Real-time Optomechanical Detection of Vortex Dynamics in 2D Superfluid Helium
2Department of Physics, University of Otago
Two-dimensional superfluids exhibit rich quantum behaviour such as topological quantum phase transitions. The key role in these phase transitions is played by quantized vortices, as acknowledged by the 2016 Physics Nobel Prize. However, none of the experimental techniques available so far have been capable of probing the microscopic dynamics of strongly-interacting 2D superfluids in real-time and resolving single quantized vortices.
Here we introduce a novel approach to probe the microscopic physics of 2D quantum fluids based on the optomechanical interaction between a nanometer-thick superfluid helium film and an optical Whispering Gallery Mode microcavity [1-3]. This method enables sound waves in the superfluid helium film to be confined to the surface of a microscale optical resonator, where they can interact with quantized vortices (Fig.1 A&B). The small area of confinement enhances the interactions between sound waves, vortices, and light (Fig 1C).
In the presence of the background flow field created by a quantized vortex, the degeneracy between clockwise and counter-clockwise propagating superfluid sound waves is lifted. The presence of quantized vortices manifests therefore as a vortex-position dependent splitting in the sound modes (see Fig1. D), which affects each sound mode in a unique fashion. By tracking this effect on multiple sound modes simultaneously, we can therefore track both the number of vortices, as well as their spatial distribution in real-time.
This capability provides a new tool to explore the microscopic behaviour of 2D strongly interacting quantum fluids. Our results will enable a deeper understanding of 2D quantum phase transitions, dissipation mechanisms in 2D superfluid helium systems, quantum turbulence and the realization of optomechanics with quantized vortices.
[1] Harris, G. I., D. L. McAuslan, E. Sheridan, Y. Sachkou, C. Baker, and W. P. Bowen. “Laser Cooling and Control of Excitations in Superfluid Helium.” Nature Physics 12, 788–93, 2016.
[2] Baker, Christopher G., Glen I. Harris, David L. McAuslan, Yauhen Sachkou, Xin He, and Warwick P. Bowen. “Theoretical Framework for Thin Film Superfluid Optomechanics: Towards the Quantum Regime.” New Journal of Physics 18, 12, 123025, 2016.
[3] McAuslan, D. L., G. I. Harris, C. Baker, Y. Sachkou, X. He, E. Sheridan, and W. P. Bowen. “Microphotonic Forces from Superfluid Flow.” Physical Review X 6, 2, 021012, 2016.
[4] Y. Sachkou, C. Baker, G. Harris et al. Observation of a negative-temperature vortex state in a strongly interacting two-dimensional superfluid, in preparation, 2018.
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