Just as a thin water jet emerging from a tap, a quantum atomic filament may break up into droplets: surprisingly, under some circumstances, quantum gases behave like liquids. In our lab, we prepare a quantum droplet of ⁴¹K and ⁸⁷Rb, and release it in an optical waveguide. The droplet expands along the waveguide up to a critical length, and then splits into two or more sub-droplets. Our results can be explained in terms of capillary instability, previously observed in a variety of physical systems, including ordinary liquids and superfluid helium, but not yet in the ultracold gas realm. Getting curious? Look it up on the arxiv!

L. Cavicchioli et al.
Dynamical formation of multiple quantum droplets in a Bose-Bose mixture
arXiv:2409.16017 (2024)

Quantum gases of paramagnetic polar molecules, namely compounds that combine a large electric dipole moment with a magnetic one, associated with a nonzero electronic spin, are regarded as pristine platforms for a wealth of quantum-technological applications and fundamental studies ranging from quantum simulation and computation to controlled quantum chemistry and precision measurements. Yet realization of quantum gases of doubly polar molecules, based on biatomic systems considered so far, remains an unsurpassed task. In our joint experimental and theoretical work, we solve this two-decade-old challenge by exploring a new class of paramagnetic polar molecules, obtained by binding lithium alkali and transition-metal chromium elements. Starting from an ultracold mixture of 6⁢Li and 53⁢Cr fermionic atoms, we efficiently produce a high phase-space-density, long-lived gas of bosonic 6⁢Li53⁢Cr dimers, prepared within a single, weakly bound vibrational level. Through state-of-the-art techniques and novel probing methods, we reveal the paramagnetic nature of this diatomic species, gain experimental control over its internal quantum state, and identify the main inelastic mechanisms that may limit the system stability. In parallel, we develop quantum-chemical calculations to build a complete model for the LiCr molecule. We predict a large electric dipole moment together with high electronic spin in the absolute ground state, and we identify suitable transitions both for the coherent transfer of our weakly bound LiCr dimers to their lowest rovibrational level and for their subsequent optical manipulation. Our studies establish an unparalleled new pathway to realize quantum gases of doubly polar molecules, with countless future applications in quantum science and technology.

S. Finelli et al.
Ultracold Li⁢Cr: A New Pathway to Quantum Gases of Paramagnetic Polar Molecules
PRX Quantum (2024)

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)