Substrate material imperfections and surface losses are one of the major factors limiting superconducting quantum circuitry from reaching the scale and complexity required to build a practicable quantum computer. One potential path towards higher coherence of superconducting quantum devices is to explore new substrate materials with a reduced density of imperfections due to inherently different surface chemistries. Here, we examine two ternary metal oxide materials, spinel (MgAl2O4) and lanthanum aluminate (LaAlO3), with a focus on surface and interface characterization and preparation. Devices fabricated on LaAlO3 have quality factors three times higher than earlier devices, which we attribute to a reduction in interfacial disorder. MgAl2O4 is a new material in the realm of superconducting quantum devices and, even in the presence of significant surface disorder, consistently outperforms LaAlO3. Our results highlight the importance of materials exploration, substrate preparation, and characterization to identify materials suitable for high-performance superconducting quantum circuitry.

Brillouin scattering has applications ranging from signal processing1,2, sensing3 and microscopy4 to quantum information5 and fundamental science6,7. Most of these applications rely on the electrostrictive interaction between light and phonons3,7,8. Here we show that in liquids optically induced surface deformations can provide an alternative and far stronger interaction. This allows the demonstration of ultralow-threshold Brillouin lasing and strong phonon-mediated optical coupling. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits9,10, for photonic quantum interfaces11 and to generate synthetic electromagnetic fields12,13. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope14 this provides the prospect of measuring superfluid circulation with unprecedented precision, and exploring the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence15,16. Light-induced deformations in a film of superfluid helium covering an optical microresonator can greatly enhance Brillouin interactions, enabling strong coupling between counter-propagating modes as well as Brillouin lasing.

Two-dimensional superfluidity and quantum turbulence are emergent phenomena native to various condensed matter systems ranging from ultracold quantum gases to superfluid helium. However, to date, there is no complete microscopic model for these phenomena in strongly interacting systems. Therefore, experiments are crucial for understanding of their rich behaviour. Both two-dimensional superfluidity and quantum turbulence are directly connected to the microscopic dynamics of quantized vortices.

Superfluidity is an emergent quantum phenomenon that arises in a number of condensed matter systems at temperatures very close to absolute zero. The first system that was discovered to exhibit superfluidity was cryogenically cooled liquid helium-4 [1, 2]. Below a certain critical temperature helium atoms undergo Bose-Einstein condensation (BEC) into a superfluid state that exhibits strong atom-atom interactions, long-range quantum phase coherence and is described by a macroscopic quantum wave function [3]. The most famous macroscopic manifestation of superfluidity is flow without friction—a phenomenon that closely resembles the resistance-less transport of electrons in superconductors [4].

This books reports the first observation of coherent vortex dynamics, opening the door to a better understanding of two dimensional superfluid helium, and to new quantum technologies based upon it. It also sheds light on the dynamics of astrophysical superfluids, thought to exist in neutron stars

In this chapter we present a paradigm of cavity optomechanics with thin films of superfluid helium. Two-dimensional superfluid films offer a number of desirable properties for both optomechanical protocols and investigation of quantum fluids. These properties include, but are not limited to, ultra-low mechanical dissipation and optical absorption, self-assembling nature of films on an optical cavity, strong coupling of superfluid mechanical excitations to quantized vortices, etc. We describe the mechanism of dispersive optomechanical coupling between an optical field confined within a whispering-gallery-mode resonator and mechanical vibrations of the superfluid film uniformly coating the resonator. The film naturally forms on surfaces due to a combination of ultra-low viscosity and attractive van der Waals forces.

Photons counter-propagating in a silica microcavity exchange excitations at a rate faster than their decay rate, by scattering off superfluid surface oscillations.

Vortices play an essential role in two-dimensional superfluids since they are responsible for the onset of superfluidity in these systems, as described by the theory of Berezinskii-Kosterlitz-Thouless phase transition [1–4], and determine the dynamics of superfluids, giving rise to dissipation [5, 6], quantum turbulence [7], and generation of exotic vortex configurations [8]. Interaction between vortices and sound is of broad significance in Bose-Einstein condensates of dilute gases and superfluid helium [9–13]. However, quantifying and modelling the vortex flow field and its interaction with sound are sophisticated hydrodynamic problems, with analytic solutions available only in special cases [14]. In this work we develop methods to compute both the vortex and sound flow fields within an arbitrary two-dimensional domain. Furthermore, we theoretically investigate the dispersive vortex-sound interaction in two-dimensional superfluids and develop a model which quantifies this interaction for any vortex distribution on any two-dimensional bounded domain, not necessarily simply-connected. To achieve this, we utilize analogies between superfluid and vortex dynamics and respectively fluid dynamics of an ideal gas and electrostatics. Using this technique, we propose an experiment leading to an unambiguous detection of single circulation quanta in two-dimensional superfluid helium.

