Laser cooling and control of excitations in superfluid helium

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Superfluidity is a quantum state of matter that exists macroscopically in helium at low temperatures. The elementary excitations in superfluid helium have been probed with great success using techniques such as neutron and light scattering. However, measurements of phonon excitations have so far been limited to average thermodynamic properties or the driven response far out of thermal equilibrium. Here, we use cavity optomechanics to probe the thermodynamics of phonon excitations in real time. Furthermore, strong light–matter interactions allow both laser cooling and amplification. This represents a new tool to observe and control superfluid excitations that may provide insight into phonon–phonon interactions, quantized vortices and two-dimensional 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 the prospect of femtogram masses, high mechanical quality factors, strong phonon–phonon and phonon–vortex interactions, and self-assembly into complex geometries with sub-nanometre feature size.

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Figure 1: Optomechanics with superfluid helium films.
Figure 2: Superfluid helium mechanics.
Figure 3: Real-time measurements of superfluid motion.
Figure 4: Optomechanical heating and cooling of two third sound modes.


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This research was funded by the Australian Centre for Engineered Quantum Systems (CE110001013). Micro-fabrication was performed at the Queensland Node of the Australian National Fabrication Facility (ANFF-Q). W.P.B. was supported by the ARC Future Fellowship FT140100650. The authors thank G. A. Brawley, M. J. Davis, B. J. Powell and Z. Duan for valuable discussions.

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G.I.H. and D.L.M. conducted the experiments and contributed equally to this work. G.I.H., D.L.M. and W.P.B. formulated the theory, analysed the data and wrote the manuscript. Micro-fabrication was performed by D.L.M. and E.S., and C.B. and Y.S. contributed to the experiments. The project was led by W.P.B.

Correspondence to W. P. Bowen.

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Harris, G., McAuslan, D., Sheridan, E. et al. Laser cooling and control of excitations in superfluid helium. Nature Phys 12, 788–793 (2016) doi:10.1038/nphys3714

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