A cavity optomechanical magnetometer is demonstrated. The magnetic field induced expansion of a magnetostrictive material is resonantly transduced onto the physical structure of a highly compliant optical microresonator, and read-out optically with ultra-high sensitivity. A peak magnetic field sensitivity of 400 nT Hz −1/2 is achieved, with theoretical modeling predicting the possibility of sensitivities below 1 pT Hz −1/2 . This chipbased magnetometer combines high-sensitivity and large dynamic range with small size and room temperature operation.Ultra-low field magnetometers are essential components for a wide range of practical applications including geology, mineral exploration, archaeology, defence and medicine [1]. The field is dominated by superconducting quantum interference devices (SQUIDs) operating at cryogenic temperatures [2]. Magnetometers capable of room temperature operation offer significant advantages both in terms of operational costs and range of applications. The state-of-the-art are magnetostrictive magnetometers with sensitivities in the range of fT Hz −1/2 [3, 4], and atomic magnetometers which achieve impressive sensitivities as low as 160 aT Hz −1/2 [5] but with limited dynamic range due to the nonlinear Zeeman effect [2,6]. Recently, significant effort has been made to miniaturize room temperature magnetometers. However both atomic and magnetostrictive magnetometers remain generally limited to millimeter or centimeter size scales. Smaller microscale magnetometers have many potential applications in biology, medicine, and condensed matter physics [7,8]. A particularly important application is magnetic resonance imaging, where by placing the magnetometer in close proximity to the sample both sensitivity and resolution may be enhanced [9], potentially enabling detection of nuclear spin noise [10], imaging of neural networks [7], and advances in areas of medicine such as magneto-cardiography[1, 6] and magneto-encephalography [11].In the past few years, rapid progress has been achieved on NV center based magnetometers. They combine sensitivities as low as 4 nT Hz −1/2 with room temperature operation, optical readout and nanoscale size [12] and are predicted theoretically to reach the fT Hz −1/2 range [13]. This has allowed three-dimensional magnetic field imaging at the micro scale using ensembles of NV-centers [7], and magnetic resonance [14] and field imaging[13] at the nanoscale using single NV centers. In spite of these extraordinary achievements applications are hampered by fabrication issues and the intricacy of the read-out schemes [15]. Furthermore miniaturization is limitied by the bulky read-out optics, the magnetic field coils for state preparation and the microwave excitation device [7].In this letter we present the concept of a cavity optomechanical field sensor which combines room temperature operation and high sensitivity with large dynamic range and small size. The sensor leverages results from the emergent field of cavity optomechanics where ultra-sensitive force and positi...

A cavity optomechanical magneto-meter operating in the 100 pT range is reported. The device operates at earth field, achieves tens of megahertz bandwidth with 60 μm spatial resolution and microwatt optical-power requirements. These unique capabilities may have a broad range of applications including cryogen-free and microfluidic magnetic resonance imaging (MRI), and investigation of spin-physics in condensed matter systems.

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Ultrasound sensors have wide applications across science and technology. However, improved sensitivity is required for both miniaturisation and increased spatial resolution. Here, we introduce cavity optomechanical ultrasound sensing, where dual optical and mechanical resonances enhance the ultrasound signal. We achieve noise equivalent pressures of 8–300 μPa Hz−1/2 at kilohertz to megahertz frequencies in a microscale silicon-chip-based sensor with >120 dB dynamic range. The sensitivity far exceeds similar sensors that use an optical resonance alone and, normalised to the sensing area, surpasses previous air-coupled ultrasound sensors by several orders of magnitude. The noise floor is dominated by collisions from molecules in the gas within which the acoustic wave propagates. This approach to acoustic sensing could find applications ranging from biomedical diagnostics, to autonomous navigation, trace gas sensing, and scientific exploration of the metabolism-induced-vibrations of single cells.

