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Experimental Observation of the Drag Force Suppression in a Flow of Superfluid Light

Michel, Claire, Omar Boughdad, Mathias Albert, Pierre-Élie Larré, Matthieu Bellec

Published Oct 10, 2017• 0 citations
OpenAlex

Abstract

We report a direct experimental detection of the frictional-superfluid transition in the flow of a fluid of light past a weakly perturbing localized obstacle in a bulk nonlinear crystal. In our cavityless all-optical system, we extract on the one hand a direct optical analog of the drag force experienced by the obstacle and measure on the other hand the associated obstacle displacement. We observe a superfluid regime characterized by a suppression of long-range radiation from the obstacle, which is, as expected, associated to the cancellation of the drag force and the absence of displacement of the obstacle.

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Journal of Fluid Mechanics•Apr 22, 2026•2 citations
We report a direct experimental detection of the frictional-superfluid transition in the flow of a fluid of light past a weakly perturbing localized obstacle in a bulk nonlinear crystal. In our cavityless all-optical system, we extract on the one hand a direct optical analog of the drag force experienced by the obstacle and measure on the other hand the associated obstacle displacement. We observe a superfluid regime characterized by a suppression of long-range radiation from the obstacle, which is, as expected, associated to the cancellation of the drag force and the absence of displacement of the obstacle.
Quantum fluids of light merge many-body physics and nonlinear optics, revealing quantum hydrodynamic features of light when it propagates in nonlinear media. One of the most outstanding evidence of light behaving as an interacting fluid is its ability to carry itself as a superfluid. Here, we report a direct experimental detection of the transition to superfluidity in the flow of a fluid of light past an obstacle in a bulk nonlinear crystal. In this cavityless all-optical system, we extract a direct optical analog of the drag force exerted by the fluid of light and measure the associated displacement of the obstacle. Both quantities drop to zero in the superfluid regime characterized by a suppression of long-range radiation from the obstacle. The experimental capability to shape both the flow and the potential landscape paves the way for simulation of quantum transport in complex systems.
During this talk, I will present our recent results on the experimental study of the frictional/superfluid transition in a flowing fluid of light in a cavityless configuration, at room temperature. Using a biased photorefractive nonlinear crystal, our approach is to study the light flow around photoinduced obstacles and probe the transition by varying both the light flow velocity and the strength of the interactions. By shaping the pump beam to increase the complexity of the 2D photoinduced potential, we aim at examining its influence on the superfluid behavior of light. Preliminary results in this direction will be presented.
Quantum fluids of light are the photonic counterpart of Bose gases. They currently attract increasing interest since they are versatile and highly tunable systems for probing many-body physics quantum phenomena, such as superfluidity. Superfluid flow of light has already been reported in microcavity exciton-polariton condensates but clear observation of this phenomenon in cavityless systems, that is, for propagating photon fluids, remains elusive. In this thesis, we study the hydrodynamical properties of light propagating close to resonance in hot rubidium vapors. Whereas photons in air are not interacting with each other, the situation is different in rubidium vapors, as an effective interaction between them, mediated by the atomic ensemble, appears. The light behaves therefore as a fluid flowing in the plane perpendicular to the optical axis. The primary purpose of this thesis was to show that light in those systems can behave as a superfluid. The first step toward the observation of superfluidity is to measure the dispersion relation of small amplitude density waves travelling onto the propagating photon fluid. I show that this dispersion exhibits a linear trend for small excitation wave-vectors, which is, according to the Landau criterion, a sufficient condition for guaranteeing superfluidity. I present then an all-optical defect experiment - where the photon fluid flows against an optically induced obstacle (namely, a local change of refractive index) - designed to measure the drag force cancellation at the fluid/superfluid threshold.
The one-dimensional flow of a continuous beam of Bose-Einstein condensed atoms in the presence of an obstacle is studied as a function of the beam velocity and of the type of perturbing potential (representing the interaction of the obstacle with the atoms of the beam). We identify the relevant regimes: stationary/time-dependent and superfluid/dissipative; the absence of drag is used as a criterion for superfluidity. There exists a critical velocity below which the flow is superfluid. For attractive obstacles, we show that this critical velocity can reach the value predicted by Landau's approach. Besides, for penetrable obstacles, it is shown that superfluidity is recovered at large beam velocity. Finally, enormous differences in drag occur when switching from repulsive to attractive potential.
Superfluidity—frictionless flow—has been observed in various physical systems such as liquid helium, cold atoms, and exciton polaritons. Superfluidity is usually realized by cooling and suppressing all dissipation. Here we challenge this paradigm by demonstrating signatures of superfluidity, enabled by dissipation, in the flow of light within a room-temperature oil-filled cavity. Dissipation in the oil mediates effective photon-photon interactions which are noninstantaneous and nonlocal. Such interactions were expected to severely limit the emergence of superfluidity in conservative photonic systems. Surprisingly, when launching a photon fluid with sufficiently high density and low velocity against an obstacle in our driven-dissipative cavity, we observe a record suppression of backscattering. Our experiments also reveal the reorganization dynamics of photons into a nonscattering steady state and a qualitatively changing behavior of the optical phase as light propagates around the obstacle. The phase is locked between the laser and the obstacle but evolves with the intensity in the wake of the obstacle where the density of the photon fluid and its mean-field interaction energy decrease. Using a generalized Gross–Pitaevskii equation for photons coupled to a thermal field, we model our experiments and elucidate how the noninstantaneous and nonlocal character of interactions influences the suppression of scattering associated with superfluidity. Beyond providing the first signatures of cavity photon superfluidity, and of any superfluid both at room temperature and in steady state, our results pave the way for probing photon hydrodynamics in arbitrary potential landscapes using structured mirrors. Published by the American Physical Society 2024
The motion of a body through a superfluid can generate phenomena distinct from a normal viscous fluid. This includes the absence of drag below a critical body speed with the shedding of quantised vortices and non-zero drag above this speed. These phenomena are often modelled using the Gross–Pitaevskii (GP) equation, which describes the wavefunction of a weakly interacting Bose–Einstein condensate and its superfluid dynamics. We study the drag experienced by a penetrable circular disk of radius, aaa , in the form of a two-dimensional potential barrier of height, V0V_0V0​ , that is moving in a superfluid of bulk density, n∞n_\inftyn∞​ , by solving the GP equation. The drag is found to exhibit a unimodal dependence on the disk speed for a fixed value of its barrier height. This behaviour is quantified analytically and confirmed using direct numerical solution. The maximum drag per unit length of Fdragmax≈5an∞V0F_{\textit{drag}}^{\textit{max}} \approx 5 a n_\infty V_0Fdragmax​≈5an∞​V0​ occurs when the barrier height coincides with the relative kinetic energy of the fluid particles. Flow excitations are diminished at a high particle kinetic energy with a commensurate reduction in the drag. This generates a saddle-node bifurcation in the dynamics of a moving disk under an applied force. These results advance understanding of the motion of bodies with finite penetrability, which is relevant to laser experiments probing the superfluidity of Bose–Einstein condensates.