Shintaro Taie

Topological Thouless pumping of ultracold fermions

Charge transport in a cyclically time-modulated periodic potential, also known as a topological Thouless pump, has been realized in an ultracold gas of fermionic atoms. An electron gas in a one-dimensional periodic potential can be transported even in the absence of a voltage bias if the potential is slowly and periodically modulated in time. Remarkably, the transferred charge per cycle is sensitive only to the topology of the path in parameter space. Although this so-called Thouless charge pump was first proposed more than thirty years ago1, it has not yet been realized. Here we report the demonstration of topological Thouless pumping using ultracold fermionic atoms in a dynamically controlled optical superlattice. We observe a shift of the atomic cloud as a result of pumping, and extract the topological invariance of the pumping process from this shift. We demonstrate the topological nature of the Thouless pump by varying the topology of the pumping path and verify that the topological pump indeed works in the quantum regime by varying the speed and temperature.

Submicron spatial modulation of an interatomic interaction in a Bose-Einstein condensate.

We demonstrate submicron spatial control of interatomic interactions in a Bose-Einstein condensate of ytterbium (Yb). A pulsed optical standing wave, tuned near an optical Feshbach resonance, varies the s-wave scattering length continuously across the standing wave pattern. The modulated mean-field energy with a spatial period every 278 nm is monitored by a diffraction pattern in a time-of-flight image. We observe a wide scattering length control of up to 250 nm. The demonstrated spatial modulation of the scattering length proves that high resolution control of atomic interactions is possible.

Realization of a SU(2) X SU(6) System of Fermions in a Cold Atomic Gas

We report the realization of a novel degenerate Fermi mixture with an SU(2)×SU(6) symmetry in a cold atomic gas. We successfully cool the mixture of the two fermionic isotopes of ytterbium Yb with the nuclear spin I = 1/2 and Yb with I = 5/2 below the Fermi temperature TF as 0.46TF for Yb and 0.54TF for Yb. The same scattering lengths for different spin components make this mixture featured with the novel SU(2)×SU(6) symmetry. The nuclear spin components are separately imaged by exploiting an optical Stern-Gerlach effect. In addition, the mixture is loaded into a 3D optical lattice to implement the SU(2)×SU(6) Hubbard model. This mixture will open the door to the study of novel quantum phases such as a spinor Bardeen-Cooper-Schrieffer-like fermionic superfluid.

An SU(6) Mott insulator of an atomic Fermi gas realized by large-spin Pomeranchuk cooling

A multicomponent gas of ytterbium atoms accommodates more entropy in its spin degrees of freedom than does its two-component analogue, leading to a lower effective temperature, and an easy route for cooling ultracold fermions towards a Mott-insulating state.

Coherent driving and freezing of bosonic matter wave in an optical Lieb lattice

Matter-wave dynamics reveals a flat energy band engineered in a novel optical lattice. Although kinetic energy of a massive particle generally has quadratic dependence on its momentum, a flat, dispersionless energy band is realized in crystals with specific lattice structures. Such macroscopic degeneracy causes the emergence of localized eigenstates and has been a key concept in the context of itinerant ferromagnetism. We report the realization of a “Lieb lattice” configuration with an optical lattice, which has a flat energy band as the first excited state. Our optical lattice potential has various degrees of freedom in its manipulation, which enables coherent transfer of a Bose-Einstein condensate into the flat band. In addition to measuring lifetime of the flat band population for different tight-binding parameters, we investigate the inter-sublattice dynamics of the system by projecting the sublattice population onto the band population. This measurement clearly shows the formation of the localized state with the specific sublattice decoupled in the flat band, and even detects the presence of flat-band breaking perturbations, resulting in the delocalization. Our results will open up the possibilities of exploring the physics of flat bands with a highly controllable quantum system.

Mott insulator of ultracold alkaline-earth-metal-like atoms

The transition from a superfluid to a Mott-insulator (MI) phase has been observed in a Bose-Einstein condensate (BEC) of ytterbium (Yb) atoms in an optical lattice. An all-optically produced BEC of

Interaction-Driven Shift and Distortion of a Flat Band in an Optical Lieb Lattice

^{174}\text{Y}\text{b}

Feshbach-Resonance-Enhanced Coherent Atom-Molecule Conversion with Ultranarrow Photoassociation Resonance.

atoms was loaded into three-dimensional optical lattices produced by a 532 nm laser beam. The interference pattern was measured after releasing the quantum gas from the trapping potential. As increasing the optical lattice depth, we observed the disappearance of the interference patterns, which is a signature of entering the MI regime. This result is an important step into studies by using a combination of the MI state and the ultranarrow optical transition of ultracold alkaline-earth-metal-like atoms.

Observation of ap-wave optical Feshbach resonance

We report the momentum-resolved measurement of Bloch bands in an optical Lieb lattice for a Bose-Einstein condensate (BEC). A BEC in the lattice is transported to a desired quasimomentum by applying a constant force. The energy dispersion of the lowest band is obtained by integrating measured group velocities. We also measure the gap from the lowest band to the higher bands with the same quasimomentum, which can be extracted from the oscillation of the sublattice populations after preparing a superposition of the band eigenstates. We show that the experimental results agree with a band calculation based on the Bogoliubov approximation. It is revealed that the second band, which should be flat in a single-particle description, is shifted and, in particular, distorted around the Brillouin zone edge as the interaction strength increases.

ULTRACOLD YTTERBIUM ATOMS IN OPTICAL LATTICES

We reveal the existence of high-density Feshbach resonances in the collision between the ground and metastable states of ^{171}Yb and coherently produce the associated Feshbach molecules by photoassociation. The extremely small transition rate is overcome by the enhanced Franck-Condon factor of the weakly bound Feshbach molecule, allowing us to observe Rabi oscillations with long decay time between an atom pair and a molecule in an optical lattice. We also perform the precision measurement of the binding energies, which characterizes the observed resonances. The ultranarrow photoassociation will be a basis for practical implementation of optical Feshbach resonances.