A recent study led by Monash University has revealed that light behaves like a liquid. Physicists predicted self-bound “quantum droplets” made of light, an entirely new state of matter within semiconductor microcavities. While droplets and light belong to separate realms in daily life, researchers have shown that light organizes itself into a behavior resembling that of a tiny liquid within a microcavity only billionths of a meter thick.1,2
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Classical-Quantum Droplet Physics
Classical liquids emerge from the balance between a dipole-dipole attraction induced by long range and a short-range repulsion/two competing interatomic forces, and can form dense droplets that are held together by surface tension.2
In the quantum realm, similar configurations appear as dilute nanodroplets of liquid helium. Recently, the existence of a unique, self-bound, ultradilute quantum droplet in ultracold mixtures of bosonic atoms with tunable interactions was predicted.2
Later, researchers experimentally realized quantum droplets in dipolar condensates and in both heteronuclear and homonuclear binary mixtures. Although quantum droplets have become a highly active research field, the focus has primarily been on ultracold atomic gases.2
Quantum Droplets of Light
In a paper recently published in Physical Review Letters, researchers predicted the presence of self-bound quantum droplets within a solid-state system involving matter and light.2
In particular, researchers considered exciton polaritons, as they exhibit exceptional collective coherent phenomena such as quantum vortices, superfluidity, and Bose-Einstein condensation owing to their small effective mass and bosonic nature.2
Exciton-polaritons are hybrid light-matter quasiparticles arising from the robust coupling between photons and excitons (electron-hole bound states) in a two-dimensional (2D) semiconductor microcavity. Most investigations have been performed in the semiclassical regime in the past two decades.2
In this regime, a mean-field (MF) theory of nonlinear classical waves can accurately account for the collective effects. Yet, efforts have been growing recently to access the regime of strongly correlated polaritons. These are crucial for different quantum photonic applications in scalable semiconductor systems.2
In this direction, common proposals involve effectively confining the cavity photon mode or improving polariton-polariton repulsion by exploiting Rydberg polaritons or dipolaritons.2
Researchers in this work proposed improving quantum fluctuations by effectively weakening the MF effects. The proposal involves tuning the interactions between polaritons, similar to ultracold atomic gases.2
Particularly, they exploited the polariton Feshbach resonance, which emerges when the energy of two opposite-spin polaritons equals the energy of a biexciton bound state. This results in an effective improvement in interactions and an abrupt change in their sign, allowing researchers to access a regime in which quantum fluctuations dominate.2
Recently, the Feshbach resonance has been observed in transition metal dichalcogenide (TMD) monolayers and quantum well semiconductors. A spin mixture of exciton-polaritons near a biexciton Feshbach resonance enables tuning of interspecies interactions to be attractive and of equal magnitude to the intraspecies repulsion.2
Demonstration of Quantum Droplet Phase
A liquid-like phase of light was observed. The droplets were self-bound and maintained a fixed density in the absence of external confinement. Their presence depended on quantum fluctuations overpowering MF forces, an effect only observed in ultracold atomic gases. This indicated quantum-driven stability.1
Additionally, the phenomenon emerged in materials compatible with chip-scale photonics, providing a practical route to strongly correlated photonic states and quantum light sources. Researchers predicted distinctive excitation spectra, droplet sizes of 10 μm, and flat-topped density profiles.1
Researchers showed that the quantum droplet phase is robust across several experimentally accessible parameter regimes and could exist whenever MF attraction is present, using a Bogoliubov approximation combined with a bosonic pairing approach.1,2
Multiple experimental signatures of this distinct quantum polariton phase like the spatial profile, the saturation density, and the excitation spectrum were obtained. Researchers predicted the presence of the droplet for parameters within reach of existing studies on semiconductor microcavities.1,2
Significance of this Development
Researchers in their work displayed that exciton-polaritons can host the ultimate liquidlike behavior of a quantum fluid of light/quantum droplets, complementing earlier research of quasiequilibrium photon fluids in plasma and semiconductor environments and liquid light in the context of nonlinear optics with competing nonlinearities.2,3
Polariton droplets do not suffer from three-body recombination owing to the absence of deep two-body bound states, and a higher range of particle numbers can be accessed. Additionally, the Bogoliubov excitation spectrum is probed readily in polariton condensates, which provides a thorough understanding into the broken symmetries and associated Goldstone modes of a Bose mixture.2
Thus, the work showed that quantum droplets of light can be realized for realistic ranges of parameters in existing TMD experiments, with key indicators of the droplet phase including a condensate size of ∼10 μm, an unchanging Bogoliubov spectrum, the absence of a blueshift with varying pump power, and an ultra-low threshold for condensation.2
Single Law of Motion
A physical homology exists between a fluid and light, that light propagates like a fluid whose propagation properties are different but strictly represent the same concepts. This homology leads to the derivation of a single law of motion, the same for both fluid and light.4
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The key differences in the physical properties of these two fields have led to the emergence of specific approaches like quantum mechanics, geometric optics, and fluid mechanics, even though these approaches could be fundamentally replaced by a single law.4
In recent work, a new law of motion was presented that resolved the paradox while remaining consistent with the results of relativity. Then, the concepts revisiting special and general relativity and those of classical mechanics were applied to quantum mechanics.4
By operating without the notion of mass, a potential unification of the laws of mechanics of material media and those of particles with or without mass was envisioned. Thus, a unified law of motion was derived in both deterministic transport and wave forms.4
Light Behaving as a Fluid
Light can exhibit fluid-like behavior under certain conditions, especially when strong interactions and quantum effects dominate. In such regimes, light-matter systems may be described by unified dynamical principles in which wave and transport descriptions become equivalent.
References and Further Reading
- Light behaves like a liquid, new Monash-led study finds [Online] Available at https://www.monash.edu/science/news-events/news/2026/light-behaves-like-a-liquid,-new-monash-led-study-finds (Accessed on 11 June 2026)
- Caldara, M., Bleu, O., Marchetti, F. M., Levinsen, J., & Parish, M. M. (2026). Quantum droplets of light in semiconductor microcavities. Physical Review Letters, 136(11), 116902. DOI: 10.1103/qbz5-df6g, https://journals.aps.org/prl/abstract/10.1103/qbz5-df6g
- Figueiredo, J. L., Mendonça, J. T., & Terças, H. (2024). Quantum kinetic theory of light-matter interactions in degenerate plasmas. Physical Review A, 110(6), 063519. DOI: 10.1103/PhysRevA.110.063519, https://journals.aps.org/pra/abstract/10.1103/PhysRevA.110.063519
- Caltagirone, J. (2025). Is light a fluid like any other?. https://hal.science/hal-05228402/
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