Publications

Kui-Tian Xi, Tim Byrnes, and Hiroki Saito,
Fingering instabilities and pattern formation in a two-component dipolar Bose-Einstein condensate,
Physical Review A 97, 023625/1-9 (2018).

[Summary] We study fingering instabilities and pattern formation at the interface of an oppositely polarized two-component Bose-Einstein condensate with strong dipole-dipole interactions in three dimensions. It is shown that the rotational symmetry is spontaneously broken by fingering instability when the dipole-dipole interactions are strengthened. Frog-shaped and mushroom-shaped patterns emerge during the dynamics due to the dipolar interactions. We also demonstrate the spontaneous density modulation and domain growth of a two-component dipolar BEC in the dynamics. Bogoliubov analyses in the two-dimensional approximation are performed, and the characteristic lengths of the domains are estimated analytically. Patterns resembling those in magnetic classical fluids are modulated when the number ratio of atoms, the trap ratio of the external potential, or tilted polarization with respect to the z direction is varied.

Naoki Irikura, Yujiro Eto, Takuya Hirano, and *Hiroki Saito,
Ground-state phases of a mixture of spin-1 and spin-2 Bose-Einstein condensates,
Physical Review A 97, 023622/1-10 (2018).

[Summary] We investigate the ground-state phases of a mixture of spin-1 and spin-2 Bose-Einstein condensates at zero magnetic field. In addition to the intraspin interactions, two spin-dependent interaction coefficients are introduced to describe the interspin interaction. We systematically explore the wide parameter space, and obtain phase diagrams containing a rich variety of phases. For example, there exists a phase in which the spin-1 and spin-2 vectors are tilted relative to each other breaking the axial symmetry.

Yujiro Eto, Hitoshi Shibayama, Hiroki Saito, and Takuya Hirano,
Spinor dynamics in a mixture of spin-1 and spin-2 Bose-Einstein condensates,
Physical Review A 97, 021602(R)/1-5 (2018).

[Summary] The spinor dynamics of Bose-Einstein condensates of 87Rb atoms with hyperfine spins of 1 and 2 areinvestigated. A mixture of spinor condensates is generated in the transversely polarized spin-1 and thelongitudinally polarized spin-2 states using simultaneous Ramsey interferometry. In the subsequent dynamics,temporal modulation of spin-1 magnetization is observed in the spinor mixture due to the spin-dependentinteraction with the spin-2 component. By comparison with numerical simulations based on the Gross-Pitaevskiiequation, we find that the observed nonferromagnetic dynamics reflect the ground-state properties of the spinormixture.

*Hiroki Saito, Masaya Kato,
Machine Learning Technique to Find Quantum Many-Body Ground States of Bosons on a Lattice,
Journal of the Physical Society of Japan 87, 014001/1-8 (2017).

[Summary] We have developed a variational method to obtain many-body ground states of the Bose–Hubbard model using feedforward artificial neural networks. A fully connected network with a single hidden layer works better than a fully connected network with multiple hidden layers, and a multilayer convolutional network is more efficient than a fully connected network. AdaGrad and Adam are optimization methods that work well. Moreover, we show that many-body ground states with different numbers of particles can be generated by a single network.

Masaya Kato, Xiao-Fei Zhang, and *Hiroki Saito,
Moving obstacle potential in a spin-orbit-coupled Bose-Einstein condensate,
Physical Review A 96, 033613/1-8 (2017).

[Summary] We investigate the dynamics around an obstacle potential moving in the plane-wave state of a pseudospin-1/2 Bose-Einstein condensate with Rashba spin-orbit coupling. We numerically investigate the dynamics of the system and find that it depends not only on the velocity of the obstacle but also significantly on the direction of obstacle motion, both of which are verified by the Bogoliubov analysis. The excitation diagram with respect to the velocity and direction is obtained. The dependence of the critical velocity on the strength of the spin-orbit coupling and the size of the obstacle is also investigated.

