# Ultracold Atoms Japan 2024

The Ultracold Atoms Japan was a great sucess! Thank you everyone. Have a safe travel back!

## 9 - 12 April 2024, Okinawa, Japan

Ultracold atoms have in the past two decades become one of the most active and exciting research areas in physics. Cold atomic systems are low energy, pure and highly configurable and therefore allow to create setups in which new and advanced quantum mechanical effects can be studied and engineered. In fact, cold atomic systems are not only at the forefront for discovering fundamental new physics, they are also at the frontier in the development of advanced quantum technologies, such as quantum simulators or quantum metrology devices. The awarding of the Nobel Prizes in 2012, 2001 and 1997 to work in this area can be seen as a recognition of its groundbreaking potential.

The aim of the workshop is to bring together experts representing leading theoretical and experimental groups working in the area of ultracold atoms. In particular it will focus on the connection between Japanese groups and Japan and the international community, and provide platform for exchange and the creation of sustainable connections.

**Location**:OIST Main Campus, OIST Seaside House, Okinawa, Japan

Comfirmed Speakers**:**

Artur Widera, University of Kaiserslautern-Landau (RPTU), Germany

**Quantum engine cycles in ultracold atoms**

Quantum control enables the precise control of quantum systems down to individual particles, bringing the realization, operation and understanding of the smallest quantum machines within reach. A central question is whether quantum effects can improve these machines. I will discuss our recent progress in experimentally answering such questions in two different ultracold-atom setups. First, we immerse single Cs atoms as impurities in an ultracold gas of Rb atoms. Spin-exchange collisions allow a very controlled transfer of energy quanta between bath and impurity. In this way, we realize a non-thermal quantum engine cycle that simultaneously offers high efficiency, high power and low power fluctuations. Second, in a different experimental setup, we investigate whether the significant energy difference between ensembles of different fermionic and bosonic quantum statistics resulting from the Pauli exclusion principle can be utilized as a novel form of energy to drive a quantum engine. We show that an Otto-inspired Pauli cycle can be driven in an ultracold lithium gas by alternately changing the trap frequency and the interaction-induced change in quantum statistics along the BEC-BCS transition, which is superior to comparable cycles based on a change in interactions.

Jae-yoon Choi, Korea Advanced Institute of Science & Technology (KAIST), South Korea

**Far from equailbrium dynamics and quantum Kelvin-Helmholtz instabiltiy in strongly ferromagentic spinor condensates.**

Understanding and classifying out-of-equilibrium dynamics in a closed quantum many-body system have been outstanding problems in modern physics. In this talk, we will introduce our recent experimental results on the universal coarsening dynamics in spin-1 Bose-Einstein condensate. Initially prepared polar condensate is quenched to ferromagnetic phases by microwave dressing. Right after the quench, we observe the emission of spin 1/-1 pairs due to dynamical instability, forming microdomains, which are coarse to form a larger domain as time evolves. We find distinctive scaling behavior depends on the symmetry of the Hamiltonian and associated dynamics of topological defects like domain walls and spin vortices. In the second part of this talk, I will also introduce our recent experiment on the quantum Kelvin Helmholtz instability (KHI). After preparing a single magnetic domain wall, we impose a counterflow by applying a magnetic field gradient. The flutter-finger pattern, the hall mark of the KHI, is observed on the magnetic domain boundary. In the nonlinear dynamic stage, a magnetic droplet is emitted from the tip of the figure, and further analysis shows that it has a fractional skyrmion charge with breaking axis symmetry.

Zhi-Fang Xu, Southern University of Science and Technology, China

**Unconventional superfluidity in orbital optical lattices**

Ultracold atoms trapped in laser-induced periodic potentials have become well-established systems capable of simulating elementary many-body lattice models. In this talk, I will discuss our recent progress in optical lattice simulators, going beyond conventional s-band Hubbard physics. Our approach involves implementing orbital degrees of freedom by utilizing higher Bloch bands. Notable examples include the observation of a chiral p+ip superfluid in a hexagonal lattice and a quantum stripe phase in a triangular lattice. Moreover, we predict the emergence of exotic properties, such as topological quasiparticle excitations and intrinsic anomalous Hall effects.

