In March 2025 a group in Italy and Austria reported making light behave partly like a solid. The headline that followed said the light had been frozen.
It was not frozen. The condensate in that experiment is 54 percent matter, and it exists only while a laser is being shone on it.
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| A Bose-Einstein condensate of rubidium. Every convincing supersolid before March 2025 was built from atoms like these, which need dilution refrigeration to a few hundred nanokelvin. Producing the phase without that machinery is the actual novelty. NIST, JILA and the University of Colorado, public domain, via Wikimedia Commons |
The earlier version of this post ran to about 370 words and claimed the result was the first of its kind ever. That part is not right, and the correction is worth making carefully.
What a supersolid is, and why it is hard
A supersolid is a phase of matter that is supposed to do two things that look incompatible.
It should be a solid, which means its particles sit in a repeating pattern. And it should be a superfluid, which means they flow without resistance.
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| Liquid helium II. Superfluidity is the half of "supersolid" that is not new at all; helium in a test tube has flowed without resistance since 1938. Bmatulis, public domain, via Wikimedia Commons |
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| Superfluid helium climbing the wall of its container. This is what "flows without resistance" actually looks like, and it is the property the polariton condensate was tested for. AlfredLeitner, public domain, via Wikimedia Commons |
The idea is old. Physicists were writing about it more than fifty years ago, and for most of that time it was a curiosity nobody could make.
The reason is that solids are usually made of things that sit still, and fluids are made of things that move. Getting both at once requires a system with a very unusual balance of forces.
Starting around 2020 the balance was found in the laboratory, mostly using ultracold atoms held in optical traps.
The three ways it had been done
Atoms coupled to an optical cavity, where light can mediate an interaction between distant atoms and hold them in place together.
Atoms with large magnetic dipole moments, whose long-range forces favor a striped pattern while leaving the stripes able to slide.
And spin-orbit coupled systems, which produce a striped phase where two internal states alternate in space.
All of these share one property that matters for what came next. They are all cold, all isolated, and all close to equilibrium.
What the March 2025 result actually did
The paper behind the headlines is titled "Emerging supersolidity from a polariton condensate in a photonic crystal waveguide."
It was led by CNR Nanotec in Lecce, with colleagues in Innsbruck, Pavia, Trento and Princeton.
That title word "Emerging" is doing real work. The authors call the result experimental evidence for a new implementation. They do not call it the first supersolid, because it is not.
| Claim | What is actually true |
| first supersolid ever | supersolids had been made from ultracold atoms since about 2020 |
| light was frozen | the condensate is 54 percent matter, and it flows |
| more stable qubits | there are no qubits anywhere in the experiment |
| a photonic platform | correct, and this is the real contribution |
The earlier version of this post against the paper. Three of its four headline claims do not survive contact.
So what did they do that had not been done before? They got the same phase out of a completely different kind of material.
Every previous supersolid was a gas of atoms in a vacuum. This one is light in a semiconductor waveguide, held in a steady state by a pump laser.
Why that is harder than it sounds
The device is a planar waveguide made from gallium arsenide, a semiconductor, with a pattern etched into it finer than the wavelength of the light.
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| A two-inch gallium arsenide wafer. The earlier version of this post named the material correctly, which was its single best detail. La2O3, CC BY-SA 3.0, via Wikimedia Commons |
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| The zincblende unit cell of gallium arsenide. Its band gap is what lets a photon and a bound electron share a single quantum state instead of passing through each other. Benjah-bmm27, public domain, via Wikimedia Commons |
Light alone cannot condense. Photons do not interact with each other, so a beam of light passes straight through another beam of light without noticing it.
The semiconductor fixes that. A photon in this waveguide can combine with a bound electron, an exciton, to make an object that is part light and part matter.
Those hybrid objects, polaritons, do interact with each other. That is the entire reason a condensate can form here at all.
