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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DIPPHASE (Exotic quantum phases with dipolar Fermi gases of spin-polarized Erbium atoms in reduced dimensions)

Teaser

Summary

The achievement of quantum degeneracy in dilute gases of neutral atoms marked the advent of a new era in atomic physics. Such systems provide a pristine and powerful platform to study problems usually encountered in condensed-matter, nuclear, or even astro-physics, with high level of control on both external and internal degrees of freedom. For many years, mostly ultracold gases of alkali atoms (Li, Na, K, Rb, Cs) have been available. More recently, the field moved towards more complex and rich scenarios, by using unconventional atomic species, such as alkaline-earths (Sr), magnetic (Dy, Er) and non-magnetic (Yb) lanthanides. Magnetic atoms are of particular interest as they do not simply interact via short-range interactions, as do most of the other species, but also exhibit prominent dipole-dipole interactions (DDI). The DDI, by being both long-ranged and anisotropic, yields exotic few- and many-body effects at the quantum level.
The Innsbruck group hosting the Dipphase project pioneered quantum degeneracy of Er atoms in 2012 (folowing that of Dy (Lev\'group, USA, 2011)). Within the Dipphase project, we successfully demonstrated the impact of the DDI on the behavior of such gases, on the phases of matter and their excitations.

Work performed

In our work, we have study the dipolar effect on quantum gases of either of the two fundamental classes of particles of quantum physics, namely bosons and fermions.
First, we focused on quantum gases of bosons, the famous Bose-Einstein condensates (BECs). Here we focused on the regime where the DDI is made dominant over the other (short-range) interactions.
In this regime, a breakthrough was made in late 2015 in the Pfauâ€™s group (Stuttgart): They observed that a BEC of Dy atoms, instead of collapsing formed stable structures made of tiny droplets. This discovery has raised huge attention. By extending the study of the novel droplet state in our setup, we contributed to its understanding as well as in demonstrating its universality by using a distinct species, Er. We observed the first isolated and large droplet, containing all the atoms of the BEC. This was important step, enabling a direct study of the droplet stateâ€™s properties. Thanks to our collaboration with the Santosâ€™s group (Hannover), we also quantitatively proved the role of quantum fluctuations, whose strength is promoted by the DDI, in stabilizing the new state of matter against the collapse promoted by an average attractive interaction.
Distinctly from the droplet state, an experimental proof of the existence of a special kind of collective excitation of the BEC, so called roton excitation, was awaited in the dipolar community for many years. This mode correspond to an excitation of low energy but of large wavenumber, and, while usually the energy increases with the wavenumber, here it forms a minimum. Such a roton mode has been fundamental in understanding heliumâ€™s counterintuitive behavior since the early 1940â€™s. Despite this long-standing interest, the roton mode still raises unanswered questions. In our experiment, also benefiting from theory collaborations (Santos, van Bijnen), we observed for the first time the dipolar roton mode and investigated the characteristic scalings of its energy and wavenumber.
Second, we studied quantum gases of fermions. Here we have also added a degree of freedom in the physics at play, that of the internal state of the atom, its so-called spin.
In a first study, we isolated two of this internal states, making thus an effective spin-1/2 system. Spin-1/2 is a fundamental component in quantum physics, corresponding to the case of electrons, thus sketching a parallel to conduction physics in solids. In our system, we first characterize the interactions between the two spin states and observe numerous resonant features in their scattering. We identified one broader and isolated feature which can be used to probe the strongly-interacting regime. This bring a first step toward studying a possible superfluid pairing of atoms in different spin states. In the future, we will be interested to study the impact of the DDI on this physics.
In a second study, we consider the full space of the 20 spin states of Er. To perform a first study of the long-range DDI on these spin state, we played a trick and confined the gas in a deep lattice, pinning each atom to one lattice well. Here spins evolve under the effect of the mere DDI between lattice sites. In collaboration with the Rey group (USA), we demonstrate the quantum many-body nature of the effect and the underlying growth of correlations.
These works have been published in high-impact peer-reviewed journals, while available in open access on Arxiv. Press releases for a broader audience were published on various websites and newspapers. In addition, these results have been presented in various international conferences, workshops, and seminars.

Final results

All these results have opened up broad opportunities within our field and beyond it, as proven by the number of citations to our articles.
For instance, our pioneer realization of an isolated droplet state has experimentally paved the way to the demonstration of its self-bound character and to a better control of their assemblies by the Pfauâ€™s group. Our contribution in demonstrating the universal quantum-fluctuation stabilization mechanism has also helped to open a blossoming field, again enlarged by the recent observation of sibling droplet states in mixtures of contact interacting gases. Many properties of these droplets remain to be explored.
Our observation of the roton mode has revived the huge interest in the specialty of this excitation, its impact on superfluidity, as well as, more recently, its connection to the formation of droplet assemblies. A special reason of interest in the dipolar roton lies in the fact that it uniquely resembles the helium roton, arising from interparticle interactions. Yet the dipolar roton do not requires strong interactions, contrarily to the helium case, and can be distinctly understood from first principles and within simple theories, thus bringing new insights into the roton physics. Finally, our work strongly connects to the possibility of a supersolid state arising from the interparticle interactions, which has been a long-lasting goal of physicists since its prediction, yet unsuccessful realization, in superfluid helium.
Turning to fermions, our pioneer study of effective spin-1/2 degenerate dipolar raises huge interest as the DDI may importantly change the nature of superfluid pairing, compared to the case of alkali atoms, connection to some long-lasting issue in electrons systems. For instance, its intrinsic anisotropy may yield an anisotropic pairing mechanism, and connects to proposed mechanisms for high-temperature superconductivity.
Our study of dipolar spin mixtures in lattices shows an unprecedented degree of control. The DDI brings an added complexity compared to lattice systems of non-magnetic atoms, getting closer to their electronic counterparts and of their famous models (e.g. Heisenberg, t-J modelsâ€¦). Our realization opens possibilities to study exotic magnetic phases or more intricate dynamics. Last but not least, this lattice system opens novel prospects for quantum computation, as it realizes a very large particle assembly, with an additional possibility of encoding information into the huge spin (i.e. beyond spin-1/2 qubits), while correlations can be engineered via the DDI.