arXiv subjects

Masahito Ueda

Publications and source records attributed to Masahito Ueda.

At least 145 records · Page 8Linked to original sources

arXiv2016

Harmonic trap resonance enhanced synthetic atomic spin-orbit coupling

Spin-orbit coupling (SOC) plays an essential role in many exotic and interesting phenomena in condensed matter physics. In neutral-atom-based quantum simulations, synthetic SOC constitutes a key enabling element. The strength of SOC realized so far is limited by various reasons or constraints. This work reports tunable SOC synthesized with a gradient magnetic field (GMF) for atoms in a harmonic trap. Nearly ten-fold enhancement is observed when the GMF is modulated near the harmonic-trap resonance in comparison with free-space atoms. A theory is developed that well explains the experimental results. Our work offers a clear physical insight into and analytical understanding of how to tune the strength of atomic SOC synthesized with GMF using harmonic trap resonance.

cond-mat.quant-gas↗

arXiv2016

Emergent Electromagnetic Induction and Adiabatic Charge Pumping in Weyl Semimetals

The photovoltaic effect in a Weyl semimetal due to the adiabatic quantum phase is studied. We particularly focus on the case in which an external ac electric field is applied to the semimetal. In this setup, we show that a photocurrent is induced by the ac electric field. By considering a generalized Weyl Hamiltonian with nonlinear terms, it is shown that the photocurrent is induced by circularly, rather than linearly, polarized light. This photovoltaic current can be understood as an emergent electromagnetic induction in the momentum space; the Weyl node is a magnetic monopole in the momentum space, of which the electric field is induced by the circular motion. This result is distinct from conventional photovoltaic effects, and potentially useful for experimentally identifying Weyl semimetals in chiral crystals.

cond-mat.mes-hall↗

arXiv2016

$μ$-Symmetry breaking: an algebraic approach to finding mean fields of quantum many-body systems

One of the most fundamental problems in quantum many-body systems is the identification of a mean field in spontaneous symmetry breaking which is usually made in a heuristic manner. We propose a systematic method of finding a mean field based on the Lie algebra and the dynamical symmetry by introducing a class of symmetry broken phases which we call $μ$-symmetry breaking. We show that for $μ$-symmetry breaking the quadratic part of an effective Lagrangian of Nambu-Goldstone modes can be block-diagonalized and that homotopy groups of topological excitations can be calculated systematically.

cond-mat.quant-gas↗

arXiv2016

P-Wave Contact Tensor -- Universal Properties of Axisymmetry-Broken P-Wave Fermi Gases

We investigate universal properties of a p-wave Fermi gas with a resonant interaction in which the axisymmetry is broken spontaneously or externally. Here, the short-range correlations can be completely characterized by the nine-component p-wave contact tensor, which can be measured by applying a generalized adiabatic sweep theorem. The distinctive features of the p-wave contact tensor emerge in a normal p-wave Fermi gas in an anisotropic trap and in a superfluid phase. An experimental scheme to measure the p-wave contact tensor and test the adiabatic sweep theorem is also discussed.

cond-mat.quant-gas↗

arXiv2016

Quantum Trajectory Thermodynamics with Discrete Feedback Control

We employ the quantum jump trajectory approach to construct a systematic framework to study the thermodynamics at the trajectory level in a nonequilibrium open quantum system under discrete feedback control. Within this framework, we derive quantum versions of the generalized Jarzynski equalities, which are demonstrated in an isolated pseudospin system and a coherently driven two-level open quantum system. Due to quantum coherence and measurement backaction, a fundamental distinction from the classical generalized Jarzynski equalities emerges in the quantum versions, which is characterized by a large negative information gain reflecting genuinely quantum rare events. A possible experimental scheme to test our findings in superconducting qubits is discussed.

cond-mat.stat-mech↗

arXiv2016

Chiral magnetic effect by synthetic gauge fields

We study the dynamical generation of the chiral chemical potential in a Weyl metal constructed from a three-dimensional optical lattice and subject to synthetic gauge fields. By numerically solving the Boltzmann equation with the Berry curvature in the presence of parallel synthetic electric and magnetic fields, we find that the spectral flow and the ensuing chiral magnetic current emerge. We show that the spectral flow and the chiral chemical potential can be probed by time-of-flight imaging.

cond-mat.quant-gas↗

arXiv2016

Topological origin of universal few-body clusters in Efimov physics

Efimov physics is renowned for the self-similar spectrum featuring the universal ratio of one eigenenergy to its neighbor. Even more esoteric is the numerically unveiled fact that every Efimov trimer is accompanied by a pair of tetramers. Here we demonstrate that this hierarchy of universal few-body clusters has a topological origin by identifying the numbers of universal 3- and 4-body bound states with the winding numbers of the renormalization-group limit cycle in theory space. The finding suggests a topological phase transition in mass-imbalanced few-body systems which should be tested experimentally.

cond-mat.quant-gas↗

arXiv2015

Precise multi-emitter localization method for fast super-resolution imaging

We present a method that can simultaneously locate positions of overlapped multi-emitters at the theoretical-limit precision. We derive a set of simple equations whose solution gives the maximum likelihood estimator of multi-emitter positions. We compare the performance of our simultaneous localization analysis with the conventional single-molecule analysis for simulated images and show that our method can improve the time-resolution of superresolution microscopy an order of magnitude. In particular, we derive the information-theoretical bound on time resolution of localization-based superresolution microscopy and demonstrate that the bound can be closely attained by our analysis.

