arXiv subjects

Masahito Ueda

Publications and source records attributed to Masahito Ueda.

At least 217 records · Page 12Linked to original sources

arXiv2010

Spontaneous Magnetic Ordering in a Ferromagnetic Spinor Dipolar Bose-Einstein Condensate

We study the spin dynamics in a spin-1 ferromagnetic Bose-Einstein condensate with magnetic dipole-dipole interaction (MDDI) based on the Gross-Pitaevskii and Bogoliubov theories. We find that various magnetic structures such as checkerboards and stripes emerge in the course of the dynamics due to the combined effects of spin-exchange interaction, MDDI, quadratic Zeeman and finite-size effects, and non-stationary initial conditions. However, the short-range magnetic order observed by the Berkeley group [Phys. Rev. Lett. {\bf 100}, 170403 (2008)] is not fully reproduced in our calculations; the periodicity of the order differs by a factor of three and the checkerboard pattern eventually dissolves in our numerical simulations. Possible reasons for the discrepancy are discussed.

cond-mat.quant-gas↗

arXiv2010

Elongated Fermi superfluid: absence of critical imbalance enhancement at equilibrium

We show that the maximum population imbalance ratio $P_\mathrm{CC}$ for a two-component Fermi gas near the unitarity limit to condense does not increase with the trap aspect ratio $λ$, by two methods of 1) solving the Bogoliubov-de Gennes equations with coupling-constant renormalization, and 2) studying the pairing susceptibility by the real-space self-consistent $T$-matrix approximation. The deviation of the cloud shape from what is expected from the trap shape increases but stays minor with increasing $λ$ up to 50. This finding indicates that despite the apparent discrepancy between the MIT and Rice experiments over the value of $P_\mathrm{CC}$ and the validity of local density approximation, the equilibrium state of the system for the aspect ratio in the Rice experiment should be consistent with that of MIT.

cond-mat.other↗

arXiv2010

Ground states and dynamics of population-imbalanced Fermi condensates in one dimension

By using the numerically exact density-matrix renormalization group (DMRG) approach, we investigate the ground states of harmonically trapped one-dimensional (1D) fermions with population imbalance and find that the Larkin-Ovchinnikov (LO) state, which is a condensed state of fermion pairs with nonzero center-of-mass momentum, is realized for a wide range of parameters. The phase diagram comprising the two phases of i) an LO state at the trap center and a balanced condensate at the periphery and ii) an LO state at the trap center and a pure majority component at the periphery, is obtained. The reduced two-body density matrix indicates that most of the minority atoms contribute to the LO-type quasi-condensate. With the time-dependent DMRG, we also investigate the real-time dynamics of a system of 1D fermions in response to a spin-flip excitation.

cond-mat.quant-gas↗

arXiv2010

Faraday Rotation with Single Nuclear Spin Qubit in a High-Finesse Optical Cavity

When an off-resonant light field is coupled with atomic spins, its polarization can rotate depending on the direction of the spins via a Faraday rotation which has been used for monitoring and controlling the atomic spins. We observed Faraday rotation by an angle of more than 10 degrees for a single 1/2 nuclear spin of 171Yb atom in a high-finesse optical cavity. By employing the coupling between the single nuclear spin and a photon, we have also demonstrated that the spin can be projected or weakly measured through the projection of the transmitted single ancillary photon.

quant-ph↗

arXiv2010

Single Nuclear Spin Cavity QED

We constructed a cavity QED system with a diamagnetic atom of 171Yb and performed projective measurements on a single nuclear spin. Since Yb has no electronic spin and has 1/2 nuclear spin, the procedure of spin polarization and state verification can be dramatically simplified compared with the pseudo spin-1/2 system. By enhancing the photon emission rate of the 1S0-3P1 transition, projective measurement is implemented for an atom with the measurement time of T_meas = 30us. Unwanted spin flip as well as dark counts of the detector lead to systematic error when the present technique is applied for the determination of diagonal elements of an unknown spin state, which is delta|beta|^2 < 2 * 10^-2. Fast measurement on a long-lived qubit is key to the realization of large-scale one-way quantum computing.

quant-ph↗

arXiv2010

Generalized Jarzynski Equality under Nonequilibrium Feedback Control

The Jarzynski equality is generalized to situations in which nonequilibrium systems are subject to a feedback control. The new terms that arise as a consequence of the feedback describe the mutual information content obtained by measurement and the efficacy of the feedback control. Our results lead to a generalized fluctuation-dissipation theorem that reflects the readout information, and can be experimentally tested using small thermodynamic systems. We illustrate our general results by an introducing "information ratchet," which can transport a Brownian particle in one direction and extract a positive work from the particle.

cond-mat.stat-mech↗

arXiv2010

Optimal Measurement on Noisy Quantum Systems

We identify the optimal measurement for obtaining information about the original quantum state after the state to be measured has undergone partial decoherence due to noise. We quantify the information that can be obtained by the measurement in terms of the Fisher information and find its value for the optimal measurement. We apply our results to a quantum control scheme based on a spin-boson model.

quant-ph↗

arXiv2009

Hermitian conjugate measurement

We propose a new class of probabilistic reversing operations on the state of a system that was disturbed by a weak measurement. It can approximately recover the original state from the disturbed state especially with an additional information gain using the Hermitian conjugate of the measurement operator. We illustrate the general scheme by considering a quantum measurement consisting of spin systems with an experimentally feasible interaction and show that the reversing operation simultaneously increases both the fidelity to the original state and the information gain with such a high probability of success that their average values increase simultaneously.

