A pair of quantized vortices within a fluid of light and matter can evolve along trajectories analogous to the interaction of two point-like scattering particles. This was demonstrated by the experimental team at Institute of nanotechnology of Cnr in Lecce, Italy in collaboration with SDSU's Prof. Carretero in a recent study published in Nature Communications.
Vortices have a special charm, and not surprisingly, show up everywhere in nature, from cosmological scales of galaxies and black holes to large scale atmospheric tornadoes and cyclones, from smaller eddies and whirlpools in rivers and streams, up to the world of art and of spiraling shapes that have inspired artists like Van Gogh and Klimt. Their spiral patterns opening from their core to infinity invite us to reflect on the origins of the universe and on the physical meaning of their existence. In fact, a more fundamental class of vortices corresponds to microscopic, quantized, vortices, such as those excitable in electromagnetic waves or electron beams. In particular, the rotational dynamics of vortices imprinted in a polaritonic fluid -a planar (super)fluid composed of both light and electrons- were the object of an experimental and theoretical study, published in Nature Communications and lead by a group of researchers at the Institute of nanotechnology of the National research council (Nanotec-CNR) in Lecce, Italy, in collaboration with Prof. Ricardo Carretero, professor of Applied Mathematics at SDSU, and a team of collaborators from the Universities of Seville, Porto, Athens, and Massachusetts, published in Nature Communications.
"To understand the quantized vortices, let's keep in mind that in these vortices something counter-intuitive happens, in fact the fluid moves faster near the center, that is where there is a narrower curve, than out on the periphery," says Lorenzo Dominici, researcher of the Nanotec-Cnr in Lecce. "Moreover, its center (called phase singularity) has a null density, is practically point-like and at the same time has a quantized charge (that of angular orbital rotation). It is precisely these characteristics that perhaps suggested a possible analogy between the quantum vortex and an elementary particle, as was already suggested by Lord Kelvin himself, a scientist and philosopher at the end of the 1800s." Quantum vortices have been the subject of intense research since the 1900s, because their regular and turbulent distributions in condensates of ultracold atoms are the basis of important phase transitions, such as those of a superfluid or a superconductor. Furthermore, nowadays, photonic vortices are also used to pin and control individual cells or particles using the so-called optical tweezers and they are being proposed as means to increase the resolution and robustness in astronomical observations and optical telecommunications. At the level of fundamental physics, however, their rotational dynamics are not yet fully explained and strongly depend on the type of quantum fluid they are embedded in.
"We have imprinted vortex pairs with the same sign, rotating in the same direction, in the polaritonic fluid of an optical microcavity, and as expected, the two vortices began to evolve by rotating around each other by the mutual influence on each other. However, we also observed unusual interactions whereby, in addition to their mutual rotational interactions, vortices tended to approach each other and then rebound as in a scattering event of colliding particles", says Prof. Ricardo Carretero, who continues "this unusual behavior of quantum vortices had not been observed before even in the related atomic condensates of ultracold atoms [that share various properties with the polaritons, Ed.]". "We need to clarify" adds Lorenzo Dominici, "that the whole fluid around the vortices is responsible to mediate these unusual dynamics, which can be described, however, as generated by an effective repulsive potential interaction between the two point vortexes. It seems that the theory of lord Kelvin can be equally meaningful in a physical as well as in a philosophical sense, as perhaps one day it will be discovered", he concludes.
Daniele Sanvitto, coordinator of the experimental team at Nanotec-Cnr in Lecce, adds that: "There are already numerous theoretical proposals to use quantum vortices in a wide range of cutting-edge technologies such as ultrasensitive gyroscopes and gravitometers, information storage and processing in optical or quantum memories and computers. Therefore, understanding the dynamics of vortices in a polaritonic fluid subject to internal forces is fundamental to propose and design such futuristic devices".
Original manuscript: Interactions and scattering of quantum vortices in a polariton fluid. Lorenzo Dominici, R. Carretero-González, Antonio Gianfrate, J. Cuevas-Maraver, A.S. Rodrigues, D.J. Frantzeskakis, P.G. Kevrekidis, Giovanni Lerario, Dario Ballarini, Milena De Giorgi, Giuseppe Gigli, and Daniele Sanvitto. Nat. Comm. 9 (2018) 1467. https://doi.org/10.1038/s41467-018-03736-5
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