Supersolids: unveiling an extraordinary quantum phase of matter

Project Information

Supersolids

Grant agreement ID: 101055319

DOI

10.3030/101055319

EC signature date 30 August 2022

Start date 1 November 2022

End date 31 October 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 065 000,00

EU contribution

€ 2 065 000,00

Coordinated by

UNIVERSITA DEGLI STUDI DI FIRENZE
Italy

Periodic Reporting for period 1 - Supersolids (Supersolids: unveiling an extraordinary quantum phase of matter)

Reporting period: 2022-11-01 to 2025-04-30

Summary of the context and overall objectives of the project

Supersolids are a paradoxical quantum phase of matter that combines the properties of superfluids and crystals, searched for long time in quantum solids and many other systems. Recently, we discovered a novel cluster phase in a quantum gas of magnetic atoms which realizes a supersolid. However, the limited size, the inhomogeneity, and the lack of appropriate detection methods have allowed so far to assess only very basic properties of supersolids.
Here I propose an innovative density-phase microscope and original ideas that combine the best of matter-wave and condensed-matter methods to unveil the extraordinary properties of supersolids. With a two-layer superfluid-supersolid configuration we will measure both density and phase of the supersolid. With controllable optical potentials we will realize large, homogeneous crystal geometries in 1D and 2D. With high-resolution optical addressing we will manipulate locally the wavefunction, e. g. creating phase patterns or force fields, and we will follow the local dynamics.
Our main goal is to explore fundamental properties that are largely unknown even theoretically: variable superfluid density under rotation; variable angular momentum of quantized vortices; dissipation-less deformation of the crystal; Josephson effect without barriers; quantum entanglement properties.
We will also attempt the realization of new types of supersolid, to prove the generality of the phenomena: with coupled supersolid layers, we will move towards supersolidity in 3D; using a quasi-2D environment, we will attempt to realize two proposed types of strongly interacting and strongly correlated supersolids.
Our work will establish connections between supersolids and other patterned quantum phases, such as pair-density waves in superconductors and in helium superfluids, intertwined phases in low-dimensional superfluids, and pasta phases in neutron stars. Our work might also open directions for the realization of materials with novel functionalities.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the first reporting period we have progressed towards four of the five planned objectives:
Objective 1: Large, homogeneous supersolids from 1D towards 3D - We designed and partially realized a new experimental setup based on novel techniques, which will be soon able to produce larger dipolar supersolids, in a faster and more reliable manner and with the possibility of adding a high-resolution microscope and various types of optical lattices. In the new setup we have achieved laser cooling of dysprosium atoms, and we are proceeding towards Bose-Einstein condensation and supersolidity.
Objective 2: Supersolid microscope - On the original experimental setup, we designed and implemented a novel imaging system with a resolution of about 1.5 microns, which is sufficiently large to resolve the individual sites of the supersolids.
Objective 3: An extraordinary superfluid - We implemented a repulsive optical potential for the dipolar supersolid and shaped it through a DMD-based optical system. The main goal is to realize an annular supersolid to study the peculiar phase-density profiles of a rotating supersolid. First characterizations of a dipolar Bose-Einstein condensate in an annular optical potential are in progress.
Objective 4: An extraordinary crystal - We discovered that a supersolid can sustain coherent Josephson oscillations, similarly to an array of Josephson junctions, and that the superfluid dynamics of the junctions allows to determine directly the so-called superfluid fraction, i.e. the key quantity that distinguishes a supersolid from ordinary superfluids and crystals. We developed a method to excite locally the Josephson junction by phase imprinting on our inhomogeneous (trapped) supersolids, and we surprisingly found that the subsequent Josephson dynamics is the same that would appear in a fully homogeneous systems, simply due to the parity of both trap potential and phase excitation. The peculiar behaviour of the superfluid fraction as a function of the depth of the lattice modulation confirmed that supersolids are indeed a quantum phase of matter distinct from ordinary crystals and superfluids.
The experimental work was complemented by theoretical studies of rotating supersolids and of the roton instability leading to the quantum phase transition from superfluids to supersolids.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The most significant achievement of the project, so far, is the discovery of the Josephson effect in supersolids and the subsequent measurement of the superfluid fraction, as described in the publication:
Measurement of the superfluid fraction of a supersolid by Josephson effect, G. Biagioni, N. Antolini, B. Donelli, L. Pezzè, A. Smerzi, M. Fattori, A. Fioretti, C. Gabbanini, M. Inguscio, L. Tanzi, G. Modugno, Nature, 629, 773–777 (2024).
We consider the discovery of the Josephson effect in supersolids and the subsequent measurement of the superfluid fraction a breakthrough, for various reasons. 1) The supersolid is the only spontaneous crystal in nature that exhibits the Josephson effect, a proof of its superfluid character. 2) The Josephson effect allowed to measure for the first time the superfluid fraction of a supersolid. 3) Since the Josephson-array structure of supersolids is spontaneous, it possesses a Goldstone mode that opens the way to a new class of Josephson junctions with novel entanglement properties.