Experiments

Quantum degenerate dipolar gas of dysprosium atoms

Long-range interactions, such as the Coulomb interaction between electrons and the dipolar interaction between magnetic spins, govern the behavior of most physical systems. In this experiment, a joint project between CNR-INO (sede di Pisa) and LENS, we employ a quantum simulator based on dysprosium atoms, to explore the physics arising from long-ranged dipolar interactions. Contrary to alkali atoms, usually employed in cold atoms experiments, Dy has a huge magnetic dipole moment, 10 Bohr magnetons, the largest among all elements. Therefore, besides interacting via the van der Waals interaction, which has a contact nature, Dy atoms interact via the dipole-dipole interaction, which is both long-range and anisotropic. The combination of these two ingredients leads to the appearance of peculiar quantum phenomena, which we are currently studying in the Dy Lab.

The main achievements so far have been:
In July 2016, we have observed and characterized the magneto-optical trap of Dy-162 [1].
In November 2017, we have achieved our first Bose-Einstein Condensate (BEC) of Dy-162 and we have studied its scattering properties [2].
In 2018, we have discovered a novel quantum phase induced by dipole-dipole interactions in a dipolar quantum gas, the supersolid. Supersolidity is a paradoxical phase of matter that combines the apparently incompatible properties of crystals and superfluids. From 2018, we are characterizing the properties of dipolar supersolids.
In ref. [3], we have reported the first observation of a novel regime that presents supersolid properties, due to the coexistence of periodic density modulation and phase coherence. In a combined experimental and theoretical analysis (with the University of Hannover), we have determined the parameter regime for the formation of coherent droplets, whose lifetime of a few tens of milliseconds is limited by the eventual destruction of the modulated pattern due to three-body losses. Our results open intriguing prospects for the development of long-lived dipolar supersolids.
In ref. [4], we have investigated the excitation properties of the supersolid. A crucial feature of one-dimensional supersolids is the occurrence of two gapless excitations reflecting the Goldstone modes associated with the spontaneous breaking of two continuous symmetries: the breaking of phase invariance, at the origin of superfluidity, and the breaking of translational invariance due to the lattice structure. In our trapped system, the symmetry breaking appears as two distinct compressional oscillation modes, reflecting the gapless Goldstone excitations of the homogeneous system. In cooperation with the University of Trento theoretical group, we have observed that the two modes have different natures, with the higher frequency mode associated with an oscillation of the periodicity of the emergent lattice and the lower one characterizing the superfluid oscillations. Our work paves the way to explore the two quantum phase transitions between the superfluid, supersolid and crystal-like configurations that can be accessed by tuning a single interaction parameter.
We are currently studying the superfluid properties of a dipolar supersolid, by measuring its moment of inertia under slow rotations, in analogy with seminal experiments performed on solid He (see preprint [5]). In particular, we study a peculiar rotational oscillation mode in a harmonic potential, the scissors mode, already employed for standard superfluids. From the measured moment of inertia, we infer a superfluid fraction that is different from zero and of order of unity, providing a direct evidence of the superfluid nature of the dipolar supersolid.

References:
[1] Lucioni E., Masella G., Fregosi A., Gabbanini C., Gozzini S., Fioretti A., Del Bino L., Catani J., Modugno G., Inguscio M., A new setup for experiments with ultracold dysprosium atoms, Eur. Phys. J. Sp. T. 226, 2775–2780 (2017).
[2] Lucioni E., Tanzi L., Fregosi A., Catani J., Gozzini S., Inguscio M., Fioretti A., Gabbanini C., Modugno G., Dysprosium dipolar Bose-Einstein condensate with broad Feshbach resonances, Phys. Rev. A 97, 060701(R) (2018).
[3] Tanzi L., Lucioni E., Famà F., Catani J., Fioretti F., Gabbanini C., Bisset R.N, Santos L. and Modugno G., Observation of a dipolar quantum gas with metastable supersolid properties, Phys. Rev. Lett. 122, 130405 (2019).
[4] Tanzi L., Roccuzzo S.M., Lucioni E., Famà F., Fioretti A., Gabbanini C., Modugno G., Recati A. and Stringari S., Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas, Nature 574, 382-385 (2019).
[5] Tanzi L., Maloberti J.G., Biagioni G., Fioretti A., Gabbanini C., and Modugno G., Evidence of superfluidity in a dipolar supersolid from non-classical rotational inertia, arXiv:1912.01910.

Experimental observation of a dipolar supersolid. Typical momentum distribution of the atomic cloud for different evolution times in three regimes: BEC (top row), supersolid (middle row), and incoherent regime (bottom row).

In a supersolid, the axial breathing mode bifurcates: the higher frequency reflects the oscillation of the periodicity of the emergent lattice, the lower one is associated with the superfluid flow between different density maxima.