Abstract
Paraxial fluids of light represent an alternative platform to atomic Bose-Einstein condensates and superfluid liquids for the study of the quantum behaviour of collective excitations. A key step in this direction is the precise characterization of the Bogoliubov dispersion relation, as recently shown in two experiments. However, the predicted interferences between the phonon excitations that would be a clear signature of the collective superfluid behaviour have not been observed to date. Here, by analytically, numerically, and experimentally exploring the phonon phase-velocity, we observe the presence of interferences between counter-propagating Bogoliubov excitations and demonstrate their critical impact on the measurement of the dispersion relation. These results are evidence of a key signature of light superfluidity and provide a novel characterization tool for quantum simulations with photons.
Highlights
The weakly beyond-mean-field description of a Bose quantum fluid, initially introduced by Bogoliubov, relies on small collective excitations on top of a time-independent condensate [1,2]
We have experimentally demonstrated a previously hidden phenomenon whereby the propagation of plane-wave excitations in the fluid does not tend to the geometric prediction for the displacement, namely, the product of the sound velocity cs by the effective time L, but keeps growing linearly with the excitation wavelength
This is shown to be a direct consequence of the interference between counterpropagating Bogoliubov modes that are generated at an interaction quench and have only been observed in atomic superfluids [36]
Summary
The weakly beyond-mean-field description of a Bose quantum fluid, initially introduced by Bogoliubov, relies on small collective excitations on top of a time-independent condensate [1,2] These excitations are described as noninteracting quasiparticles with a specific energy spectrum: soundlike at low momenta and free-particle-like at large momenta. Experimental implementations rely on the propagation of an intense laser beam within a negative third-order, Kerr nonlinear medium such as photorefractive crystals [10,11], thermo-optic media [12,13], and hot atomic vapors [14,16] In this (2+1)-dimensional geometry, the system is two dimensional (2D) in the transverse direction and the propagation coordinate is analogous to an effective time. We propose that extracting the contrast of constructive interference fringes in the output plane as a function of the probe parameters will give access to the efficiency at which we can excite phonons, known as the static structure factor [22]
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