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 Pfau’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 state’s properties. Thanks to our collaboration with the Santos’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 helium’s counterintuitive behavior since the early 1940’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.
