Yosuke Takasu

Degenerate Fermi gases of ytterbium.

Evaporative cooling was performed to cool fermionic 173Yb atoms in a crossed optical dipole trap. The large elastic collision rate leads to efficient evaporation and we have successfully cooled the atoms to 0.37+/-0.06 of the Fermi temperature, that is to say, to a quantum degenerate regime. In this regime, a plunge of evaporation efficiency was observed as a result of Fermi degeneracy.

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.

Interaction-Controlled Transport of an Ultracold Fermi Gas

We explore the transport properties of an interacting Fermi gas in a three-dimensional optical lattice. The center of mass dynamics of the atoms after a sudden displacement of the trap minimum is monitored for different interaction strengths and lattice fillings. With increasingly strong attractive interactions the weakly damped oscillation, observed for the noninteracting case, turns into a slow relaxational drift. Tuning the interaction strength during the evolution allows us to dynamically control the transport behavior. Strong attraction between the atoms leads to the formation of local pairs with a reduced tunneling rate. The interpretation in terms of pair formation is supported by a measurement of the number of doubly occupied lattice sites. This quantity also allows us to determine the temperature of the noninteracting gas in the lattice to be as low as (27+/-2)% of the Fermi temperature.

Quantum Degenerate Mixtures of Alkali and Alkaline-earth-like Atoms

We realize simultaneous quantum degeneracy in mixtures consisting of the alkali and alkaline-earth-like atoms Li and Yb. This is accomplished within an optical trap by sympathetic cooling of the fermionic isotope {sup 6}Li with evaporatively cooled bosonic {sup 174}Yb and, separately, fermionic {sup 173}Yb. Using cross-thermalization studies, we also measure the elastic s-wave scattering lengths of both Li-Yb combinations, |a{sub {sup 6}Li-{sup 174}Yb}|=1.0{+-}0.2 nm and |a{sub {sup 6}Li-{sup 173}Yb}|=0.9{+-}0.2 nm. The equality of these lengths is found to be consistent with mass-scaling analysis. The quantum degenerate mixtures of Li and Yb, as realized here, can be the basis for creation of ultracold molecules with electron spin degrees of freedom, studies of novel Efimov trimers, and impurity probes of superfluid systems.

All-Optical Formation of Quantum Degenerate Mixtures

Ultracold atomic gases have provided deep insight intoquantum many-body systems, since the realization of aquantum degenerate gas, such as a Bose-Einstein con-densate (BEC) [1] and a degenerate Fermi gas [2]. Oneof the intriguing new developments in this field is a studyof quantum degeneracy in a mixed gas. An advantage inthe study using ultracold atomic mixed gases is the abil-ity to select the statistics of atomic gases: a Bose-Bose,Fermi-Bose and Fermi-Fermi mixture. The interactions,which determine the stability and the dynamics of themixed-gas system, can be also tuned by changing a com-bination of a mixed gas or by using magnetic and opticalFeshbach resonances [3, 4, 5]. So far, Fermi-Bose mix-tures in the degenerate regime have been realized withvarious combinations of atoms:

Injection-Locking of Blue Laser Diodes and Its Application to the Laser Cooling of Neutral Ytterbium Atoms

We present a simple, laser-diode-based ultraviolet light source with sufficient power and narrow linewidth. In this scheme, an injection-locking technique was applied, for the first time to our knowledge, to a GaN blue laser diode (LD). By injecting ~5 mW output light of the external cavity laser diode (ECLD) into two free-running blue slave LDs, we could obtain an output power of over 30 mW from the injection-locked slave LDs. The characterisitics of the injection-locked slave LD were studied in detail. We also present an absolute frequency-locking scheme with about a 500 MHz locking bandwidth, where we achieved offset-locking of the frequencies of two ECLDs. As an interesting example of the application of these developments, we demonstrated laser cooling and trapping of ytterbium atoms.

Controlled production of subradiant states of a diatomic molecule in an optical lattice.

We report the successful production of subradiant states of a two-atom system in a three-dimensional optical lattice starting from doubly occupied sites in a Mott insulator phase of a quantum gas of atomic ytterbium. We can selectively produce either a subradiant 1(g) state or a superradiant 0(u) state by choosing the excitation laser frequency. The inherent weak excitation rate for the subradiant 1(g) state is overcome by the increased atomic density due to the tight confinement in a three-dimensional optical lattice. Our experimental measurements of binding energies, linewidth, and Zeeman shift confirm the observation of subradiant levels of the 1(g) state of the Yb(2) molecule.

A Three-Dimensional Optical Lattice of Ytterbium and Lithium Atomic Gas Mixture

We develop an optical lattice system for an ultracold atomic gas mixture of ytterbium (174Yb) and lithium (6Li), which is an ideal system for studying disorder and impurity problems. We load a Bose–Einstein condensate of 174Yb into a three-dimensional optical lattice and observe its interference patterns in time-of-flight (TOF) images. Furthermore, we perform laser spectroscopy of 174Yb in an optical lattice using the ultranarrow optical transition 1S0–3P2 in both cases with and without 6Li. Owing to the weak interspecies interaction, we do not observe clear effects of 6Li in the obtained interference patterns or excitation spectra. However, this is an important first step in the optical control of atomic impurity in ultracold fermions. We also measure the polarizabilities of the 3P2 state of 174Yb atoms in an optical trap with a wavelength of 1070 nm. We reveal that the polarizability can be tuned to be positive, zero, or the same as the ground state, which is useful for certain applications.

Observation of the Mott insulator to superfluid crossover of a driven-dissipative Bose-Hubbard system

We engineer the on-site dissipation to reveal its impact on the quantum phase transition from Mott insulator to superfluid. Dissipation is ubiquitous in nature and plays a crucial role in quantum systems such as causing decoherence of quantum states. Recently, much attention has been paid to an intriguing possibility of dissipation as an efficient tool for the preparation and manipulation of quantum states. We report the realization of successful demonstration of a novel role of dissipation in a quantum phase transition using cold atoms. We realize an engineered dissipative Bose-Hubbard system by introducing a controllable strength of two-body inelastic collision via photoassociation for ultracold bosons in a three-dimensional optical lattice. In the dynamics subjected to a slow ramp-down of the optical lattice, we find that strong on-site dissipation favors the Mott insulating state: The melting of the Mott insulator is delayed, and the growth of the phase coherence is suppressed. The controllability of the dissipation is highlighted by quenching the dissipation, providing a novel method for investigating a quantum many-body state and its nonequilibrium dynamics.

Quantum Degenerate Gases of Ytterbium Atoms

We describe in detail our recent experiments on quantum degenerate gases of ytterbium (Yb) atoms. Up to now Bose–Einstein condensates of 174 Yb and 170 Yb, and a Fermi degenerate gas of 173 Yb have been realized by efficient evaporative cooling in a crossed far-off resonant trap. Our studies showed that the efficient escape from the trap and rapid thermalization are important for efficient evaporative cooling. We also describe some recent experiments using Yb Bose–Einstein condensates such as the achievement of a scalar atom laser, the observation of an optical coherent transient, and successful loading into an optical lattice. Finally, the future prospects using Yb quantum degenerate gases are discussed in detail.