Hiroki Saito,
Solving the Bose-Hubbard model with machine learning,
Journal of the Physical Society of Japan 86, 093001/1-4 (2017).

[Summary] Motivated by the recent successful application of artificial neural networks to quantum many-body problems [G. Carleo and M. Troyer, Science 355, 602 (2017)], a method to calculate the ground state of the Bose–Hubbard model using a feedforward neural network is proposed. The results are in good agreement with those obtained by exact diagonalization and the Gutzwiller approximation. The method of neural-network quantum states is promising for solving quantum many-body problems of ultracold atoms in optical lattices.

Masaya Kato, Xiao-Fei Zhang, and *Hiroki Saito,
Vortex pairs in a spin-orbit coupled Bose-Einstein condensate,
Physical Review A 95, 043605/1-7 (2017).

[Summary] Static and dynamic properties of vortices in a two-component Bose-Einstein condensate with Rashba spin-orbit coupling are investigated. The mass current around a vortex core in the plane-wave phase is found to be deformed by the spin-orbit coupling, and this makes the dynamics of the vortex pairs quite different from those in a scalar Bose-Einstein condensate. The velocity of a vortex-antivortex pair is much smaller than that without spin-orbit coupling, and there exist stationary states. Two vortices with the same circulation move away from each other or unite to form a stationary state.

Xiao-Fei Zhang, Masaya Kato, Wei Han, Shou-Gang Zhang, and Hiroki Saito,
Spin-orbit-coupled Bose-Einstein condensates held under a toroidal trap,
Physical Review A 95, 033620 (2017).

[Summary] We study a quasispin-1/2 Bose-Einstein condensate with synthetically generated spin-orbit coupling in a toroidal trap and show that the system has a rich variety of ground states. As the central hole region increases, i.e., the potential changes from harmoniclike to ringlike, the condensate exhibits a variety of structures, such as a modified stripe, an alternately arranged stripe, and countercircling states. In the limit of a quasi-one-dimensional ring, the quantum many-body ground state is obtained, which is found to be the fragmented condensate.

*Xiao-Fei Zhang, Wei Han, Hai-Feng Jiang, Wu-Ming Liu, Hiroki Saito, and Shou-Gang Zhang,
Topological defect formation in rotating binary dipolar Bose-Einstein condensate,
Annals of Physics 375, 368-377 (2016).

[Summary] We investigate the topological defects and spin structures of a rotating binary Bose–Einstein condensate, which consists of both dipolar and scalar bosonic atoms confined in spin-dependent optical lattices, for an arbitrary orientation of the dipoles with respect to their plane of motion. Our results show that the tunable dipolar interaction, especially the orientation of the dipoles, can be used to control the direction of stripe phase and its related half-vortex sheets. In addition, it can also be used to obtain a regular arrangement of various topological spin textures, such as meron, circular and cross disgyration spin structures. We point out that such topological defects and regular arrangement of spin structures arise primarily from the long-range and anisotropic nature of dipolar interaction and its competition with the spin-dependent optical lattices and rotation.

Masaya Kato, Xiao-Fei Zhang, Daichi Sasaki, and Hiroki Saito,
Twisted spin vortices in a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction,
Physical Review A 94, 043633/1-6 (2016).

[Summary] We consider a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction confined in a cigar-shaped trap. Due to the combined effects of spin-orbit coupling, dipole-dipole interaction, and trap geometry, the system exhibits a rich variety of ground-state spin structures, including twisted spin vortices. The ground-state phase diagram is determined with respect to the strengths of the spin-orbit coupling and dipole-dipole interaction.

Hiroki Saito and Rina Kanamoto,
Self-rotation and synchronization in exciton-polariton condensates,
Physical Review B 94, 165306 (2016).