Two-dimensional superfluidity and quantum turbulence are directly connected to the microscopic dynamics of quantized vortices. However, surface effects have prevented direct observations of coherent vortex dynamics in strongly-interacting two-dimensional systems. Here, we overcome this challenge by confining a twodimensional droplet of superfluid helium at microscale on the atomically-smooth surface of a silicon chip. An on-chip optical microcavity allows laser-initiation of vortex clusters and nondestructive observation of their decay in a single shot. Coherent dynamics dominate, with thermal vortex diffusion suppressed by six orders-of-magnitude. This establishes a new on-chip platform to study emergent phenomena in strongly-interacting superfluids, test astrophysical dynamics such as those in the superfluid core of neutron stars in the laboratory, and construct quantum technologies such as precision inertial sensors. arXiv:1902.04409v1 [cond-mat.other] 7 Feb 2019Strongly-interacting many-body quantum systems exhibit rich behaviours of significance to areas ranging from superconductivity [1] to quantum computation [2, 3], astrophysics [4][5][6], and even string theory [7]. The first example of such a behaviour, superfluidity, was discovered more than eighty years ago in cryogenically cooled liquid helium-4 [8]. Quite remarkably, it was found to persist even in thin two-dimensional films [9], for which the well-known Mermin-Wagner theorem precludes condensation into a superfluid phase in the thermodynamic limit [10]. This apparent contradiction was resolved by Berezinskii, Kosterlitz and Thouless (BKT), who predicted that quantized vortices allow a topological phase transition into superfluidity [11,12]. It is now recognized that quantized vortices also dominate much of the out-of-equilibrium dynamics of two-dimensional superfluids, such as quantum turbulence [13].Recently, laser control and imaging of vortices in ultracold gases [14, 15] and semiconductor exciton-polariton systems [16,17] has provided rich capabilities to study superfluid dynamics [18] including, for example, the formation of collective vortex dipoles with negative temperature and large-scale order [19, 20] as predicted by Lars Onsager seventy years ago [21]. However, these experiments are generally limited to the regime of weak interactions, where the Gross-Pitaevskii equation provides a microscopic model of the dynamics of the superfluid. The regime of strong interactions can be reached by tuning the atomic scattering length in ultracold gases [22, 23]. However, technical challenges have limited investigations of nonequilibrium phenomena [22]. The strongly-interacting regime defies a microscopic theoretical treatment and is the relevant regime for superfluid helium as well as for astrophysical superfluid phenomena such as pulsar glitches [24] and superfluidity of the quark-gluon plasma in the early universe [6]. The vortex dynamics in this regime are typically predicted using phenomenological vortex models. However, whether the vortices...

Brillouin scattering has applications ranging from signal processing [1, 2], sensing [3] and microscopy [4], to quantum information [5] and fundamental science [6, 7]. Most of these applications rely on the electrostrictive interaction between light and phonons [3, 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 for the first time. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits [9, 10], for photonic quantum interfaces [11], and to generate synthetic electromagnetic fields [12, 13]. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope [14] this provides the prospect of measuring superfluid circulation with unprecedented precision, and to explore the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence [15, 16].Brillouin scattering is an optomechanical process that couples two optical waves via their interaction with travelling acoustic phonons. In the electrostrictive interaction usually employed, the optical electric field induces strain in a bulk medium, and the generated phonons scatter light between the two optical waves via refractive index changes caused by the medium's photoelasticity. However, the inherent weakness of this interaction presents a significant challenge [3], necessitating the use of high optical powers and prohibiting some applications. This can be alleviated by resonant enhancement in an optical cavity, which has allowed recent demonstrations of ultralow linewidth lasers [17, 18], Brillouin lasing in liquid droplets [8], non-reciprocal optical transport [19], Brillouin gyroscopes [14] and low-noise microwave oscillators [2]. Alternatively, optically-induced deformations of the boundary of the medium can be leveraged to provide a Brillouin interaction, with scattering induced by the effective refractive index-modulation caused by the deformation. In purpose-engineered solid structures these surface interactions can be made comparable to, or even exceed, the native electrostriction [20][21][22].Here, we transfer the concept of deformation-induced Brillouin scattering to liquid media, specifically a fewnanometer-thick superfluid helium film that coats the surface of a silica microdisk cavity and couples to its whispering gallery modes (see Fig. 1a,b) via perturbation of their evanescent field. Similar to other liquids [23], the superfluid film has an exceedingly weak restoring force, affording a compliant dielectric interface that easily deforms in the presence of optical forces [24, 25], as illustrated in Fig. 1c. This offers the potential for very large surface deformations and consequently extreme interaction strengths. We show that it allows radiation-pressure interactions with acoustic phonons that are over two orders of magnitude ...

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. 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.

The dual-resonant enhancement of mechanical and optical response in cavity optomechanical magnetometers enables precision sensing of magnetic fields. In previous working prototypes of such magnetometers, a cavity optomechanical system is functionalized by manually epoxy-bonding a grain of magnetostrictive material. While this approach allows proof-of-principle demonstrations, practical applications require more scalable and reproducible fabrication pathways. In this work, we developed a multiple-step method to scalably fabricate optomechanical magnetometers on a silicon chip, with reproducible performance across different devices. The key step is to develop a process to sputter coat a magnetostrictive film onto high quality toroidal microresonators, without degradation of the optical quality factor. A peak sensitivity of 585 pT/Hz is achieved, which is comparable with previously reported results using epoxy-bonding. Furthermore, we demonstrate that thermally annealing the sputtered film can improve the magnetometer sensitivity by a factor of 6.3.