Heidelberg physicists show through experiments that driven quantum systems can behave like supersolids

In everyday life, all matter exists as either a gas, liquid, or solid. In quantum mechanics, however, it is possible for two distinct states to exist simultaneously. An ultracold quantum system, for instance, can exhibit the properties of both a fluid and a solid at the same time. The Synthetic Quantum Systems research group at Heidelberg University has now demonstrated this phenomenon using a new experimental approach, by feeding a small amount of energy into a superfluid. They showed that, in a driven quantum system of this kind, sound waves propagate at two different speeds, which points towards coexisting liquid and solid states, a hallmark of supersolidity.

This surprising and seemingly contradictory behavior of two states of matter existing at the same time does not occur at room temperature. But at ultralow temperatures, quantum mechanics takes over, and matter can exhibit fundamentally different properties. When atoms are cooled to such low temperatures, their wave-like nature is dominant. If brought close enough together, many particles merge into one large wave, known as a Bose-Einstein condensate. This state is a superfluid, a fluid that flows without friction.

In rare cases, superfluids can also exhibit periodic density modulations. Driven by external forces, these modulations cause the superfluid to effectively “crystallize” and take on solid-like properties. Despite this crystallization, the atoms in the system continue to behave as one collective wave, retaining their superfluid characteristics. In quantum mechanics, this coexistence of fluid and solid states is known as supersolidity.

Periodic density modulations in superfluids can, for example, be generated by shaking the system. Much like ripples forming on the surface of water when a bucket is shaken, energy is introduced into the superfluid by “shaking” the interaction between atoms. It becomes a dynamic, externally driven quantum system and is no longer in a state of equilibrium. Previous studies have demonstrated that crystalline order can nevertheless arise in such systems. However, as Prof. Dr Markus Oberthaler, head of the Synthetic Quantum Systems group, explains, the connection between these crystallization patterns and supersolidity had not yet been investigated through experiments.

A defining feature of supersolids is the presence of two types of sound waves: one that perturbs the superfluid, and another that perturbs the crystalline order. Using advanced experimental techniques, the Heidelberg physicists have now successfully triggered each of these perturbations separately. As part of this, they examined how the sound waves moved through the driven quantum system. They found that the resulting defects travelled at different speeds, indicating that the system exhibits both liquid and solid characteristics, making it supersolid.

“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,” says Prof. Oberthaler. “The excited superfluid supports oscillations like a solid does, with atoms vibrating in sync around their equilibrium positions as a sound wave passes,” explains the researcher, who works with his group at Heidelberg University’s Kirchhoff Institute for Physics.

According to Nikolas Liebster, this work represents the first observation of supersolid sound waves in a system far from equilibrium. “Typically, supersolids are discussed in terms of equilibrium physics, meaning everything is static in time,” says the physicist and member of Prof. Oberthaler’s research group. “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.”

This research is part of the work carried out within the Collaborative Research Centre “Isolated Quantum Systems and Universality in Extreme Conditions” (ISOQUANT) and the STRUCTURES Cluster of Excellence at Heidelberg University. It was funded by the German Research Foundation. The results have been published in “Nature Physics”.