[Summary] Self-rotation occurs in an exciton-polariton condensate in a two-dimensional semiconductor microcavity pumped by a nonresonant Gaussian laser beam. A wave packet of the condensate spontaneously rotates around the center of the pumped region at a constant frequency breaking the rotation symmetry of the system. When two self-rotating condensates are created with an appropriate distance, synchronization occurs between the dynamics of the self-rotating condensates.

*Wei Han, Xiao-Fei Zhang, Shu-Wei Song, Hiroki Saito, Wei Zhang, Wu-Ming Liu, and Shou-Gang Zhang,
Double-quantum spin vortices in SU(3) spin-orbit coupled Bose gases,
Physical Review A 94, 033611/1-9 (2016).

[Summary] We show that double-quantum spin vortices, which are characterized by doubly quantized circulating spin currents and unmagnetized filled cores, can exist in the ground states of SU(3) spin-orbit-coupled Bose gases. It is found that the SU(3) spin-orbit coupling and spin-exchange interaction play important roles in determining the ground-state phase diagram. In the case of effective ferromagnetic spin interaction, the SU(3) spin-orbit coupling induces a threefold degeneracy to the magnetized ground state, while in the antiferromagnetic spin interaction case, the SU(3) spin-orbit coupling breaks the ordinary phase rule of spinor Bose gases and allows the spontaneous emergence of double-quantum spin vortices. This exotic topological defect is in stark contrast to the singly quantized spin vortices observed in existing experiments and can be readily observed by the current magnetization-sensitive phase-contrast imaging technique.

*Yujiro Eto, Masahiro Takahashi, Masaya Kunimi, Hiroki Saito, and Takuya Hirano,
Nonequilibrium dynamics induced by miscible-immiscible transition in binary Bose-Einstein condensates,
New Journal of Physics 17, 0703029/1-6 (2016).

[Summary] We have observed and characterized the nonequilibrium spatial dynamics of a two-component 87Rb Bose–Einstein condensate (BEC) that is controllable switched back and forth between the miscible and immiscible phases of the phase separation transition by changing the internal states of the 87Rb atoms. The subsequent evolution exhibits large scale oscillations of the spatial structure that involve component mixing and separation. We show that the larger total energy of the miscible system results in a higher oscillation frequency. This investigation introduces a new technique to control the miscibility and the spatial degrees of freedom in atomic BECs.

Hiroki Saito,
Path-integral Monte Carlo study on a droplet of a dipolar Bose-Einstein condensate stabilized by quantum fluctuation,
Journal of the Physical Society of Japan 85, 053001 (2016).

[Summary] Motivated by recent experiments [H. Kadau et al., Nature (London) 530, 194 (2016); I. Ferrier-Barbut et al., arXiv:1601.03318] and theoretical prediction (F. Wächtler and L. Santos, arXiv:1601.04501), the ground state of a dysprosium Bose–Einstein condensate with strong dipole–dipole interaction is studied by the path-integral Monte Carlo method. It is shown that quantum fluctuation can stabilize the condensate against dipolar collapse.

*Yujiro Eto, Masahiro Takahashi, Keita Nabeta, Ryotaro Okada, Masaya Kunimi, Hiroki Saito, and Takuya Hirano,
Bouncing motion and penetration dynamics in multicomponent Bose-Einstein condensates,
Physical Review A 93, 033615/1-6 (2016).

[Summary] We investigate the dynamic properties of bouncing and penetration in colliding binary and ternary Bose-Einstein condensates comprised of different Zeeman or hyperfine states of 87Rb. Through the application of magnetic field gradient pulses, two- or three-component condensates in an optical trap are spatially separated and then made to collide. The subsequent evolutions are classified into two categories: repeated bouncing motion and mutual penetration after damped bounces. We experimentally observed mutual penetration for immiscible condensates, bouncing between miscible condensates, and domain formation for miscible condensates. From numerical simulations of the Gross-Pitaevskii equation, we find that the penetration time can be tuned by slightly changing the atomic interaction strengths.

