In a significant discovery published in Nature, researchers from the University of Innsbruck have finally seen quantized vortices in a rotating two-dimensional supersolid. This confirms the long-awaited irrotational superfluid flow into a supersolid and represents a significant advancement in the study of modulated quantum matter.
Simulation of quantum vortices superimposed with experimental data. Image Credit: University of Innsbruck, Austria
Supersolids, a recently realized state of quantum matter, exhibit both crystalline order and superfluidity. This duality was first predicted over five decades ago, suggesting that quantum mechanics allows for a phase where particles simultaneously display solid-like and fluid-like characteristics. Researchers have successfully engineered such supersolid states in ultracold dipolar quantum gases.
A team at the University of Innsbruck has made a remarkable advancement in this field by experimentally observing quantized vortices within a supersolid, a definitive indicator of superfluid behavior. These vortices, tiny whirlpools in the quantum fluid, exhibited unexpected dynamics compared to prior theoretical predictions. This discovery not only confirms the superfluid nature of supersolids but also opens new avenues for exploring quantum hydrodynamics in systems that manifest both solid and fluid properties.
It is a bit like Schrödinger’s cat, which is both alive and dead, a supersolid is both rigid and liquid.
Francesca Ferlaino, Department of Experimental Physics, University of Innsbruck
While the crystalline structure underlying the "solid" nature of supersolids has been directly observed, confirming their superfluid properties has proven more elusive. Researchers have investigated various indicators of superfluidity, such as phase coherence and gapless Goldstone modes, yet direct evidence of quantized vortices—a key signature of superfluidity—has remained out of reach until now.
In this major advancement, quantized vortices have now been observed in a rotating two-dimensional supersolid, delivering long-sought confirmation of irrotational superfluid flow within this state of matter. This achievement represents a significant milestone in understanding modulated quantum matter.
Challenging Experiment
In this study, researchers merged theoretical models with advanced experimental techniques to create and observe vortices in dipolar supersolids—a particularly complex task. The Innsbruck team had previously made a breakthrough in 2021 by producing the first long-lived two-dimensional supersolid in an ultracold gas of erbium atoms, which was already a difficult achievement.
The next step—developing a way to stir the supersolid without destroying its fragile state—required even greater precision.
The next step—developing a way to stir the supersolid without destroying its fragile state—required even greater precision.
Eva Casotti, Study Lead Author and PhD Student, University of Innsbruck
Using high-precision techniques guided by theoretical insights, the researchers carefully rotated the supersolid using magnetic fields. Since liquids do not rotate rigidly, this stirring formed quantized vortices, which serve as the hydrodynamic fingerprint of superfluidity.
“This work is a significant step forward in understanding the unique behavior of supersolids and their potential applications in the field of quantum matter,” Ferlaino stated.
The experiment took nearly a year and revealed substantial differences between the dynamics of vortices in supersolids and unmodulated quantum fluids, offering fresh insights into how superfluid and solid characteristics coexist and interact in these exotic quantum states.
Exploring New Physics
The implications of this discovery extend far beyond the lab, impacting fields from condensed matter physics to astrophysics, where similar quantum phases may exist under extreme conditions.
Our findings open the door to studying the hydrodynamic properties of exotic quantum systems with multiple broken symmetries, such as quantum crystals and even neutron stars.
Thomas Bland, Postdoctoral Researcher, University of Innsbruck
He added, “For instance, it is assumed that the change in rotational speed observed in neutron stars - so-called glitches - are caused by superfluid vortices trapped inside neutron stars. Our platform offers the opportunity to simulate such phenomena right here on Earth. Superfluid vortices are also believed to exist in superconductors, which can conduct electricity without loss.”
Ferlaino further added, “Our work is an important milestone on the way to investigating new physics. We can observe physical phenomena here in the lab that occur in nature only under very extreme conditions, such as in neutron stars.”
The Austrian Science Fund (FWF), the Austrian Research Promotion Agency (FFG), and the European Union provided funding for the study, which was published in Nature.
Journal Reference:
Casotti, E. et. al. (2024) Observation of vortices in a dipolar supersolid. Nature. doi.org/10.1038/s41586-024-08149-7