quant-ph↗

arXiv2009

Spin-dependent inelastic collisions in spin-2 Bose-Einstein condensates

We studied spin-dependent two-body inelastic collisions in F=2 87Rb Bose-Einstein condensates both experimentally and theoretically. The 87Rb condensates were confined in an optical trap and selectively prepared in various spin states in the F=2 manifold at a magnetic field of 3.0 G. Measured atom loss rates are found to depend on spin states of colliding atoms. We measured two fundamental loss coefficients for two-body inelastic collisions with the total spin of 0 and 2; the coefficients determine loss rates for all the spin pairs. The experimental results for mixtures of all the spin combinations are in good agreement with numerical solutions of the Gross-Pitaevskii equations that include the effect of a magnetic field gradient.

cond-mat.quant-gas↗

arXiv2009

Collision Dynamics and Rung Formation of Non-Abelian Vortices

We investigate the collision dynamics of two non-Abelian vortices and find that, unlike Abelian vortices, they neither reconnect themselves nor pass through each other, but create a rung between them in a topologically stable manner. Our predictions are verified using the model of the cyclic phase of a spin-2 spinor Bose-Einstein condensate.

cond-mat.other↗

arXiv2009

A possible Efimov trimer state in a 3-component lithium-6 mixture

We consider the Efimov trimer theory as a possible framework to explain recently observed losses by inelastic three-body collisions in a three-hyperfine-component ultracold mixture of lithium 6. Within this framework, these losses would arise chiefly from the existence of an Efimov trimer bound state below the continuum of free triplets of atoms, and the loss maxima (at certain values of an applied magnetic field) would correspond to zero-energy resonances where the trimer dissociates into three free atoms. Our results show that such a trimer state is indeed possible given the two-body scattering lengths in the three-component lithium mixture, and gives rise to two zero-energy resonances. The locations of these resonances appear to be consistent with observed losses.

physics.atom-ph↗

arXiv2009

Independent Control of Scattering Lengths in Multicomponent Quantum Gases

We develop a method of simultaneous and independent control of different scattering lengths in ultracold multicomponent atomic gases, such as $ \mathrm{40}}$K or $^{\mathrm{40}} $K-$^{\mathrm{6}}$Li mixture. Our method can be used to engineer multi-component quantum phases and Efimov trimer states.

physics.atom-ph↗

arXiv2009

Minimal Energy Cost for Thermodynamic Information Processing: Measurement and Information Erasure

The fundamental lower bounds of the thermodynamic energy cost (work) needed for the measurement and the erasure of information are found. The lower bound for the erasure vindicates the "Landauer's principle" for a special case, but otherwise implies its breakdown, indicating that no unique relationship exists between logical reversibility and physical one. Our results constitute the second law of "information thermodynamics", in which the information content and thermodynamic variables are treated on an equal footing.

quant-ph↗

arXiv2008

Ferrofluidity in a two-component dipolar Bose-Einstein Condensate

It is shown that the interface in a two-component Bose-Einstein condensate (BEC) in which one component exhibits a dipole-dipole interaction spontaneously forms patterns similar to those formed in a magnetic liquid subject to a magnetic field. A hexagonal pattern, hysteretic behavior, and soliton-like structure are numerically demonstrated. A phenomenon similar to the labyrinthine instability is also found. These phenomena may be realized using a 52Cr BEC. The periodic density modulation in the superfluid ground state offers evidence of supersolidity.

cond-mat.other↗

arXiv2008

d-wave collapse and explosion of a dipolar Bose-Einstein condensate

We investigate the collapse dynamics of a dipolar condensate of 52Cr atoms when the s-wave scattering length characterizing the contact interaction is reduced below a critical value. A complex dynamics, involving an anisotropic, d-wave symmetric explosion of the condensate, is observed. The atom number decreases abruptly during the collapse. We find good agreement between our experimental results and those of a numerical simulation of the three-dimensional Gross-Pitaevskii equation, including contact and dipolar interactions as well as three-body losses. The simulation indicates that the collapse induces the formation of two vortex rings with opposite circulations.

cond-mat.other↗

arXiv2008

Dynamical symmetry in spinor Bose-Einstein condensates

We demonstrate that dynamical symmetry plays a crucial role in determining the structure of the eigenspectra of spinor Bose-Einstein condensates (BECs). In particular, the eigenspectra of spin- 1 and spin-2 BECs in the single-mode approximation are shown to be completely determined by dynamical symmetries, where a spin-2 BEC corresponds to the U(5) limit of the interacting boson model in nuclear physics. The eigenspectrum of a spin-3 BEC is solved analytically for a specific class of coupling constants, while it is shown that dynamical symmetry alone is not enough to determine the spectrum for arbitrary coupling constants. We also study the low-lying eigenspectra of spin-1 and spin-2 BECs in the absence of external magnetic field, and find, in particular, that the quasidegenerate spectra emerge for antiferromagnetic and cyclic phases. This implies that these systems are highly susceptible to external perturbations and may undergo symmetry-breaking transitions to other states upon increasing the size of system.

cond-mat.other↗

arXiv2008

Critical Temperature and Condensate Fraction of a Fermion Pair Condensate

We report on measurements of the critical temperature and the temperature dependence of the condensate fraction for a fermion pair condensate of 6Li atoms. The Bragg spectroscopy is employed to determine the critical temperature and the condensate fraction after a fast magnetic field ramp to the molecular side of the Feshbach resonance. Our measurements reveal the level-off of the critical temperature and the limiting behavior of condensate fraction near the unitarity limit.

cond-mat.other↗

arXiv2008

Collisional Properties of p-Wave Feshbach Molecules

We detected the formation of p-wave Feshbach molecules for all three combinations of the two lowest atomic spin states of 6Li. By creating a pure molecular sample in an optical trap, we measured the inelastic collision rates of p-wave molecules. The elastic collision rate was measured from the thermalization rate of a breathing mode excited spontaneously upon molecular formation.

cond-mat.other↗