Ana Maria Rey, University of Colorado Boulder, USA

**Observation of Itinerant magnetism in polar molecule confined in optical lattices**

Photon mediated interactions in polar molecules give rise to strong long-range and anisotropic dipolar interactions. Dipolar interactions provide opportunities for the exploration of a wider range of many-body phenomena which remain difficult to manifest and probe in systems with just contact interactions. One such phenomenon falls under the general heading of itinerant quantum magnetism, where magnetic moments (spins) interact with one another with coupling strength J as they hop in a periodic potential at a rate t. Their dynamics is described by the so-called t-J model, a model originally emerging from the large interaction energy expansion of the Hubbard Model, which is believed to describe the fundamental physics behind high Temperature superconductors. In this talk I plan to report on the first realization of a generalized t-J spin model with dipolar interactions using a system of ultracold fermionic molecules with a spin-1/2 encoded in the two lowest rotational states. To systematically explore the parameter regimes of the model, we apply both microwave and dc electric fields to tune the dipolar interactions which control the strength of the Ising and spin-exchange couplings dipolar interactions independently from each other and from the molecular motion, which is regulated by optical lattices. Using Ramsey spectroscopy, we observe many-body dynamics that depend strongly on the form of the spin couplings and the molecular motional confinement. These observations are supported by theory models established in different motional regimes, and they provide physically intuitive pictures of the phenomena. This study paves the way for future exploration of itinerant quantum magnetism with the tunability of molecular platforms.

Yu-Ju Lin, Institute of Atomic and Molecular Sciences, Taiwan

**Vortex nucleations in spinor Bose condensates under localized synthetic magnetic fields**

Gauge fields are ubiquitous in modern quantum physics. In superfluids, quantized vortices can be induced by gauge fields. Here we demonstrate the experimental observation of vortex nucleations in spinor Bose-Einstein Condensates under radially-localized synthetic magnetic fields. The gauge potentials are azimuthal and created by light-induced spin-orbital-angular-momentum coupling, generating circulating azimuthal velocity fields. A sufficiently large azimuthal velocity peaked near the condensate center results in a dynamically unstable localized excitation that initiates vortex nucleations. This excitation appears as a spontaneously-formed vortex-antivortex pair near the cloud center. Following the initially developed instability, the dynamics is governed by the asymmetry and dissipation, where the atomic orbital angular momentum evolves and can reach the value of the ground state. Our system exhibits dynamical and Landau instabilities and agrees reasonably with time-dependent Gross-Pitaevskii simulations.

Yuki Miyazawa, Tokyo Institute of Technology, Japan

**Bose-Einstein Condensate of europium**

Spinor Bose-Einstein condensates (BECs) with dipole-dipole interaction (DDI), namely spinor dipolar BECs, are attractive research subjects that potentially exhibit many fascinating phenomena, including spin textures and vortices. These phenomena arise from the interplay between spin-dependent contact interactions and DDIs, which provide the coupling between spin and orbital angular momentum from their long-range and anisotropic nature. Europium (Eu), a magnetic lanthanide with a large magnetic dipole moment of 7 Bohr magnetons, features a highly symmetric electronic ground state. This unique property suggests that Eu is a promising candidate for realizing DDI-dominant spinor dipolar BEC, which has never been realized. In this session, I will talk about the realization of Eu BEC and its characteristics, followed by Matsui’s presentation on his recent experimental works using Eu BEC.

Hiroki Matsui, Tokyo Institute of Technology, Japan

**Einstein–de Haas effect in europium Bose–Einstein Condensate**

The magnetic dipole-dipole interaction in an atomic Bose–Einstein condensate (BEC) couples the spin to the orbital degree of freedom due to its anisotropy. Combined with its long-range nature, quantized vortices can appear in an initially spin-polarized BEC in conjunction with the atomic spin relaxation, conserving the total angular momentum, which is considered the equivalent of the Einstein–de Haas effect. We observed this effect in a europium atomic BEC. We prepared a BEC in the spin-polarized state (m=−6) along an external magnetic field and quenched the magnetic field below 100 µG. We let the system evolve and then froze the spins by suddenly increasing the magnetic field. We then performed a Stern–Gerlach experiment and observed spin relaxation in the BEC and a ring structure in the m=−5 spin component in the absorption image. We confirmed the presence of a quantized mass current in the ring-shaped component through matter-wave interference.

Masahito Ueda, The University of Tokyo, Japan

**Beyond-Hermitian Quantum Physics**

Beyond-hermitian physics has recently attracted a great deal of attention due to remarkable advances in experimental techniques and theoretical methods in AMO, condensed matter and nonequilibrium statistical physics. Complete knowledge about quantum jumps allows a description of quantum dynamics at the single-trajectory level. A subclass thereof without quantum jumps can be described by a non-hermitian Hamiltonian. Here, symmetry, topology and many-body effects are fundamentally altered. Importantly, transposition and complex conjugation, which are equivalent in hermitian physics, become inequivalent, leading to proliferation of new topological phases and symmetry classes. In random matrix theory, transposition symmetry leads to two new universality classes of level-spacing statistics other than the Ginibre ensemble. In many-body physics, non-hermiticity leads to the dynamical sign reversal of magnetism in dissipative Hubbard models, violation of the g-theorem in the Kondo problem, and quantum phase transitions without gap closing. In the talk, I will provide an overview on the fundamentals and new frontiers about beyond-Hermitian quantum physics.

