Light always travels at 3 lakh km per second in a vacuum. It cannot be trapped and solidified because the particles of light, photons, have no rest mass and don’t interact strongly with each other. Generally, light exists only as a particle or wave. But recently, a team of researchers from Italy’s University of Pavia and CNR Nanotec reported successfully ‘freezing’ light by manipulating photons in a meticulously arranged ultra-cold environment.
This groundbreaking research shows light can be turned into a supersolid and that it can flow with almost zero viscosity. The findings were published in Nature.
Slowing light
A supersolid is an exotic phase of matter in which particles are arranged in a crystalline structure but also move like a non-viscous fluid. In other words, it combines the friction-free flow of a superfluid with the ordered structure of a crystalline solid. Usually solids don’t move on their own, but supersolids change direction and density depending on their particles’ interactions while maintaining an organised internal structure.
Physicists predicted the idea of a supersolid in the 1960s and first created it in a laboratory in 2017. In a precursor, the Danish physicist Lene Hau and her team used a Bose-Einstein condensate — another exotic state of matter — of ultra-cold atoms to slow a beam of light to 17 m/s in 1999. Two years later, the team managed to completely ‘stop’ a light pulse. They accomplished this feat by ‘transporting’ information about the light to the atoms in the condensate. This technically froze the light, which could be ‘released’ from the atoms as needed.
These studies showed light could be stored as an ‘excitation’ in matter. But this technique only used matter to temporarily store the light rather than turning the light into a solid structure.
In 2010, a group in Germany created a Bose-Einstein condensate composed purely of photons by confining light in a dye-filled optical microcavity. This allowed photons to form a coherent quantum state, essentially liquid light, but it still wasn’t a solid structure.
Polaritons in the mix
The new study marks the first time scientists have managed to couple light with matter to create a supersolid. The feat opens up new possibilities in condensed matter physics, the field that gave us optical fibres, lasers, semiconductors, and quantum computing.
Solids and liquids are two common phases of matter. Solids turn into liquids when they are heated and liquids turn to solids when they are cooled. There are also certain quantum states of matter that turn into each other in peculiar ways.
The new research used a quantum mechanical approach that relied on the properties of polaritons. These are hybrid particles that behave sometimes like light and sometimes like matter. They are created by coupling photons with packets of energy inside materials, like phonons (vibrational energy) or excitons (electron-hole pairs).
The researchers used an aluminium gallium arsenide semiconductor platform as a waveguide — a channel through which waves can pass — fit with a source of excitons and a laser. The waveguide had a microscopic structure with a periodic grating. The etched ridges influenced the polaritons’ motion, trapping them in a regular pattern. The team used a pulsed laser to maintain a dense polariton condensate at a temperature of about –269º C.
‘Quantum theatre’
When the laser light entered the semiconductor, polaritons were created and then confined by the grating. They subsequently settled into a periodic lattice of hybrid light-matter waves, resulting in a density modulation corresponding to a crystal-like structure. This low-loss condition allowed the polaritons to survive long enough to organise themselves in particular ways. These polaritons were found to behave like a supersolid.
This was a state of light rather than matter: a polariton condensate exhibiting a crystalline structure and superfluid coherence. The polaritons were organised into a periodic pattern in space, just like how carbon atoms are organised in a diamond or silicon dioxide molecules are arranged in a quartz crystal.
The researchers used quantitative methods to confirm the state’s supersolid character. For example, they found the system could lower its energy by spontaneously producing a density wave and ‘freezing’ into an ordered pattern while maintaining a single, coherent quantum state, making it a quintessential supersolid.
In a Nature briefing, the researchers described this phenomenon as “quantum theatre”. Imagine sitting in a packed auditorium with only three seats left, all in the first row. Everyone wants the centre seat because they want the best view, but it can accommodate only one person. In a “quantum theatre”, all bosonic particles — I.e. force-carrying particles — can simultaneously occupy the middle seat. When they do, a Bose-Einstein condensate is formed.
(I.e. the bosons in an isolated quantum system are all allowed to have the same energy at the same time. A Bose-Einstein condensate is formed when they do. Matter-making particles, or fermions, in a system can’t have the same energy at the same time, however. This is called Pauli’s exclusion principle. This is what creates the different elements in the periodic table.)
Potential applications
This said, interactions between particles also limit the number that can occupy that one seat. Beyond a point, pairs of particles get pushed to the seats on the left and right. So as particles collect in the quantum theatre, two “satellite condensates” take shape on either side of the central one. This way, a supersolid state emerges from the condensate.
The experiment demonstrated that light can exhibit certain states of matter in the right — if also laboratory-engineered — conditions. The possibility of turning light into a quantum structure could render photonic supersolids more accessible for experimentation and potential applications such as lossless optical energy transport and optical computing elements.
Shamim Haque Mondal is a researcher in the Physics Division, State Forensic Science Laboratory, Kolkata.
Published – June 29, 2025 06:30 am IST
