In everyday life, all matter exists in one of three states: gas, liquid, or solid. However, in quantum physics, two different states can exist at the same time. An ultracold quantum system, for example, can display fluid and solid characteristics at the same time. The Synthetic Quantum Systems research group at Heidelberg University has now demonstrated this phenomenon using a novel experimental method: infusing a modest amount of energy into a superfluid. The study was published in Nature Physics.
A schematic of two sound waves propagating through a supersolid. One sound wave perturbs the crystalline order (solid line), while the other perturbs the superfluid (dashed line). This behavior has now been observed in a new experiment. Image Credit: Nikolas Liebster
They showed that, in a driven quantum system like this, sound waves travel at two distinct speeds, which is evidence of coexisting liquid and solid phases, a hallmark of supersolidity .
At room temperature, this unexpected and seemingly incongruous action of two states of matter existing simultaneously does not happen. However, quantum mechanics takes control at extremely low temperatures, allowing matter to have essentially distinct characteristics.
At extremely low temperatures, the wave-like nature of atoms becomes dominant. When multiple particles merge into a single, collective wave, they form what's known as a Bose-Einstein condensate. This unique state of matter is a superfluid, meaning it can flow without any friction.
Rarely, periodic density modulations can also be seen in superfluids. These modulations, which are fueled by outside forces, essentially “crystallize” the superfluid and give it solid-like characteristics. The system's atoms maintain their superfluid properties and operate as a single collective wave in spite of this crystallization. Supersolidity is the term used to describe the coexistence of fluid and solid phases in quantum mechanics.
For instance, shaking the system can produce periodic density modulations in superfluids. Energy is added to the superfluid by “shaking” the interaction between atoms, much as ripples appear on the water's surface when a bucket is shaken. It stops being in equilibrium and turns into a dynamic, externally driven quantum system.
Nevertheless, prior research has shown that crystalline organization may develop in these types of systems. However, the relationship between these crystallization patterns and supersolidity has not yet been explored experimentally, as explained by Prof. Dr. Markus Oberthaler, leader of the Synthetic Quantum Systems group.
The existence of two distinct types of sound waves (one that disturbs the crystalline order and the other that disturbs the superfluid) is a characteristic that distinguishes supersolids. The Heidelberg scientists have now successfully triggered each of these disturbances independently using sophisticated experimental methods.
They investigated the motion of the sound waves in the driven quantum system as part of this. They discovered that the resultant flaws traveled at varying rates, suggesting that the system is supersolid since it had both liquid and solid properties.
It is fascinating to see that simply by adding a little bit of energy to a superfluid, we can give it the properties of a solid. The excited superfluid supports oscillations like a solid does, with atoms vibrating in sync around their equilibrium positions as a sound wave passes.
Markus Oberthaler, Professor, Heidelberg University
According to Nikolas Liebster, this is the first observation of supersolid sound waves in a system that is far from equilibrium.
Typically, supersolids are discussed in terms of equilibrium physics, meaning everything is static in time. Now we are shaking the superfluid, thereby injecting energy into it, and we’ve discovered that the concept of supersolidity remains valid even well outside equilibrium conditions.
Nikolas Liebster, Physicist, Heidelberg University
The study is part of the Collaborative Research Centre “Isolated Quantum Systems and Universality in Extreme Conditions” (ISOQUANT) and the STRUCTURES Cluster of Excellence at Heidelberg University. It was financed by the German Research Foundation.
Journal Reference:
Liebster, N., et al. (2025) Supersolid-like sound modes in a driven quantum gas. Nature Physics. doi.org/10.1038/s41567-025-02927-4.