The spiral motion of a fluid around a defined axis is part of our daily life but in the quantum world it behaves differently
They accelerate technological advances in quantum simulation with ultracold atoms. A group of researchers from the National Optics Institute of Cnr at the Lens (European laboratory of non-linear spectroscopy) of the University of Florence studied the dynamics of quantum vortices using a programmable simulator based on ultra-cold lithium atoms. This work, published in the prestigious journal Nature, “offers a new window into the elementary mechanisms underlying the relaxation of vortex quantum states such as turbulence, which still remains difficult to understand and model”, underline the researchers.
Vortices, or the spiral motion of a fluid around a defined axis, are part of our daily life. We observe them when we mix coffee or when we swim, and they appear in the atmosphere as cyclones. In our body, the dynamics of the vortices promotes the transfer of oxygen in blood vessels and plays a crucial role in heart tissue, in connection with the onset of pathologies and malfunctions. In classical fluids, the vortex motion tends to disappear thanks to the diffusion of the rotational energy of the fluid caused by the frictional forces that give it viscosity.
The researchers point out that in the quantum world, in which the particles behave like waves and the laws of physics are dictated by discrete steps, i.e. quantized, the vortices behave differently. The speed of their rotary motion it cannot assume any value, but only “discrete” values. In superfluids and superconductors, the dynamics of quantum vortices play a fundamental role in the onset of dissipative processes that limit their conduction properties without viscosity and resistance.
The quantum nature of vortices also affects how they lose energy when interacting with each other. In particular, dissipation can occur by transforming the rotational energy into sound waves in the quantum fluid, which are also quantized into “quasi-particles” called phonons. The study of the conversion mechanism between vortex energy and sound waves in quantum fluids is the subject of an intense multidisciplinary research activity, however, made difficult by the complexity of ordinary materials in which inhomogeneity and imperfections prevent the direct observation of this fundamental mechanism. The team of researchers from the National Institute of Optics of the National Research Council (Cnr-Ino) at the Lens (European laboratory for non-linear spectroscopy), in collaboration with the BioMedico Campus of Rome and the University of Newcastle (UK), he observed for the first time the decay of quantum vortices into sound waves, in samples of lithium atoms cooled to temperatures close to absolute zero (-273 ° C). The work was published in the prestigious journal Nature. In order to observe this phenomenon, the Florentine team developed a completely new approach.
“We used innovative optical techniques to build a quantum vortex accelerator, which are created in controlled numbers and collided with defined energy “explains Woojin Kwon, researcher of Cnr-Ino at the Lens.” Our protocol is the analogue at the atomic level of a particle accelerator: introducing the vortices one by one in the atomic superfluid in a controlled manner, and observing its evolution over time, we were able to observe the generation of sound waves following the process of mutual annihilation between vortices of opposite circulation (vortex and anti-vortex) “says Francesco Scazza, now professor at University of Trieste and associated with Cnr-Ino. “Our work represents a breakthrough in understanding the fundamental mechanisms of quantum vortex dynamics connecting to the experiments carried out on liquid helium samples, and offers new scenarios for studies on neutron stars and high temperature superconductors “continues Giacomo Roati, research director Cnr-Ino at Lens and head of the research group.
“This work shows once again how quantum simulation with ultracold matter offers great potential for future investigations in various interdisciplinary research fields, from condensed matter to biological systems, with a completely new and extremely effective perspective “concludes Massimo Inguscio, professor emeritus at the Campus Bio-Medico in Rome, past president of the CNR and head of the Research Area of Quantum Science and Technology at the Lens of Florence.