In the realm of everyday experience, matter is typically recognized in one of three classical states: solid, liquid, or gas. These states define the familiar physical characteristics we observe, such as rigidity, fluidity, or expansiveness. Yet, the intricate domain of quantum mechanics reveals a much richer tapestry of possibilities, where the boundaries between these states blur and new, counterintuitive phases emerge. Recently, pioneering research at Heidelberg University has unveiled a striking manifestation of this quantum complexity: the simultaneous coexistence of solid and fluid properties within a single ultracold quantum system. This discovery not only challenges conventional wisdom but also provides new insight into the fascinating phase of matter known as a supersolid.
Supersolidity, a phase that combines the frictionless flow of a superfluid with the ordered structure of a crystal, has been a subject of intense theoretical interest for decades. However, experimentally proving its existence and understanding its nuanced behavior have proven exceptionally challenging. The Synthetic Quantum Systems team at Heidelberg has now leveraged innovative experimental methodologies to produce and observe this elusive phase in a driven quantum gas, marking a milestone in condensed matter physics. Their approach revolves around the delicate injection of energy into a superfluid to induce crystallization-like properties without destroying the superfluid’s coherence.
At ultralow temperatures, where quantum effects dominate, atoms behave not as distinct particles but rather as interconnected wave-like entities. When cooled near absolute zero, bosonic atoms can occupy the same quantum ground state, forming a Bose-Einstein condensate (BEC). In this state, these atoms collectively behave as a single quantum entity—a superfluid—with unique properties such as the ability to flow without viscosity or dissipation. Yet, under specific conditions, this smooth fluid can develop periodic modulations in its density, effectively imbuing it with solid-like characteristics. This remarkable coexistence, where the system simultaneously supports crystalline order and superfluid flow, is the hallmark of supersolidity.
The Heidelberg researchers took this phenomenon a step further by investigating supersolidity in a system driven far from equilibrium. Unlike traditional studies that consider quantum phases in static, equilibrium states, this work involved continuously “shaking” the interaction parameters between atoms within the superfluid. This dynamic excitation acts somewhat like ripples generated on a water surface when stirred but occurs within the quantum mechanical landscape governing atomic interactions. By externally driving the system, the researchers created a non-equilibrium environment where the interplay between order and quantum coherence could be probed with unprecedented detail.
One of the most compelling signatures of supersolidity lies in its sound modes. In conventional solids and fluids, sound waves propagate at characteristic speeds determined by the material’s elastic and hydrodynamic properties. However, in a supersolid, two distinct types of sound waves coexist: one associated with perturbations of the superfluid component and another related to fluctuations of the crystalline order. By exciting and tracking these modes individually, the Heidelberg team demonstrated that sound waves within their driven quantum system propagate at two different velocities, providing direct evidence of the supersolid phase.
These observations were made possible through state-of-the-art experimental techniques involving ultracold atoms confined and manipulated in optical traps. The researchers could finely control atomic interactions via magnetic and optical means and introduce controlled oscillations to shake the system. Through careful measurement, they detected the propagation characteristics of defects and excitations traveling through the condensate, revealing the dual nature of the sound waves in the supersolid—a behavior fundamentally distinct from classical solids or fluids.
Prof. Dr. Markus Oberthaler, who leads the Synthetic Quantum Systems group, emphasizes the novelty of these findings: “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,” he said. The experimental results show that the excited superfluid supports collective oscillations akin to atomic vibrations in a crystal lattice, indicating that atoms move in synchronized patterns around equilibrium positions, which is a direct manifestation of solid-like behavior embedded within a fluid framework.
What makes this study particularly groundbreaking is its focus on driven, non-equilibrium systems. Supersolids have traditionally been considered within the confines of equilibrium physics, where the system’s parameters are static and time-independent. However, Nikolas Liebster, a key member of the research team, explains that “this work represents the first observation of supersolid sound waves in a system far from equilibrium.” By continuously feeding energy into the quantum gas, the team demonstrated that the defining characteristics of supersolidity survive and can be robustly explored even in dynamic, time-evolving conditions.
The implications of this are profound for quantum science and technology. Understanding phases of matter under dynamic conditions is crucial for the development of quantum simulators, sensors, and other applications where controlled non-equilibrium dynamics play a vital role. Moreover, the principles underlying supersolidity might inspire future materials engineered to exploit coexistence of fluid and solid properties, potentially unlocking new functionalities unattainable in traditional condensed matter systems.
This work also complements a larger international effort to explore exotic states of matter in isolated quantum systems, often referred to as quantum simulators. By creating artificial environments where fundamental physical laws can be tested in regimes previously inaccessible, researchers not only validate abstract theoretical models but also pave the way toward harnessing quantum phases for practical use. The Heidelberg experiments stand as a testament to the power of precision control and measurement in modern quantum physics laboratories.
Funded by the German Research Foundation and carried out within the frameworks of the Collaborative Research Centre “Isolated Quantum Systems and Universality in Extreme Conditions” (ISOQUANT) and the STRUCTURES Cluster of Excellence at Heidelberg University, these results have been documented in the journal Nature Physics. The findings open new avenues for investigating the interplay between order, coherence, and dynamics in quantum gases, and steer the quest for understanding nature’s most enigmatic phases into novel territory.
In summary, the Heidelberg team’s experimental demonstration of supersolid-like sound modes in a driven quantum gas not only confirms theoretical predictions but also challenges conventional paradigms about matter’s states under dynamic manipulation. This breakthrough marks a pivotal advancement in quantum many-body physics by revealing that matter can simultaneously exhibit the frictionless flow of a superfluid and the rigid structure of a solid in a single, coherent quantum state—pushing the frontier of our understanding of quantum phases beyond equilibrium.
Subject of Research: Supersolidity and sound modes in driven ultracold quantum gases
Article Title: Supersolid-like sound modes in a driven quantum gas
News Publication Date: 2-Jun-2025
Web References: 10.1038/s41567-025-02927-4
Keywords
Physics, Quantum mechanics, Superfluidity, Supersolidity
