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Novel phases in quantum gases: from few-body to many-body physics

Final 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.
We have done the work on itinerant ferromagnetism in two-component Fermi gases (Phys.Rev.A 94,011601(2016)). 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. We have published the works on interlayer resonances in bilayer systems of bosonic polar molecules (Phys.Rev.Lett,112,103201(2014)) and found 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 leads to repulsive three-body interaction which prevents the collapse and, 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. Our most pronounced work in this domain shows 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 extended to dipolar gases, where it explains the emergence of dipolar droplet arrays with supersolid properties observed in recent remarkable experiments (G. Modugno, Florence; T.Pfau Stuttgart; F. Ferlaino, Innsbruck). The recent work of our team (J. Exp. Theor. Phys. 127, 865 (2018)) calculates the drag force acting on an impurity moving in the supersolid stripe phase of a spin-orbit coupled Bose-Einstein condensate and shows that for slow impurities superfluidity can survive on a time scale of seconds. The work on exotic superfluids proposed interlayer FFLO states in an imbalanced system of polar molecules in the bilayer geometry (Phys. Rev. A, 96, 061602 (2017)). We also described stable topological p-wave superfluids of fermionic atoms (Phys. Rev. A, 95, 043615 (2017)) and that of identical microwave-dressed fermionic polar molecules in a 2D optical lattice (Scientific Reports,6,27448(2016)).
In the domain of disordered quantum systems we completed the picture of finite-temperature localization-delocalization transitions for 1D and 2D disordered bosons (PNAS 113, E4455(2016); Phys. Rev. Lett., 121, 030403 (2018); Phys. Rev. A, 100, 013628 (2019) ). 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)). In the most profound work in this domain we considered disordered quantum systems with long-range hops between the lattice sites. In an irregular one-dimensional lattice most of single-particle (single-excitation) states remain localized (Phys. Rev. Lett. 120, 110602 (2018)). However, for dipolar excitations propagating via a long-range dipole-induced exchange among immobile molecules randomly spaced in a 3D lattice all eigenstates are extended, but not always ergodic (Phys.Rev.Lett.117,020401(2016)). We identified the bands of ergodic and non-ergodic states and indicated a possibility of ergodic-nonergodic phase transition by changing the filling factor or energy. This novel phase transition was clearly identified (through the dynamics of expansion of initially localized wave packet) in our work on excitations in a quasiperiodic 1D lattice with long-range power-law hops (Phys. Rev. Lett. 123, 025301 (2019)), which can be created for trapped ions.
Our work on resonant interaction between atoms was largely focused on the creation of two-qubit gates in small traps for quantum information processing and it was performed 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 provided a theoretical description of the first experiment on the creation of the CNOT gate and entanglement of non-identical atoms (85Rb and 87Rb), which allows a strong reduction of the crosstalk (Phys. Rev. Lett. 119, 160502 (2017)).