By César Ordóñez, Hugo A. Lara, Arturo Camacho and Giuseppe Pirruccio
When millions of identical quantum particles synchronize, occupying the same state and behaving as a single entity, one of the most fascinating phenomena in modern physics emerges: Bose-Einstein condensation, also known as the fifth state of matter.
In the 1920s, physicists Satyendra Nath Bose and Albert Einstein predicted that, at extremely low temperatures, atoms could behave collectively and form a new phase of matter, distinct from the solid, liquid, gaseous, and plasma states.
Decades later, in 1995, physicists Eric Cornell, Wolfgang Ketterle, and Carl Wieman produced this state for the first time. The new state was named the Bose-Einstein condensate in honor of the scientists who predicted its existence. Their discovery earned them the 2001 Nobel Prize in Physics.
Today, scientists are seeking to reproduce this collective behavior in hybrid light-matter systems that can form even at room temperature (25 °C), without having to cool atoms to temperatures near absolute zero.
In a new study, a team from the Institute of Physics at the National Autonomous University of Mexico (UNAM) observed the formation of a polariton condensate—an unusual state in which millions of quasiparticles combining properties of light and matter act in sync, as though they were a single entity.
The researchers also detected tiny quantum vortices, known as quantized vortices, inside a cesium-lead-bromide perovskite microcrystal (CsPbBr₃), a semiconductor material that emits intense green light and can be synthesized using relatively simple, low-cost methods.
Polaritons: A Mix of Light and Matter
Simply put, a polariton is a quasiparticle that emerges when light and matter interact so strongly that they behave as a single system.
It forms when a mode of light inside a material—known as an optical mode—strongly interacts with an exciton, a state in which an electron in a semiconductor absorbs energy and becomes bound to a hole created in the material.
As a result of this coupling, a new hybrid state emerges with both photonic components, related to light, and excitonic components, related to the material’s electronic properties. A polariton is therefore neither light nor matter alone, but a combination of both that can behave as a single entity.
Because they inherit an extremely small effective mass from light, polaritons can reach coherent collective states at much higher temperatures than those required for conventional atomic condensates.
To generate these states in the laboratory, the researchers used square CsPbBr₃ microcrystals measuring between 2 and 4 micrometers on each side. The crystals were synthesized through chemical vapor deposition.
The geometry of the crystal allows it to function as an optical microcavity, essentially trapping light. Inside the crystal, light becomes confined and repeatedly travels along its edges through internal reflections, a phenomenon known as whispering gallery behavior. The same effect can occur with sound and light waves as they propagate along a curved surface through successive reflections.
Because the light remains confined, it has more opportunities to interact with excitons in the material. This interaction produces polaritons and, under certain conditions, can cause them to accumulate in the same quantum state and form a condensate.
From Chaos to Order
The researchers then illuminated the microcrystals with ultrafast laser pulses lasting approximately 100 femtoseconds and analyzed the emitted light as they gradually increased the excitation power.
Below a threshold power, the emission is weak and incoherent. Once that threshold is exceeded, however, the light intensity rises abruptly and nonlinearly, the emission spectrum narrows considerably, and the polariton population becomes concentrated in specific states of the system.
The presence of these characteristics—including the collective synchronization of polaritons and the emergence of quantized vortices—provides some of the main experimental evidence of polariton condensation.
To confirm that a polariton condensate had formed, the team looked for signs that the system had transitioned from disordered behavior to a collective, synchronized state. One of these was a blue shift in the emitted energy, meaning a shift toward higher frequencies caused by interactions among the polaritons themselves.
The researchers also observed that once the number of these quasiparticles exceeded a critical value, known as the condensation threshold, the system began emitting light much more rapidly.
Taken together, these signals indicate that the polaritons stopped behaving as independent particles and began acting as a single quantum entity, a fundamental characteristic of polariton condensation.
Light Vortices: Quantum Vortices
One of the study’s most significant findings was determining how the condensate was organized inside the crystal. To do this, the researchers used a modified Michelson interferometer, an instrument that makes it possible to compare the light emitted by the condensate with a reflected copy of itself.
The resulting pattern showed clearly visible interference fringes across the entire microcrystal, indicating that the polaritons maintained large-scale spatial synchronization. This coherence is one of the characteristics that distinguishes a polariton condensate from a collection of independently behaving particles.
In other words, the experiment showed that millions of these quasiparticles could maintain the same collective rhythm inside a tiny crystal.
The images, however, also revealed fork-shaped structures, a characteristic signature of quantized vortices. These quantum vortices are tiny swirling structures that appear when the collective state of the polaritons undergoes a localized disturbance.
Around each vortex, the phase of the quantum wave changes continuously until completing a full 360-degree turn (±2π), as if the system were making one complete rotation around itself.
The researchers found that the vortices appeared in pairs: a vortex and an antivortex with opposite circulation, similar to two whirlpools rotating in opposite directions. Most remarkably, they emerged spontaneously, without an external force being applied to make the system rotate. Their appearance resulted from the combined effects of the microcrystal’s geometry, the condensate’s need to exchange energy with its surroundings, and small irregularities in the material.
Disorder as an Ally
In most photonic devices, structural imperfections are considered undesirable. In this system, however, disorder plays a constructive role.
Irregularities in the microcrystal locally modify the cavity’s energy landscape, favoring the formation of regions where the condensate tends to become localized and where vortices can stabilize. At the same time, these imperfections allow some of the confined light to escape, making it possible to observe the system’s internal dynamics.
This feature makes it possible to directly study the vortices forming within the condensate.
In other words, disorder not only influences vortex formation but also makes the vortices easier to detect. Far from being an obstacle, it becomes an essential element for understanding the physics of the condensate.
This work establishes self-assembled perovskite microcavities as a promising platform for studying quantum fluids of light at room temperature, opening new opportunities for experimental research into collective quantum phenomena.
In the long term, a better understanding of these systems could contribute to the development of polaritonic devices based on quantum coherence, as well as photonic technologies that do not require cooling systems.
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Reference
Montagnac, M.; García Jomaso, Y. A.; Robledo Ibarra, E.; Sánchez-Martínez, R.; Santiago García, M.; Ordóñez-Romero, C. L.; Lara-García, H. A.; Camacho-Guardian, A.; Pirruccio, G. Macroscopic coherence and vorticity in room-temperature polariton condensate confined in a self-assembled perovskite microcavity. ACS Photonics (2026).
Authors
César Ordóñez, Hugo A. Lara García, Arturo Camacho-Guardian, and Giuseppe Pirruccio are researchers at the Institute of Physics of the National Autonomous University of Mexico (UNAM). Their work focuses on quantum fluids of light, polariton condensates, nonequilibrium collective phenomena, and quantum photonics.