Reference: Y. Ashida, Z. Gong, and M. Ueda, Adv. Phys. 69, 249 (2021)

Ippei Danshita, Kindai University, Japan

**Hilbert space fragmentation and quantum many-body scars in Bose-Hubbard systems**

Thanks to their controllability and near-perfect isolation from environment, cold-atom systems has offered unique opportunities for studying far-from-equilibrium dynamics of isolated quantum many-body systems. Understanding how an isolated quantum system thermalizes through unitary time evolution is one of fundamental issues of quantum statistical physics. For this purpose, it is meaningful to investigate mechanisms of systems exhibiting non-ergodic dynamics, such as integrability, many-body localization, quantum many-body scar (QMBS) states, and the Hilbert space fragmentation (HSF). We propose how to experimentally realize non-ergodic dynamics due to QMBS states and HSF, respectively, in systems of optical lattices loaded with ultracold Bose gases, which is paradigmatic among cold-atom systems and can be well described by Bose-Hubbard models. We present actual experimental attempts for observing the latter one.

Kousuke Shibata, Gakushuin University, Japan

**Precise magnetometer with a Bose-Einstein condensate**

Takao Aoki, Waseda University, Japan

**Cavity Quantum Electrodynamics with Optical Nanofibers and Trapped Atoms for Quantum Network **

A quantum network, which consists of many quantum nodes connected by quantum channels, has a wide variety of applications from the implementation of quantum computation to fundamental studies on quantum many-body systems. Quantum nodes are required to be capable of storing and controlling local quantum information as well as to be efficiently interfaced with the quantum channels through which flying quantum information is transmitted. Fiber-coupled cavity quantum electrodynamics (QED) systems in the strong coupling regime are one of the most promising candidates for the quantum nodes. We present our experimental research on a nanofiber cavity QED system with a trapped single atom in the strong coupling regime[1], and the setting of coupled-cavities QED, where two nanofiber cavity QED systems are coherently connected by a meter-long low-loss channel in an all-fiber fashion[2,3], and development of high-finesse nanofiber cavities for achieving high cooperativity in nanofiber cavity QED[4,5]. We also show our recent progress toward distributed quantum computing with nanofiber cavity QED systems.

References:

[1] S. Kato and T. Aoki, Phys. Rev. Lett. 115, 093603 (2015).

[2] S. Kato et al., Nature Communications 10, 1038 (2019).

[3] D. White et al., Phys. Rev. Lett. 122, 253603 (2019).

[4] S. K. Ruddell et al., Opt. Lett. 45, 4875 (2020).

[5] S. Kato and T. Aoki, Opt. Lett. 47, 5000 (2022).

Shoichi Okaba, The University of Tokyo, Japan

**Continuous generation of an ultracold atomic beam using crossed moving optical lattices**

Ultracold atoms in an optical lattice are important in quantum measurements, such as optical lattice clocks, because they have a suppressed Doppler effect and can keep their quantum state for a longer time. However, the fluorescence produced during the cooling process usually disturbs these quantum states, thus requiring that the cooling process and measurements are conducted alternately.

Our study presents a novel method that generates ultracold atoms and transports them continuously with orthogonally crossed moving optical lattices. This approach allows us to realize an ultracold atomic beam in regions unaffected by the fluorescence from the cooling process, making it possible to conduct the cooling process and measurements simultaneously.

This technique enables continuous tracking of the frequency fluctuation of the clock laser of optical lattice clocks. By utilizing this method, the frequency uncertainty Δν of measurements can be improved as Δν∝τ^(-1) for averaging time τ while the improvement is limited to Δν∝τ^(-1/2) in the conventional method due to the untracked clock laser noise. This advancement enhances the precision in optical lattice clocks in shorter measurement time.

**Important Dates:**

Application Open:** 29 November 2023**

Application Deadline:** 9 January 2024 Extended to 12 February**

Notification of Acceptance:

**the end of February**

Participants arrive in Okinawa:

**8 April 2024**

Participants depart Okinawa:

**13 April 2024**

**Main Details: **

Participation fee: 20,000JPY (This is part of the workshop operating budget and a breakdown cannot be provided.)

Accommodation in shared twin rooms is available for free and food will be provided.

Accommodation: OIST Seaside House

*Kindly be advised that OIST does not provide paid accommodations, thus family members or companions of participants are unable to stay at the Seaside House

**OIST is deeply committed to the advancement of women in science, in Japan and worldwide.
Women are strongly encouraged to apply.**

**Organisers:**

Thomas Busch, OIST Graduate University

Shin Inouye, Osaka Metropolitan University

Yuki Kawaguchi, Nagoya University

Munekazu Horikoshi, Osaka Metropolitan University

Takeshi Fukuhara, Riken

Hiroki Takahashi, OIST Graduate University

Poster Sessions Sponsor