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Recent physics studies have found that light can sometimes flow in unexpected ways, behaving like a so-called "superfluid." Superfluids, such as ultracold atomic gases or helium-4 below specific temperatures, are phases of matter characterized by flowing behavior with zero viscosity (i.e., with no resistance).

Researchers at Laboratoire Kastler Brossel, Sorbonne Université -CNRS recently demonstrated that photons in a two-component fluid of light can exhibit both spin and density modes, which are signatures of mixtures of quantum superfluids. Their paper, in Â鶹ÒùÔºical Review Letters, could open new and exciting possibilities for the simulation and investigation of quantum many-body physics using optical systems.

"We study quantum fluids of light, or in other words, optical systems where light behaves like a superfluid, which are much like Bose-Einstein condensates or superconductors," Quentin Glorieux, senior author of the paper, told Â鶹ÒùÔº.

"The goal of this study was to see whether it's possible to push this analogy further by creating a mixture of two interacting fluids of light. Mixtures are very interesting since they support rich collective dynamics and offer a new platform for the study of quantum phase transitions, topological structures or even analog gravity."

To confirm that a system is a two-component quantum fluid, physicists need to demonstrate that it hosts two different types of collective oscillations. The first of these are oscillations in the total density of photons (i.e., density mode), while the second relates to the difference between its two underlying components (i.e., spin mode).

"This is exactly what we observed in our experiment," explained Clara Piekarski, first author of the paper. "For us, the equivalent of the quantum fluid wavefunction is the electric field envelope of a laser beam propagating through a hot atomic vapor of rubidium. Within this nonlinear medium, photons effectively start to interact with each other. The two fluid components correspond to the two circular polarizations of light which act as distinct 'species' of particles in the mixture."

Essentially, Glorieux, Piekarski and their colleagues split a laser beam into two parts, each of which had a different circular polarization. They sent these two polarizations of light through a hot vapor of rubidium atoms, where they behave as two interacting boson gases.

The researchers found that when inside the gas, the light started behaving like a superfluid (i.e., started flowing with no resistance). They can produce two types of excitations that propagate without dissipation, by perturbing the fluid-like optical system with a weaker light beam of controlled polarization and orientation.

"Our most notable achievement is the clear observation of spin and density modes in a two-component fluid of light," said Glorieux. "We showed that these modes can be selectively excited, and we observed two distinct speeds of sound, one for each mode. This is the first experimental realization of a binary superfluid made of photons. This clearly opens widely the field of quantum fluids of light."

In addition to observing spin and density modes of the two-component fluid of light, the researchers showed that the relative speeds of these two modes could be controlled by tuning the photon density. This tunability, which is enabled by the saturation of the atomic vapor, is unique to fluids of light and was found to be inaccessible in other known superfluids.

"Our results open many possibilities," added Piekarski. "We can now work in a regime where only the spin is superfluid and not the density, or the opposite. At the moment, we are studying the non-miscible regime, where the spin mode is unstable and the two components separate to occupy different regions of space. We could then study the quantum version of hydrodynamical instabilities, for instance, by making the two components collide."

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