When Crystals Flow: The Emergence of Supersolid Quantum States

The exploration of superfluidity has fascinated scientists for decades, spanning a wide range of systems—from solids and liquids to gases, and even light. Much of our modern understanding of superfluid behavior is rooted in the theoretical framework developed by pioneers such as Lev Pitaevskii, whose work on weakly interacting Bose systems, superfluid hydrodynamics, and collective excitations laid the foundation for describing macroscopic quantum phenomena.
Traditionally, the study of superfluid order has been confined to spatially homogeneous systems, where uniform conditions provide a simpler framework for understanding this extraordinary quantum state. Yet already within this framework, the structure of the excitation spectrum—particularly the emergence and softening of roton modes—reveals a deep connection between interactions and spatial ordering.
Early theoretical insights by Lev Pitaevskii anticipated that the softening of roton excitations could signal an instability toward density-modulated phases,
foreshadowing the possibility of supersolid behavior where crystalline order and superfluid coherence coexist.
But what happens when superfluidity arises in systems with periodic density modulations? Can the inherent localization of periodic structures coexist with the fluid-like properties of a superfluid? Could a solid, with its rigid crystalline structure, exhibit superfluid behavior? Or conversely, might a superfluid reveal a crystalline order? These questions, long at the frontier of quantum many-body physics, naturally extend Pitaevskii’s theoretical vision into regimes where broken symmetries intertwine. Recent breakthroughs have provided compelling answers with the discovery of “supersolid” quantum states—phases that uniquely combine superfluid and crystalline properties.
This talk will delve into the experimental realization of supersolidity in magnetic quantum gases, enabled by the momentum-dependent, long-range, and anisotropic dipole–dipole interactions. Key topics include the dynamics of symmetry breaking, the emergence of phase-coherent density-modulated states, and the observation of quantized vortices in rotating supersolid phases, providing a direct bridge to the mean-field and hydrodynamic descriptions embodied in the Gross–Pitaevskii equation.
These advancements open new avenues for understanding many-body quantum physics and the interplay of order and coherence in complex quantum systems, carrying forward the profound legacy of Lev Pitaevskii into a new generation of quantum matter.