Superfluid Brillouin optomechanics

Abstract

Optomechanical systems couple an electromagnetic cavity to a mechanical resonator which is typically formed from a solid object. The range of phenomena accessible to these systems depends on the properties of the mechanical resonator and on the manner in which it couples to the cavity fields. In both respects, a mechanical resonator formed from superfluid liquid helium offers several appealing features: low electromagnetic absorption, high thermal conductivity, vanishing viscosity, well-understood mechanical loss, and in situ alignment with cryogenic cavities. In addition, it offers degrees of freedom that differ qualitatively from those of a solid. Here, we describe an optomechanical system consisting of a miniature optical cavity filled with superfluid helium. The cavity mirrors define optical and mechanical modes with near-perfect overlap, resulting in an optomechanical coupling rate ~ 3 kHz. This coupling is used to drive the superfluid; it is also used to observe the superfluid's thermal motion, resolving a mean phonon number as low as 11.Light confined in a cavity exerts forces on the components that form the cavity. These forces can excite mechanical vibrations in the cavity components, and these vibrations can alter the propagation of light in the cavity. This interplay between electromagnetic (EM) and mechanical degrees of freedom is the basis of cavity optomechanics. It gives rise to a variety of nonlinear phenomena in both the EM and mechanical domains, and provides means for controlling and sensing EM fields and mechanical oscillators. 1 If the optomechanical interaction is approximately unitary, it can provide access to quantum effects in the optical and mechanical degrees of freedom. 1 Optomechanical systems have been used to observe quantum effects which are remarkable in that they are associated with the motion of massive objects. 2,3,4,5,6,7,8,9,10,11,12 They have also been proposed for use in a range of quantum information and sensing applications. 13,14,15,16,17,18,19,20,21,22 Realizing these goals typically requires strong optomechanical coupling, weak EM and mechanical loss, efficient cooling to cryogenic temperatures, and reduced influence from technical noise.To date, nearly all optomechanical devices have used solid objects as mechanical oscillators.However, liquid oscillators offer potential advantages. A liquid can conformally fill a hollow EM cavity, 23 allowing for near-perfect overlap between the cavity's EM modes and the normal modes of the liquid body's vibrations. In addition, the liquid's composition can be changed in situ, an important feature for applications in fluidic sensing. 24 However, most liquids face two important obstacles to operation in or near the quantum regime: their viscosity results in strong mechanical losses, and they solidify when cooled to cryogenic temperatures. Liquid helium is exceptional in both respects, as it does not solidify under its own vapor pressure and possesses zero viscosity in its purely superfluid state. In addition, liquid He has low EM lo...

Theoretical framework for thin film superfluid optomechanics: towards the quantum regime

“…The predicted cooperativity displays a strong dependence on film thickness, reaching large values above unity, with = C 6 0 for d=30 nm. This value -on par with the state-of-the-art in optomechanical systems [2,48]-would represent a significant increase in performance compared to existing superfluid optomechanics systems; it corresponds for instance to an over four orders of magnitude increase over recently demonstrated superfluid helium filled fiber cavities [12]. High cooperativities and large optomechanical coupling rates are essential for such applications as ultra-high precision quantum-limited measurement [1] of the superfluid motion, mechanical ground state cooling [2] and accessing the strong coupling regime [49,50] between light and third sound excitations.…”

“…To date, the majority of superfluid optomechanics schemes have relied on bulk helium, with implementations taking for example the form of a gram-scale resonator coupled to a superconducting microwave resonator [9], a capacitively detected superfluid Helmholtz resonator [11] or a helium-filled fiber cavity [12].…”

High mechanical bandwidth fiber-coupled Fabry-Perot cavity

“…These cavities can achieve very small mode waists, which are advantageous for cavity quantum electrodynamics applications; so far, they have been coupled to atoms [11][12][13], ions [9,14], optomechanical systems [15,16], molecules [17], and crystalline defect centers [18][19][20]. Moreover, the light weight of the fiber mirror suggests that it should be possible to achieve a high bandwidth feedback loop for length stabilization of such a cavity.…”

Cavity optomechanics in a levitated helium drop

“…Liquid-based optomechanical devices can also be realized by filling [21][22][23][24] or coating [25] a solid electromagnetic cavity with a fluid. In this case only the mechanical degree of freedom is provided by the fluid, for example, as a density wave or surface wave that detunes the cavity by modulating the overlap between the liquid and the cavity mode.…”

“…In this case only the mechanical degree of freedom is provided by the fluid, for example, as a density wave or surface wave that detunes the cavity by modulating the overlap between the liquid and the cavity mode. This approach has been used at cryogenic temperatures with superfluid 4 He serving as the liquid [22][23][24][25][26].…”