Supersolids represent a novel category of quantum matter, a concept that has recently been brought to light. This state of matter can be artificially created in ultracold dipolar quantum gases. A research team, led by physicist Francesca Ferlaino from Innsbruck, has now identified a key feature of superfluidity that was previously observed only in theory: the existence of quantized vortices, which arise when the system is rotated. They discovered that these tiny quantum vortices within the supersolid exhibit unexpected behavior.
The concept of matter behaving simultaneously as both a solid and a superfluid might seem impossible. However, over 50 years ago, scientists theorized that quantum mechanics could enable such a state, where a group of indistinguishable particles can display seemingly opposite characteristics at once. Francesca Ferlaino, from the Department of Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW), explains, “It’s akin to Schrödinger’s cat, which exists in two states—alive and dead; similarly, a supersolid is both firm and fluid.” While researchers have accurately visualized the crystal structure that provides the “solid” aspect of supersolids, the superfluid traits have proven more elusive. Investigations into superfluid behavior have included examining phase coherence and gapless Goldstone modes, but direct proof of quantized vortices—an essential feature of superfluidity—has been difficult to obtain.
In a significant advancement, scientists have now successfully detected quantized vortices in a rotating two-dimensional supersolid. This marks the long-sought validation of irrotational superfluid flow within a supersolid and represents a pivotal progress in the exploration of modulated quantum matter.
A Complex Experiment
The research team merged theoretical models with advanced experimental techniques to engineer and study vortices in dipolar supersolids, a task that proved to be exceedingly difficult. The Innsbruck team previously achieved a monumental milestone in 2021 by creating the first long-lasting two-dimensional supersolid in an ultracold gas of erbium atoms—a complex challenge in itself. “The subsequent objective—finding a method to agitate the supersolid without compromising its delicate state—demanded even more precision,” said lead author Eva Casotti. The researchers utilized high-precision methods guided by theoretical insights, applying magnetic fields to meticulously rotate the supersolid. Unlike liquids, which do not rotate uniformly, this stirring led to the emergence of quantized vortices, which serve as a signature of superfluidity. Francesca Ferlaino commented, “This accomplishment significantly enhances our understanding of the unique properties of supersolids and their potential uses in quantum matter.”
The experiment spanned nearly a year and revealed notable differences between the behavior of vortices in supersolids versus unmodulated quantum fluids. This provided fresh perspective on how the superfluid and solid characteristics coexist and interact in these unique quantum states.
Venturing into New Physics
The ramifications of this discovery extend far beyond the confines of the laboratory, holding potential implications for various fields, including condensed matter physics and astrophysics, where analogous quantum phases could exist under extreme conditions. “Our results pave the way for investigating the hydrodynamic characteristics of exotic quantum systems with multiple broken symmetries, such as quantum crystals and potentially neutron stars,” noted Thomas Bland, who contributed to the project’s theoretical framework. “For example, the changes in rotational speed observed in neutron stars—referred to as glitches—are believed to stem from superfluid vortices ensnared within them. Our research platform allows us to simulate such phenomena right here on Earth.” Additionally, it is thought that superfluid vortices also exist in superconductors, which can conduct electricity without loss. “
Francesca Ferlaino states, “Our research marks a crucial step toward exploring new physical phenomena. We can study processes in the lab that occur naturally only under extreme conditions, such as inside neutron stars.” The findings were published in Nature and received funding from the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, and the European Union.