And the number that ends the "frozen light" headline is in the paper. The Hopfield coefficient at the working state is 0.54.
| Figure | What it is |
| 0.54 | matter fraction of the condensate, the Hopfield coefficient |
| 2.3 micrometers | period of the density modulation |
| 2.6 percent | amplitude of that modulation |
| about 30 | lattice sites the modulation spans |
| 30 micrometers | the condensate waist |
| 1.526 eV | energy of the bound state in the continuum |
| 1.5 meV | coupling between the two counter-propagating modes |
| 250 microwatts | condensation threshold |
| 430 microwatts | where parametric scattering sets in |
| 80 MHz | pump repetition rate |
| a few hundred picoseconds | condensate lifetime |
Figures from the sources listed at the foot of this article.
How the pattern forms
The etched pattern does something specific. It folds light of every wavelength in the guide back on itself, so that a light wave heading right and one heading left cross at the same point.
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| A diffraction grating. The etched pattern on the waveguide does the same job, folding light back on itself so that two waves travelling opposite ways meet and interfere. Deglr6328, public domain, via Wikimedia Commons |
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| A one-dimensional photonic crystal. This is the geometry the experiment used: a periodic index pattern along one axis, which folds the propagating modes back on themselves. Птах, public domain, via Wikimedia Commons |
Where they cross, the two can cancel their radiation away entirely while still holding energy. This is a bound state in the continuum, and it behaves oddly.
It cannot radiate away, so the light in it barely leaks. It has zero group velocity, which means it does not spread.
And it has a negative effective mass. That last one is what lets the condensate sit still instead of rolling off in some direction.
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| A two-dimensional photonic crystal, shown for contrast. Polariton supersolid work has also been done in this geometry. The experiment described here is one-dimensional. Птах, public domain, via Wikimedia Commons |
The spacing between the neighboring optical modes is what sets the scale of everything else. Here it gives a pattern period of 2.3 micrometers.
Two extra modes appear
Once enough polaritons collect at that state, a nonlinear process starts converting them into pairs that land in two neighboring modes at finite momentum.
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| A two- and three-photon parametric process. In the supersolid experiment the condensate scatters into two side modes at once, which is what lays down the density pattern. DMB, CC BY-SA 3.0, via Wikimedia Commons |
This is optical parametric oscillation, and it is a familiar piece of nonlinear optics rather than anything new.
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| Nonlinear crystals used in an optical parametric oscillator. The same third-order nonlinearity was exploited here, but inside the semiconductor waveguide rather than a bulk crystal. Kkmurray, CC BY 3.0, via Wikimedia Commons |
What matters is what it does to the density. Because the two side modes are symmetric, adding them together produces a standing density wave laid over the condensate.
That is the solid half of supersolidity: a repeating pattern at a fixed spacing, which breaks the translational symmetry of the system.
Condensation begins at roughly 250 microwatts of pump power. The parametric process takes over around 430 microwatts, about 1.7 times threshold.
The resulting modulation is shallow, about 2.6 percent, and it spans roughly thirty of the 2.3-micron cells across the 30-micron width of the condensate.
The authors also had to phase-lock it to see it. The side modes are never quite empty, because the exciton reservoir and Rayleigh scattering fill them.
A small residual coupling then fixes the phase of the pattern, which is what makes it visible in a time-integrated photograph.
Why the authors said "emerging"
Both halves of a supersolid were present. The density modulation supplies the structure, and the authors separately measured the local phase of the condensate to check the fluid half.
The phase of the pattern was also seen to rotate by about 1.1 times pi across the sample.
A pattern that is still rotating is not yet settled into a perfect lattice.
That is why the title says "emerging" and not "observed". The modulation was measured to a few parts in a thousand, but the order parameter is small and the phase is not fully settled.
There is a deeper reason for caution, and it is the one the original post missed entirely.
This system is not in equilibrium
A cold atomic gas is isolated and cools toward equilibrium. This one is pumped continuously and leaks.
The condensate lives for a few hundred picoseconds and has to be fed. The light density in the measurements was built from about eight million separate pulses at 80 megahertz.
What was observed is a steady state of a driven system, not a phase of matter in the sense most textbooks mean.
Whether "supersolid" is the right word for something that exists only while you keep shining a laser on it is genuinely open. The authors are careful. One of them is quoted saying this is only the beginning of understanding supersolidity.
What the original post claimed, and what happened to it
The qubit claim
The earlier version listed more stable qubits for quantum computers as a likely consequence.
