in underwater communication and imaging applications.

Despite extensive experimental efforts over the past two decades, the quest for Majorana fermions in superconductors remains inconclusive. We propose an experimental method that can conclusively confirm, or rule out, the existence of these quasiparticles: Firstly, we shift focus from superconductors, whose very topological nature is disputed, to the unambiguous topological superfluid helium-3. Secondly, we identify the interaction between surface waves and the chiral Majorana current in the bulk of a topological superfluid of varying density. The proposed experiment provides a path towards the detection of the Majorana fermion, an 80-year-old theoretical prediction. It is realistically achievable based on the advent of microscopic superfluid resonators coupled to optical cavities. The proposal may open the door to experiments ranging from simulations of exotic cosmological particles to topological acoustics and fault tolerant quantum computing.

Integrated acoustic circuits leverage guided acoustic waves for applications ranging from radio frequency filters to quantum state transfer, biochemical sensing, and nanomechanical computing. In many applications, it is desirable to have a method for unidirectional acoustic wave emission. In this work, we demonstrate directional emission in an integrated single-mode, on-chip membrane waveguide, demonstrating over 99.9% directional suppression and reconfigurable directionality. This avoids both loss and unwanted crosstalk, allowing the creation of more complex and compact phononic circuits.