Arrays of Josephson junctions are at the forefront of research on quantum circuitry for quantum computing, simulation, and metrology. They provide a testing bed for exploring a variety of fundamental physical effects where macroscopic phase coherence, nonlinearities, and dissipative mechanisms compete. In this work we realize finite-circulation states in an atomtronic Josephson junction necklace, consisting of a tunable array of tunneling links in a ring-shaped superfluid. We study the stability diagram of the atomic flow by tuning both the circulation and the number of junctions. We predict theoretically and demonstrate experimentally that, counterintuitively, the atomic circuit withstands higher circulations (corresponding to higher critical currents) by increasing the number of Josephson links. The increased stability contrasts with the trend of the superfluid fraction – quantified by Leggett’s criterion – which instead decreases with the number of junctions and the corresponding density depletion. Our results demonstrate atomic superfluids in mesoscopic structured ring potentials as excellent candidates for atomtronics applications, with prospects towards the observation of non-trivial macroscopic superpositions of current states.

L. Pezzè, K. Xhani, C. Daix et al.
Stabilizing persistent currents in an atomtronic Josephson junction necklace
Nat. Comm. (2024)

Supersolids exhibit unique properties halfway between traditional superfluids and crystals. In our latest study, we examined the superfluid behavior of dipolar supersolids, focusing on the superfluid fraction, a crucial concept introduced by Nobel Laureate A. Leggett in the 1970s. By investigating a cold-atom dipolar supersolid, we measured a reduced superfluid fraction, revealing non-standard superfluid dynamics. We achieved this by probing the supercurrent between adjacent sites of the supersolid, triggering Josephson oscillations. Our findings pave the way for new research into phenomena like partially quantized vortices and supercurrents in supersolid systems, enhancing our understanding of related condensed matter systems.

G. Biagioni et al.
Measurement of the superfluid fraction of a supersolid by Josephson effect
Nature (2024)

See also UNIFI press release

At the interface between two fluid layers in relative motion, infinitesimal fluctuations can be exponentially amplified, inducing vorticity and the breakdown of laminar flow. While shear-flow instabilities in classical fluids have been extensively observed in various contexts, controlled experiments in the presence of quantized circulation are quite rare. In our last work, we observe how the contact interface between two counter-rotating atomic superflows develops into an ordered circular array of quantized vortices, which loses stability and rolls up into vortex clusters. We extract the instability growth rates and find that they obey the same scaling relations across different superfluid regimes, ranging from weakly-interacting bosonic to strongly-correlated fermionic pair condensates. Our results establish connections between vortex arrays and shear-flow instabilities, suggesting a possible interpretation of the observed quantized vortex dynamics as a manifestation of the underlying unstable flow. Moreover, they open the way for exploring out-of-equilibrium phenomena such as vortex-matter phase transitions and the spontaneous emergence and decay of two-dimensional quantum turbulence.

D. Henández-Rajkov et al.
Connecting shear-flow and vortex array instabilities in annular atomic superfluids
Nat. Phys. (2024)

We are immensely proud of Giulio Biagioni, who has defended his doctoral thesis "Can a superfluid be solid? A study of the supersolid phase in a dipolar quantum gas", achieving the summa cum laude distinction. In the last few years, Giulio has given invaluable contributions to our research, enriching our group in countless ways and inspiring many experiments. We wish him all the success and fulfillment he deserves, and even more luck in his academic journey!

See also the thesis

Beatrice successfully defends her Master's thesis titled ["Realization and characterization of LiCr ultracold bosonic dimers"], and starts her PhD on few-body physics and impurity problems in resonantly interacting Fermi mixtures.

Congrats and good luck Bea!