Session B2: Focus Session: Quantum Magnetism

Magnetic Excitations in Spinor Bose-Einstein Condensates

The quantum degenerate spinor Bose gas is a new material characterized by both magnetic and superfluid order. Like other ordered magnetic materials, the gas supports magnon excitations, which are the Nambu-Goldstone bosons associated with the spontaneous breaking of rotational symmetry. We have developed techniques to create and image magnon excitations in ferromagnetic rubidium spinor condensates. At short times after their creation, magnons are observed to propagate coherently, allowing us to measure their energy dispersion with high precision through interferometry. Using high-resolution spin-sensitive imaging, we measure the magnon spectrum to be gapped due to magnetic dipole interactions (as it often is in magnetic solids). At longer times, the magnons thermalize. We show that this thermalization allows one to measure the temperature of highly degenerate gases, and to reduce this temperature further by a new form of evaporative cooling.   

Resonantly enhanced spin-spin interaction of ultracold atoms in an optical lattice for quantum information and simulation

Control of the spin-spin interactions between atoms in an optical lattice is a key ingredient for simulating quantum magnetism and also creating entanglement required for quantum computation. Here, we investigate the use of resonant enhancement of the perturbative spin interactions. First, we discuss entanglement generation with a tunable Ising interaction. Enhancing the interaction allows us to shorten operation time. However, it conflicts with the perturbative nature of the interaction and inevitably induces unwanted correlations that degrade fidelity. We propose a method for overcoming this difficulty. Next, we also discuss characteristic magnetism caused by the resonantly enhanced interaction. In the similar way to the above, the transition temperatures can be increased, which is limited by the breakdown of the perturbation. We will discuss the mechanism of the limitation. This work was partly supported by JST CREST.   

Spin-Domain and Superfluid Transition in a shaken optical lattice

Recently, we have developed a resonant lattice shaking technique to induce an effective ferromagnetic transition in a cesium Bose condensate. By phase modulating the optical lattice beam at the frequency near the band gap, we create a double well structure in the energy dispersion. Based on in situ imaging, we observe spatial domains of condensates pinned to one of the two wells. In this work, we extend this technique to form a desired domain structure based by imprinting a potential grating to the sample. We also explore the superfluid transition at finite temperatures in optical lattices.   

Detection of Antiferromagnetic Correlations in the Fermi-Hubbard Model

The Hubbard model, consisting of a cubic lattice with on-site interactions and kinetic energy arising from tunneling to nearest neighbors is a ``standard model'' of strongly correlated many-body physics, and it may also contain the essential ingredients of high-temperature superconductivity. While the Hamiltonian has only two terms it cannot be numerically solved for arbitrary density of spin-1/2 fermions due to exponential growth in the basis size. At a density of one spin-1/2 particle per site, however, the Hubbard model is known to exhibit antiferromagnetism at temperatures below the N\'{e}el temperature $T_{\mathrm{N}}$, a property shared by most of the undoped parent compounds of high-$T_{\mathrm{c}}$ superconductors. The realization of antiferromagnetism in a 3D optical lattice with atomic fermions has been impeded by the inability to attain sufficiently low temperatures. We have developed a method to perform evaporative cooling in a 3D cubic lattice by compensating the confinement envelope of the infrared optical lattice beams with blue-detuned laser beams. Evaporation can be controlled by the intensity of these non-retroreflected compensating beams. We observe significantly lower temperatures of a two-spin component gas of $^{\mathrm{6}}$Li atoms in the lattice using this method. The cooling enables us to detect the development of short-range antiferromagnetic correlations using spin-sensitive Bragg scattering of light. Comparison with quantum Monte Carlo constrains the temperature in the lattice to 2-3 $T_{\mathrm{N}}$. We will discuss the prospects of attaining even lower temperatures with this method.   

Heralded magnetism in non-Hermitian atomic systems

Cold atoms provide the opportunity to study dissipative quantum systems with novel properties. We study the non-equilibrium phase transitions of the XY model in the presence of an imaginary field. This type of non-Hermitian spin model can be implemented with cold atoms when the up state decays into an auxiliary state instead of the down state. The non-Hermitian model is ``heralded'' by the absence of population in the auxiliary state. The non-Hermitian model exhibits unique behavior compared to the Hermitian model as well as the master equation. There is a sharp phase transition already for two atoms. For a long one-dimensional chain, there is a quantum phase transition from short-range order to quasi-long-range order. The ordered phase features a non-repeating spin pattern without a classical analogue. Our results can be seen experimentally with atoms in optical lattices, trapped ions, and cavity QED.   

Magnetic ordering and dynamics of driven and dissipative spin systems

There has been increasing interest in the study of dissipative and interacting quantum systems due to the advances of experimental implementations based on trapped ions and atomic ensembles. Here, we theoretically study the Heisenberg spin model under coherent driving and dissipation. The competition among the coherent drive, spin interaction and dissipation leads to an enriched steady state phase diagram comprising of various spin orderings, bi- and tri-stabilities, as well as self-sustaining spin oscillations. The non-equilibrium nature of the dissipative spin system can render the power spectrums and the spatial correlation functions to be time dependent.   

P-orbital chiral Bose liquid and dynamical signature of chiral order

Recent experiments on p-orbital atomic bosons have suggested the emergence of a spectacular ultracold superfluid with an antiferromagnetic orbital current pattern in optical lattices. This raises fundamental questions concerning the effects of thermal fluctuations as well as possible ways of directly observing such angular momentum order. Here we show via Monte Carlo simulations that thermal fluctuations destroy this superfluid in an unexpected two-step process, unveiling an intermediate normal phase with spontaneously broken time-reversal symmetry, dubbed ``chiral Bose liquid.'' For integer fillings in the chiral Mott regime, thermal fluctuations are captured by an effective orbital Ising model, allowing us to determine transition from this intermediate liquid to the para-orbital normal phase at high temperature. A lattice quench is designed to convert the staggered angular momentum into coherent orbital oscillations, providing a direct time-resolved dynamical signature of chiral order. Such quenches may also be used to simulate spin dynamics using orbital degrees of freedom.