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.Comment: 6 pages, 4 figures. Supplementary information attache

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. DOI: 10.1103/PhysRevX.6.021012 Subject Areas: Photonics, Quantum Physics, SuperfluidityOptical forces are widely utilized in photonic circuits [1,2], micromanipulation [3,4], and biophysics [5,6]. In cavity optomechanics, in particular, optical forces enable cooling and control of microscale mechanical oscillators that can be used for ultrasensitive detection of forces, fields and mass [7][8][9], quantum and classical information systems [10], and fundamental science [11,12]. Recent progress has seen radiation pressure used for coherent state swapping [13], ponderomotive squeezing [14], and ground state cooling [15], while static gradient forces have enabled all-optical routing [16] and nonvolatile mechanical memories [17]. Likewise, photothermal forces, where the mechanical element moves in response to mechanical stress from localized optical absorption and heating, have been used to demonstrate cavity cooling of a semiconductor membrane [18,19], single molecule force spectroscopy [20], and rich chaotic dynamics in suspended mirrors [21].Here, we demonstrate an alternative photoconvective approach to optical forcing that allows an order-ofmagnitude stronger mechanical actuation than radiation pressure. In our implementation, this technique utilizes the convection in superfluids, whereby frictionless fluid flow is generated in response to a local heat source. This well-known superfluid fountain effect [22] is a direct manifestation of the phenomenological two-fluid model proposed by Landau [23] and Tisza [24]. The momentum carried by the helium-4 flow is then transferred to a mechanical element via collision and recoil of superfluid atoms. If the heat source is localized upon the mechanical element, the incident superfluid atoms are either converted to a normal fluid counterflow or evaporated [see Figs. 1(a) and 1(b)]. Alternatively, a distant heat source could be utilized with the mechanical element acting to reroute the superflow.Strong actuation forces are important for a range of techniques in quantum optomechanics, and, most particularly, for protocols that utilize precise measurements combined with feedback act...

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...