Tomoya Kaneda and Hiroki Saito,
Collision dynamics of Skyrmions in a two-component Bose-Einsteincondensate,
Physical Review A 93, 033611/1-6 (2016).

[Summary] The dynamics of skyrmions in a two-component Bose-Einstein condensate is numerically investigated in the mean-field theory. When two skyrmions collide with each other, they are first united and then scattered into various states. For head-on collisions, skyrmions with unit winding number are scattered. The collision dynamics with an impact parameter are shown to depend on the relative phase. These dynamic processes are characterized by integer winding numbers.

Kui-Tian Xi and Hiroki Saito,
Droplet formation in a Bose-Einstein condensate with strong dipole-dipole interaction,
Physical Review A 93, 011604(R) (2016).

[Summary] Motivated by the recent experiment [H. Kadau et al., arXiv:1508.05007], we study roton instability and droplet formation in a Bose-Einstein condensate of Dy164 atoms with strong magnetic dipole-dipole interaction. We numerically solve the cubic-quintic Gross-Pitaevskii equation with dipole-dipole interaction, and show that the three-body interaction plays a significant role in the formation of droplet patterns. We numerically demonstrate the formation of droplet patterns and crystalline structures, decay of droplets, and hysteresis behavior, which are in good agreement with the experiment. Our numerical simulations provide the first prediction on the values of the three-body interaction in a Dy164 Bose-Einstein condensate. We also predict that the droplets remain stable during the time-of-flight expansion. From our results, further experiments investigating the three-body interaction in dipolar quantum gases are required.

*Hiroki Saito,
Can we swim in superfluids?: Numerical demonstration of self-propulsion in a Bose-Einstein condensate,
Journal of the Physical Society of Japan 84, 114001/1-6 (2015).

[Summary] We numerically investigated whether a deformable object can propel itself in a superfluid. Articulated bodies and multicomponent condensates are examined as swimmers. An articulated two-body swimmer cannot obtain locomotion without emitting excitations. More flexible swimmers can do so without the need to excite waves.

*Yujiro Eto, Masaya Kunimi,Hidekatsu Tokita,Hiroki Saito, and Takuya Hirano,
Suppression of relative flow by multiple domains in two component Bose-Einstein condensates,
Physical Review A 92, 013611/1-5 (2015).

[Summary] We investigate flow properties of immiscible Bose-Einstein condensates composed of two different Zeeman spin states of 〈sup〉87〈/sup〉Rb. Two spatially overlapping condensates in the optical trap are prepared by application of a resonant radio-frequency pulse, and then the magnetic field gradient is applied in order to produce the atomic flow. We find that the spontaneous multiple-domain formation arising from the immiscible nature drastically changes the fluidity. The homogeneously overlapping condensates readily separate under the magnetic field gradient, and they form a stable configuration composed of the two layers. In contrast, the relative flow between two condensates is largely suppressed in the case where the magnetic field gradient is applied after spontaneous domain formation.

*Hiroki Saito and Masaya Kunimi,
Energy shift of magnons in a ferromagnetic spinor-dipolar Bose-Einstein condensate,
Physical Review A 91, 041603(R)/1-4 (2015).

[Summary] Motivated by the recent experiment performed by the Berkeley group [G. E. Marti et al., Phys. Rev. Lett. 113, 165301 (2014)], we consider the dynamics of magnons in a spin-1 spinor-dipolar Bose-Einstein condensate, using mean-field theory. We show that the effective mass of a magnon is increased by the magnetic dipole-dipole interaction, as observed in the experiment. The magnon mass is also decreased by changing the direction of the magnetic field. The increase and decrease in the magnon mass manifest themselves in the acceleration of the magnons.

Masaya Kunimi* and Hiroki Saito,
Upper bound of one-magnon excitation and lower bound of effective mass for ferromagnetic spinor Bose and Fermi gases,
Physical Review A 91, 043624/1-6 (2015).

