We study fundamental features of spin current in a very weakly interacting Fermi gas of $^6$Li. By creating a spin current and then reversing its flow, we demonstrate control of the spin current. This reversal is predicted by a spin vector evolution equation in energy representation, which shows how the spin and energy of individual atoms become correlated in the nearly undamped regime of the experiments. The theory provides a simple physical description of the spin current and explains both the large amplitude and the slow temporal evolution of the data. Our results have applications in studying and controlling fundamental spin interactions and spin currents in ultra-cold gases.

Controlling spin current in a trapped Fermi gas

We report the observation of coherent heteronuclear spin dynamics driven by interspecies spin-spin interaction in an ultracold spinor mixture, which manifests as periodical and well-correlated spin oscillations between two atomic species. In particular, we investigate the magnetic field dependence of the oscillations and find a resonance behavior which depends on both the linear and quadratic Zeeman effects and the spin-dependent interaction. We also demonstrate a unique knob for controlling the spin dynamics in the spinor mixture with species-dependent vector light shifts.

http://absimage.aps.org/image/DAMOP15/MWS_DAMOP15-2015-000475.pdf

Coherent heteronuclear spin dynamics in an ultracold spin-1 mixture

In the mixture of ultracold spin-1 atoms of two different species A and B (e.g., $^{23}\mathrm{Na}$ (A) and $^{87}\mathrm{Rb}$ (B)), the interspecies singlet-pairing process ${\mathrm{A}}_{+1}+{\mathrm{B}}_{\ensuremath{-}1}\ensuremath{\rightleftharpoons}{\mathrm{A}}_{\ensuremath{-}1}+{\mathrm{B}}_{+1}$ can be induced by the spin-dependent interatomic interaction, where subscript $\ifmmode\pm\else\textpm\fi{}1$ denotes the magnetic quantum number. Nevertheless, one cannot isolate this process from other spin-changing processes, which are usually much stronger, by tuning the bias real magnetic field. As a result, it is difficult to clearly observe the singlet-pairing process and precisely measure the corresponding interaction strength. In this work we propose to control the singlet-pairing process via combining the real magnetic field and a laser-induced species-dependent synthetic magnetic field. With our approach one can significantly enhance this process and simultaneously suppress all other spin-changing processes. We illustrate our approach for both a confined two-atom system and a binary mixture of spinor Bose-Einstein condensates. Our control scheme is helpful for the precise measurement of the weak singlet-pairing interaction strength and the entanglement generation of two different atoms.

Laser control of the singlet-pairing process in an ultracold spinor mixture

This work studies heteronuclear spinor gases and characterizes the collision channels that can transfer magnetization from one element to another, using spin relaxation from different initial states.

