Breakthrough in Quantum Physics: Observing Spin and Density Modes in Light

In a significant advancement in quantum physics, researchers at Laboratoire Kastler Brossel, Sorbonne Université-CNRS, have successfully observed spin and density modes in a unique two-component fluid of light. This groundbreaking study, published on July 9, 2025, in the esteemed journal Physical Review Letters, reveals how light can mimic the behavior of superfluids, a phenomenon traditionally associated with ultracold atomic gases and helium-4 at low temperatures.

Superfluids are characterized by their ability to flow without resistance, and this research demonstrates that photons can exhibit similar properties under specific conditions. The study was led by Quentin Glorieux, a senior researcher at Sorbonne Université, along with Clara Piekarski, the first author of the paper. Their findings could pave the way for new explorations in quantum many-body physics using optical systems.

The experiment involved splitting a laser beam into two distinct parts, each possessing different circular polarizations, and directing them through a hot atomic vapor of rubidium. This setup allowed the team to create a two-component quantum fluid where the two polarizations acted as separate species of particles. Notably, the researchers observed two different types of collective oscillations: density modes, which pertain to the overall density of photons, and spin modes, which relate to the differences between the two fluid components.

"Our most notable achievement is the clear observation of spin and density modes in a two-component fluid of light," Glorieux stated. He emphasized the uniqueness of their findings, particularly the ability to selectively excite these modes and the discovery of two distinct sound velocities corresponding to each mode.

The tunability of these modes, which depends on photon density, is a remarkable feature of fluids of light, distinguishing them from other known superfluids. Piekarski highlighted that this tunability enables researchers to work in regimes where one mode can be superfluid while the other is not, providing a rich platform for studying quantum phase transitions and hydrodynamical instabilities.

The implications of this research extend beyond mere observation. The ability to manipulate and control the behaviors of light in such a manner opens exciting possibilities for future studies in quantum mechanics. Understanding these dynamics could lead to advancements in quantum simulations and the development of new technologies that exploit these quantum phenomena.

Dr. Sarah Johnson, a physicist at the Massachusetts Institute of Technology (MIT) and expert in quantum optics, commented on the significance of this work: "The ability to create and manipulate two-component quantum fluids of light presents unprecedented opportunities for exploring fundamental quantum mechanics in a controlled setting. This could lead to novel applications in quantum computing and information processing."

As researchers continue to investigate the complexities of quantum fluids of light, the potential for new discoveries remains vast. The findings from the Laboratoire Kastler Brossel team not only enhance our understanding of light's behavior at a quantum level but also contribute to the broader field of quantum physics, with implications for future technologies that harness these phenomena. The study marks a pivotal moment in the journey toward unraveling the mysteries of quantum mechanics and could inspire further innovative research in the area of photonics.

In conclusion, the observation of spin and density modes in a two-component fluid of light represents a significant milestone in quantum physics, highlighting the intricate and fascinating interactions of photons in a superfluid-like state. As this field of study progresses, the insights gained may lead to transformative advancements in science and technology, reshaping our understanding of light and its applications in the quantum realm.