[Summary] Using a variationalmethod, we derive an exact upper bound for one-magnon excitation energy in ferromagneticspinor gases, which limits the quantum corrections to the effective mass of a magnon to be positive. We alsoderive an upper bound for one-magnon excitation energy in lattice systems. The results hold for both Bose andFermi systems in d dimensions as long as the interaction is local and invariant under spin rotation.

*Masaya Kunimi,
Metastable spin textures and Nambu-Goldstone modes of a ferromagnetic spin-1 Bose-Einstein condensate confined in a ring trap,
Physical Review A 90, 063632/1-8 (2014).

[Summary] We investigate the metastability of a ferromagnetic spin-1 Bose-Einstein condensate confined in a quasi-onedimensionalrotating ring trap by solving the spin-1 Gross-Pitaevskii equation. We find analytical solutions thatexhibit spin textures. By performing linear stability analysis, it is shown that the solutions can become metastablestates. We also find that the number of Nambu-Goldstone modes changes at a certain rotation velocity withoutchanging the continuous symmetry of the order parameter.

Tomoya Kaneda and Hiroki Saito,
Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate,
Physical Review A 90, 053632/1-7 (2014).

[Summary] The dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which magnetic phases are spatially distributed is investigated. When a vortex dipole travels from the ferromagnetic phase to the polar phase, or vice versa, it penetrates the phase boundary and transforms into one of the various spin vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a half-quantum vortex dipole. Topological connections of spin wave functions across the phase boundary are discussed.

*Yujiro Eto, Mark Sadgrove, Sho Hasegawa, Hiroki Saito, and Takuya Hirano,
Control of spin current in a Bose gas by periodic application of π pulses,
Physical Review A 90, 013626/1-6 (2014).

[Summary] We generate spin currents in an ⁸⁷Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic π pulses (bang-bang control), producing longer spin-coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.

Tsuyoshi Kadokura, Jun Yoshida, and Hiroki Saito,
Hysteresis in quantized vortex shedding,
Physical Review A 90, 013612/1-5 (2014).

[Summary] It is shown using numerical simulations that flow patterns around an obstacle potential moving in a superfluid exhibit hysteresis. In a certain velocity region, there is a bistability between stationary laminar flow and periodic vortex shedding. The bistability exists in two- and three-dimensional systems.

Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
FFLO Multi-Phase Transition in Two-Band Superconductor,
JPS Conference Proceedings 3, 015022 (2014).

[Summary] We study the phase diagram of Pauli-limiting two-band superconductors in the plane of an external magnetic field and temperature, based on the mean field approximation. In the case of a single-band superconductor, the superconducting phase diagram consists of two phases, Bardeen–Cooper–Schrieffer state in the lower field and Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state in the higher field. In the case of two-band superconductors, there are competing two length scales originated by first and second bands. Due to the competing length scales, the FFLO phase is divided into multiple — actually infinitely many — phases. We discuss the mechanism of the multiple phase transition.

*Hiroki Saito,
Comment on “Ground-state fragmentation phase transition for attractive bosons in anisotropic traps”,
Physical Review A 89, 067601/1-2 (2014).

[Summary] In a recent article [Phys. Rev. A 88, 063641 (2013)], Cizek and Kasevich claimed that quantum fragmentation occurs in the metastable state of a Bose-Einstein condensate with an attractive interaction confined in an elongated harmonic potential. This result was obtained using the two-state Gaussian variational method. However, modified methods show that no fragmentation occurs for the parameters used by Cizek and Kasevich.

*Yujiro Eto, Hiroki Saito, and Takuya Hirano,
Observation of dipole-induced spin texture in an ⁸⁷Rb spin-2 Bose-Einstein condensate,
Physical Review Letters 112, 185301/1-5 (2014).