Collisional spin transfer in an atomic heteronuclear spinor Bose gas

After decades of improvements in cooling techniques of several atomic species and in finding methods for the achievement of stable quantum mixtures, the field is now ready for an extensive use of such a versatile experimental platform for the investigation of various physical problems. Among them, relevant examples are the dynamics of impurities in a quantum gas, the miscibility condition of different gases, the study of exotic topological structures, the interplay between magnetism and superfluidity, the formation of artificial molecules or new few-body states. We illustrate the differences among possible quantum mixtures — whether homonuclear spin mixtures or heteronuclear ones — and show how they can be exploited to investigate a plethora of topics from the few-body to the many-body regime. In particular, we discuss quantum mixtures of ultracold gases under three different perspectives: systems made of a few atoms of different kinds, single impurities immersed in a host quantum gas and quantum mixtures of two interacting gases. We restrict the discussion to single harmonic or flat traps, predominantly in a 3D configuration. A selection of results on recent experiments and possible interesting future directions are given. Three-dimensional, quantum mixtures of ultracold gases of neutral atoms are ideal platforms for probing the physics of many-body systems because the interparticle interactions can be fine-tuned externally. This Review introduces a range of active topics under investigation: topological defects, the interplay of superconductivity and magnetism, novel few-body states and more. Mixtures of atoms can be realized either using different ‘spin’ states of the same atomic species (homonuclear mixtures), or using different atomic species or isotopes (heteronuclear mixtures). The main differences concern the different levels of experimental complexity, the possibility to independently manipulate the two components, the quantum statistics and mass ratio of the constituents, and the interconversion between the two components of the mixture. The combination of ultralow collision energies, quantum statistics of the scattering particles and their mass asymmetry promotes the appearance of various novel few-particle cluster states, exhibiting different types of spatial correlations and spectral properties. When one atom (impurity) is embedded in a bath of different particles with which it interacts, it dresses itself with excitations of the bath and can be described in terms of a quasiparticle (polaron), which is an object that behaves as a free particle but with renormalized properties, such as energy and mass. The behaviour of two bosonic gases is mainly given by the combination of their two-body interaction strengths, with interesting features for small values of intercomponent interactions. For repulsive interspecies interaction, the gases can mix and self-organize in spatially separated domains, whereas for increasing attractive ones they can either form stable solitons or quantum droplets or collapse. The physics of two distinguishable fermionic gases is dominated by the crossover from a superfluid of tightly bound molecules to a superfluid of loosely bounded pairs while passing from strongly repulsive to strongly attractive intercomponent interactions. Mixtures of atoms can be realized either using different ‘spin’ states of the same atomic species (homonuclear mixtures), or using different atomic species or isotopes (heteronuclear mixtures). The main differences concern the different levels of experimental complexity, the possibility to independently manipulate the two components, the quantum statistics and mass ratio of the constituents, and the interconversion between the two components of the mixture. The combination of ultralow collision energies, quantum statistics of the scattering particles and their mass asymmetry promotes the appearance of various novel few-particle cluster states, exhibiting different types of spatial correlations and spectral properties. When one atom (impurity) is embedded in a bath of different particles with which it interacts, it dresses itself with excitations of the bath and can be described in terms of a quasiparticle (polaron), which is an object that behaves as a free particle but with renormalized properties, such as energy and mass. The behaviour of two bosonic gases is mainly given by the combination of their two-body interaction strengths, with interesting features for small values of intercomponent interactions. For repulsive interspecies interaction, the gases can mix and self-organize in spatially separated domains, whereas for increasing attractive ones they can either form stable solitons or quantum droplets or collapse. The physics of two distinguishable fermionic gases is dominated by the crossover from a superfluid of tightly bound molecules to a superfluid of loosely bounded pairs while passing from strongly repulsive to strongly attractive intercomponent interactions.

Quantum mixtures of ultracold gases of neutral atoms

We observe spin transfer within a non-degenerate heteronuclear spinor atomic gas comprised of a small $^7$Li population admixed with a $^{87}$Rb bath, with both elements in their $F=1$ hyperfine spin manifolds and at temperatures of 10's of $\mu$K. Prepared in a non-equilibrium initial state, the $^7$Li spin distribution evolves through incoherent spin-changing collisions toward a steady-state distribution. We identify and measure the cross-sections of all three types of spin-dependent heteronuclear collisions, namely the spin-exchange, spin-mixing, and quadrupole-exchange interactions, and find agreement with predictions of heteronuclear $^7$Li-$^{87}$Rb interactions at low energy. Moreover, we observe that the steady state of the $^7$Li spinor gas can be controlled by varying the composition of the $^{87}$Rb spin bath with which it interacts.

pdf/collisional-spin-transfer-in-an-atomic-heteronuclear-spinor-2fl2k3f0fm.pdf

Interspecies interaction strength between ${}^{7}$Li and ${}^{87}$Rb atoms through cross-dimensional relaxation of two-element gas mixtures is measured experimentally and calculated with Monte Carlo simulations. A disagreement between those two results provides an interesting challenge for interpretation and may stimulate further work.

Cross-dimensional relaxation of Li 7 − Rb 87 atomic gas mixtures in a spherical-quadrupole magnetic trap

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Collisional spin transfer in an atomic heteronuclear spinor Bose gas

Collisional spin transfer in an atomic heteronuclear spinor Bose gas

Cross-dimensional relaxation of Li 7 − Rb 87 atomic gas mixtures in a spherical-quadrupole magnetic trap