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NOMBQUANT Report Summary

Project ID: 341197
Funded under: FP7-IDEAS-ERC
Country: France

Mid-Term Report Summary - NOMBQUANT (Novel phases in quantum gases: from few-body to many-body physics)

The project NOMBQUANT is dedicated to developing new methods to create ultracold gases with unexplored many-body properties and to solve intriguing problems, such as the existence of itinerant ferromagnets for two-component Fermi gases. The project includes the studies ranging from inducing/implementing resonant or long-range interaction between particles to describing states with non-conventional localization and diffusion properties in a long-range disorder.
In the present stage, we have published the work on itinerant ferromagnetism in two-component Fermi gases (Phys.Rev.A, 94,011601(2016)) in collaboration with X-W. Guan/Y.Jiang from Wuhan, China. In a one-dimensional (1D) two-component Fermi gas with an infinite intercomponent contact repulsion, adding an attractive nearly resonant odd-wave momentum-dependent interaction breaking the rotational symmetry can make the ground state ferromagnetic. A promising system to observe this itinerant ferromagnetic state is the 1D gas of 40K atoms, where the 3D s-wave Feshbach resonance (even-wave in 1D) is very close to the intracomponent p-wave (odd-wave in 1D) resonance, which allows a coexistence of the infinite contact repulsion and the necessary odd-wave attraction. Moreover, the 1D confinement reduces the rates inelastic decay processes. The required temperatures are about 20 nK or lower, and the experiment on achieving the ferromagnetic state in the two-component 1D gas of 40K is now underway in the lab of F. Schreck in Amsterdam.
We have published the works on interlayer resonances in bilayer systems of bosonic polar molecules (Phys.Rev.Lett,112,103201(2014)) and revealed stable supersolid states in these systems (Phys.Rev.Lett.115,075303(2015)). The resonance is provided by a virtual tunneling of colliding dipoles to the bound interlayer state in which one of the dipoles is already in the neighboring (initially empty) layer. This allows one to switch the contact two-body interaction from positive to negative or make it zero. Then the three-body interaction comes into play and it was proven to be repulsive. This repulsion, together with two-body contact and long-range dipolar (momentum-dependent and attractive) interactions may lead to a supersolid ground state, in which the condensate wavefunction is a superposition of a uniform background and lattice structure. The phase diagram contains various types of supersolid states, uniform superfluid, and collapsing regime. We have also shown that Bose-Bose mixtures, collapsing due to a strong mean-field interspecies attraction, can get to a dilute droplet state stabilized by quantum fluctuations (Phys.Rev.Lett.115,155302(2015)). Our approach was then extended to dipolar gases, where it leads to droplets with a particle number exceeding the mean-field prediction for collapse. Such droplets were observed in experiments in Stuttgart (T.Pfau).
Our work on exotic superfluids described stable topological p-wave superfluids of identical microwave-dressed fermionic polar molecules in a 2D optical lattice, which may emerge due to a long-range character of the dipole-dipole interaction (Scientific Reports,6,27448(2016)). We also predicted a p-wave interlayer superfluid of polar molecules in the bilayer geometry, which can be a quantum simulator of superconductivity in layered condensed matter systems.
In the domain of disordered quantum systems we completed the picture of finite-temperature localization-delocalization transitions for 1D disordered bosons: At any temperature an increase in the interaction strength first leads to the insulator-fluid transition and at sufficiently large interaction one has a reentrance to the insulator state (PNAS,113,E4455(2016)). Weakly interacting bosons in the 1D quasiperiodic potential exhibit a ‘’freezing with heating phenomenon’’: An increase in temperature may provide a fluid to insulator transition (Phys.Rev.Lett.113,045304(2014)). Eventually, we considered dipolar excitations propagating via a long-range dipole-induced exchange among immobile molecules randomly spaced in a lattice (Phys.Rev.Lett.117,020401(2016)). The character of the propagation is determined by long-range hops (Levy flights), and in 1D and 2D all eigenstates are localized. In contrast, in 3D all states are extended but not always ergodic, and we identified the regions of ergodic and non-ergodic states. The reduction of the lattice filling induces an ergodic to non-ergodic transition, and the excitations are mostly non-ergodic at low filling.
Our work on resonant interaction between atoms focused on the creation of two-qubit gates in small traps for quantum information processing. Together with the experimental group of M.Zhan/P.Xu (Wuhan, China) we analyzed the requirements for elastic and inelastic interactions in the creation of a collisional quantum gate (Nature Com.,6,7803(2015)). We then did the work on the creation of C-NOT gate in a heteronuclear system of 85Rb and 87Rb, based on Rydberg blockade. The calculations of the blockade shift were relying on the Forster resonance for Rydberg states.

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