There are no qubits in this experiment. There is no superconducting circuit, no trapped ion, and no two-level system being manipulated. The authors make no connection of that kind.
The claim has been removed rather than softened, because softening it would still imply a connection that does not exist.
The applications list
Ultra-fast computing and next-generation sensors were listed as likely applications.
No device, no figure of merit and no paper supports either. What the work offers is a way to study a phase of matter in a system that is small, fast to build and tunable by changing the etched pattern.
The dual nature of light
The post closed by saying this deepens our understanding of light's dual nature, as wave and particle.
That framing is not what the experiment was about, and it replaces the interesting question with a slogan.
The condensate is a hybrid object whose matter fraction can be tuned, and whose patterning is driven by a nonlinear scattering threshold. The interesting question is whether supersolidity survives at all in a system that is neither cold nor isolated.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Polariton | A hybrid object made of a photon and a bound electron. Half light, half matter, and they can interact. |
| Exciton | An electron bound to a hole. In a semiconductor this pair can absorb and emit light. |
| Photonic crystal | A structure with a repeating pattern of optical properties that shapes which wavelengths can travel. |
| Bound state in the continuum | A state that cannot escape or radiate away even though it sits inside a range of energies that normally leaks. |
| Group velocity | How fast a wave pattern appears to travel. Zero group velocity means the pattern stands still. |
| Effective mass | How a particle responds to a force. A negative value means it accelerates the opposite way, which is what lets the condensate sit still. |
| Optical parametric oscillation | A nonlinear process where a pump drops pairs of particles into two other modes at the same time. |
| Hopfield coefficient | The matter fraction of a polariton. Here 0.54, which is why "frozen light" is a poor label. |
| Supersolid | A phase that is both crystalline in structure and superfluid in flow. Speculated over fifty years ago. |
| Superfluid | A fluid that flows with no resistance, first seen in helium in 1938. |
| Bose-Einstein condensate | A large number of particles occupying a single quantum state, so they act as one. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- This was not the first supersolid. Supersolids had been made from ultracold atoms from about 2020 onward.
- The real contribution is a photonic platform, and a phase built without ultracold atoms.
- Nothing was frozen. The condensate is 54 percent matter and it flows.
- The system is pumped and leaky, with a lifetime of a few hundred picoseconds. It is not in equilibrium.
- The pattern comes from optical parametric oscillation, with a period of 2.3 micrometers and an amplitude of about 2.6 percent.
- The original post claimed more stable qubits. There are no qubits in the experiment.
Frequently Asked Questions
Was light really frozen?
No. The condensate is a polariton that is 54 percent matter by mass, and it exhibits superfluid flow, which is the opposite of being frozen.
The phrase comes from the popular coverage. The authors did not use it, and they titled their paper "Emerging supersolidity".
So was it the first supersolid?
No. Convincing evidence has existed since around 2020, in ultracold atomic systems including cavity-coupled and dipolar atoms.
The defensible version of the claim is that it is the first supersolid in a photonic system, and the first not made from ultracold atoms.
How is this different from a Bose-Einstein condensate?
An atomic condensate needs a vacuum, magnetic trapping and temperatures of a few hundred nanokelvin.
This one sits in a semiconductor waveguide and is sustained by a pump laser. It also modulates in space, which an ordinary condensate does not.
Could this lead to quantum computing?
Nothing in the result connects to qubits. That claim was in the earlier version of this post and has been removed.
The waveguide is a photonic device, so integrated photonics is a plausible direction. But a research platform is not a processor.
Why did the authors use the word "emerging"?
Because the density modulation is shallow, about 2.6 percent, and the phase of the pattern still fluctuates across the sample.
They report the measurement to a few parts in a thousand, which is precise. Precision is not the same as certainty.
Key references
• Emerging supersolidity from a polariton condensate in a photonic crystal waveguide
• A supersolid made using photons (University of Innsbruck, 10 March 2025)
• Making light supersolid in a world first (CORDIS)
• Supersolid emission in a photonic crystal waveguide (Physical Review Letters, 2025)
• Exciton-polaritons
• Photonic crystal
• Gallium arsenide
• Bose-Einstein condensate
Images: Wikimedia Commons, with the author and license named in each caption. Figures follow the sources above.
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