[Summary] We report the formation of spin texture resulting from the magnetic dipole-dipole interaction in a spin-2　⁸⁷Rb Bose-Einstein condensate. The spinor condensate is prepared in the transversely polarized spin state and the time evolution is observed under a magnetic field of 90 mG with a gradient of 3  mG/cm using Stern-Gerlach imaging. The experimental results are compared with numerical simulations of the Gross-Pitaevskii equation, which reveals that the observed spatial modulation of the longitudinal magnetization is due to the spin precession in an effective magnetic field produced by the dipole-dipole interaction. These results show that the dipole-dipole interaction has considerable effects even on spinor condensates of alkali metal atoms.

*Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
Multi-band effects on Fulde-Ferrell-Larkin-Ovchinnikov states of Pauli-limited superconductors,
Physical Review B 89, 064505/1-16 (2014).

[Summary] Multi-band effects on Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of a Pauli-limiting two-band superconductor are studied theoretically, based on self-consistent calculations of the Bogoliubov-de Gennes equation. First, we examine the phase diagrams of two-band systems with a passive band in which the intraband pairing interaction is absent and superconductivity is induced by a Cooper pair tunneling from an active band. It is demonstrated that the temperature of the Lifshitz point at which three second-order transition lines meet is independent of the Cooper pair tunneling strength. The BCS-FFLO critical field becomes lower than the Lifshitz point with increasing the interband tunneling strength, and the resultant phase diagram is qualitatively different from that in a single-band superconductor. We also study the thermodynamics of Pauli-limiting two-band superconductors with comparable intraband pairing interactions. As a consequence of a competing effect between two bands, the FFLO phase is divided into two phases: Q1- and Q2-FFLO phases. The Q1-FFLO is favored in a high field regime and the Q2-FFLO becomes stable in the lower field. In a particular case, the latter is further subdivided into a family of FFLO states with rational modulation lengths, leading to a devil's staircase structure in the field-dependence of physical quantities. The critical field, above which the FFLO is stabilized, is lower than that in a single band superconductor, while the temperature of tricritical Lifshitz point is invariant under the change of two-band parameters.

*Hiroki Saito,
Many-body dynamics of a Bose-Einstein condensate collapsing by quantum tunneling,
Physical Review A 89, 023610/1-6 (2014).

[Summary] The dynamics of a Bose-Einstein condensate of atoms having attractive interactions is studied using quantum many-body simulations. The collapse of the condensate by quantum tunneling is numerically demonstrated, and the tunneling rate is calculated. The correlation properties of the quantum many-body state are investigated.

Masahiro Takahashi, Takeshi Mizushima, and Kazushige Machida,
Fulde-Ferrell-Larkin-Ovchinnikov States in Two-Band Superconductors,
Journal of the Physical Society of Japan 83, 023703/1-5 (2014).

[Summary] We examine the possible phase diagram in an H–T plane for Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states in a two-band Pauli-limiting superconductor. We here demonstrate that, as a result of the competition of two different modulation lengthscales, the FFLO phase is divided into two phases by the first-order transition: the Q1- and Q2-FFLO phases at the higher and lower fields. The Q2-FFLO phase is further divided by successive first order transitions into an infinite family of FFLO subphases with rational modulation vectors, forming a devil’s staircase structure for the field dependences of the modulation vector and paramagnetic moment. The critical magnetic field above which the FFLO is stabilized is lower than that in a single-band superconductor. However, the tricritical Lifshitz point L at TL is invariant under two-band parameter changes.

*Yujiro Eto, Hayato Ikeda, Hirosuke Suzuki, Sho Hasegawa, Yasushi Tomiyama, Sawako Sekine, Mark Sadgrove, and Takuya Hirano,
Spin-echo-based magnetometry with spinor Bose-Einstein condensates,
Physical Review A 88, 031602(R)/1-4 (2013).

[Summary] We demonstrate detection of a weak alternate-current magnetic field by application of the spin-echo technique to F=2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/√Hz is attained for an atom number of 5×10^3 at a spatial resolution of 100 μ㎡. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.