Vorticity in two-dimensional superfluids is subject to intense research efforts due to its role in quantum turbulence, dissipation and the BKT phase transition. Interaction of sound and vortices is of broad importance in Bose-Einstein condensates and superfluid helium [1-4]. However, both the modelling of the vortex flow field and of its interaction with sound are complicated hydrodynamic problems, with analytic solutions only available in special cases. In this work, we develop methods to compute both the vortex and sound flow fields in an arbitrary two-dimensional domain. Further, we analyse the dispersive interaction of vortices with sound modes in a two-dimensional superfluid and develop a model that quantifies this interaction for any vortex distribution on any two-dimensional bounded domain, possibly non-simply connected, exploiting analogies with fluid dynamics of an ideal gas and electrostatics. As an example application we use this technique to propose an experiment that should be able to unambiguously detect single circulation quanta in a helium thin film.

Quantized vortices are fundamental to the two-dimensional dynamics of superfluids, from quantum turbulence to phase transitions. However, surface effects have prevented direct observations of coherent two-dimensional vortex dynamics in strongly interacting systems. Here, we overcome this challenge by confining a thin film of superfluid helium at microscale on the atomically smooth surface of a silicon chip. An on-chip optical microcavity allows laser initiation of clusters of quasi-two-dimensional vortices and nondestructive observation of their decay in a single shot. Coherent dynamics dominate, with thermal vortex diffusion suppressed by five orders of magnitude. This establishes an on-chip platform with which to study emergent phenomena in strongly interacting superfluids and to develop quantum technologies such as precision inertial sensors.

Using superfluid optomechanical system, here we show both that radiation pressure can greatly deform superfluid film, increasing its local thickness by over a factor of 2, and that this generates new sound modes within the film locally, that are confined by the optical mode and interact strongly with it. This demonstrates a new form of dynamical backaction between the intensity of light within a cavity and the shape of the mechanical eigenmodes that it creates.

In cavity optomechanics, radiation pressure and photothermal forces are widely utilized to cool and control micromechanical motion, with applications ranging from precision sensing and quantum information to fundamental science. Here, we realize an alternative approach to optical forcing based on superfluid flow and evaporation in response to optical heating. We demonstrate optical forcing of the motion of a cryogenic microtoroidal resonator at a level of 1.46 nN, roughly 1 order of magnitude larger than the radiation pressure force. We use this force to feedback cool the motion of a microtoroid mechanical mode to 137 mK. The photoconvective forces we demonstrate here provide a new tool for high bandwidth control of mechanical motion in cryogenic conditions, while the ability to apply forces remotely, combined with the persistence of flow in superfluids, offers the prospect for new applications.

Superfluidity is an emergent quantum phenomenon which arises due to strong interactions between elementary excitations in liquid helium. These excitations have been probed with great success using techniques such as neutron and light scattering. However measurements to-date have been limited, quite generally, to average properties of bulk superfluid or the driven response far out of thermal equilibrium. Here, we use cavity optomechanics to probe the thermodynamics of superfluid excitations in real-time. Furthermore, strong light-matter interactions allow both laser cooling and amplification of the thermal motion. This provides a new tool to understand and control the microscopic behaviour of superfluids, including phonon-phonon interactions, quantised vortices and two-dimensional quantum phenomena such as the Berezinskii-Kosterlitz-Thouless transition. The third sound modes studied here also offer a pathway towards quantum optomechanics with thin superfluid films, including femtogram effective masses, high mechanical quality factors, strong phonon-phonon and phonon-vortex interactions, and self-assembly into complex geometries with sub-nanometre feature size.

Excitations in superfluid helium represent attractive mechanical degrees of freedom for cavity optomechanics schemes. Here we numerically and analytically investigate the properties of optomechanical resonators formed by thin films of superfluid 4He covering micrometer-scale whispering gallery mode cavities. We predict that through proper optimization of the interaction between film and optical field, large optomechanical coupling rates g 0 > 2 π × 100 kHz and single photon cooperativities C 0 > 10 are achievable. Our analytical model reveals the unconventional behaviour of these thin films, such as thicker and heavier films exhibiting smaller effective mass and larger zero point motion. The optomechanical system outlined here provides access to unusual regimes such as g 0 > Ω M and opens the prospect of laser cooling a liquid into its quantum ground state.

Here we present laser based readout and control of the Brownian motion of a thin film of superfluid helium formed around an optical microresonator. Detuned laser driving allows photothermal induced laser cooling and heating, while amplitude modulation probes the nonlinear Duffing interaction.