physics.optics↗

arXiv2015

Work fluctuation-dissipation trade-off in heat engines

Reducing work fluctuation and dissipation in heat engines or, more generally, information heat engines that perform feedback control is vital to maximize their efficiency. The same problem arises when we attempt to maximize the efficiency of a given thermodynamic task that undergoes nonequilibrium processes for arbitrary initial and final states. We find that the most general trade-off relation between work fluctuation and dissipation applicable to arbitrary nonequilibrium processes is bounded from below by the information distance characterizing how far the system is from thermal equilibrium. The minimum amount of dissipation is found to be given in terms of the relative entropy and the Renyi divergence, both of which quantify the information distance between the state of the system and the canonical distribution. We give an explicit protocol that achieves the fundamental lower bound of the trade-off relation.

cond-mat.stat-mech↗

arXiv2015

Excitation band topology and edge matter waves in Bose-Einstein condensates in optical lattices

We show that Bose-Einstein condensates in optical lattices with broken time-reversal symmetry can support chiral edge modes originating from nontrivial bulk excitation band topology. To be specific, we analyze a Bose-Hubbard extension of the Haldane model, which can be realized with recently developed techniques of manipulating honeycomb optical lattices. The topological properties of Bloch bands known for the noninteracting case are smoothly carried over to Bogoliubov excitation bands for the interacting case. We show that the parameter ranges that display topological bands enlarge with increasing the Hubbard interaction or the particle density. In the presence of sharp boundaries, chiral edge modes appear in the gap between topological excitation bands. We demonstrate that by coherently transferring a portion of a condensate into an edge mode, a density wave is formed along the edge owing to an interference with the background condensate. This offers a unique method of detecting an edge mode through a macroscopic quantum phenomenon.

cond-mat.quant-gas↗

arXiv2015

Diffraction-Unlimited Position Measurement of Ultracold Atoms in an Optical Lattice

We consider a method of high-fidelity, spatially resolved position measurement of ultracold atoms in an optical lattice. We show that the atom-number distribution can be nondestructively determined at a spatial resolution beyond the diffraction limit by tracking the progressive evolution of the many-body wavefunction collapse into a Fock state. We predict that the Pauli exclusion principle accelerates the rate of wavefunction collapse of fermions in comparison with bosons. A possible application of our principle of surpassing the diffraction limit to other imaging systems is discussed.

cond-mat.quant-gas↗

arXiv2015

How accurately can the microcanonical ensemble describe small isolated quantum systems?

We numerically investigate quantum quenches of a nonintegrable hard-core Bose-Hubbard model to test the accuracy of the microcanonical ensemble in small isolated quantum systems. We show that, in a certain range of system size, the accuracy increases with the dimension of the Hilbert space $D$ as $1/D$. We ascribe this rapid improvement to the absence of correlations between many-body energy eigenstates as well as to the eigenstate thermalization. Outside of that range, the accuracy is found to scale as $1/\sqrt{D}$ and improves algebraically with the system size.

cond-mat.stat-mech↗

arXiv2015

Tradeoff Relation between Information and Disturbance in Quantum Measurement

When we extract information from a system by performing a quantum measurement, the state of the system is disturbed due to the backaction of the measurement. Numerous studies have been performed to quantitatively formulate tradeoff relations between information and disturbance. We formulate a tradeoff relation between information and disturbance from an estimation-theoretic point of view, and derive an inequality between them. The information is defined as the classical Fisher information obtained by the measurement, and the disturbance is defined as the average loss of the quantum Fisher information. We show that pure and reversible measurements achieve the equality of the inequality. We also identify the necessary condition for various divergences between two quantum states to satisfy a similar relation. The obtained relation holds not only for the quantum relative entropy but also for the maximum quantum relative entropy.

quant-ph↗

arXiv2015

Universal high-momentum asymptote and thermodynamic relations in a spinless Fermi gas with a resonant $p$-wave interaction

We investigate universal relations in a spinless Fermi gas near a $p$-wave Feshbach resonance. We show that the momentum distribution $n_{\vec{k}}$ has an asymptote proportional to $k^{-2}$ with the proportionality constant--the $p$-wave contact-- scaling with the number of closed-channel molecules. We prove the adiabatic sweep theorem for a $p$-wave resonance which reveals the physical meaning of the $p$-wave contact in thermodynamics. In contrast to the unitary Fermi gas in which Tan's contact is universal, the $p$-wave contact depends on the short-range details of the interaction.

cond-mat.quant-gas↗

arXiv2015

Quantum nonequilibrium equalities with absolute irreversibility

We derive quantum nonequilibrium equalities in absolutely irreversible processes. Here by absolute irreversibility we mean that in the backward process the density matrix does not return to the subspace spanned by those eigenvectors that have nonzero weight in the initial density matrix. Since the initial state of a memory and the postmeasurement state of the system are usually restricted to a subspace, absolute irreversibility occurs during the measurement and feedback processes. An additional entropy produced in absolute irreversible processes needs to be taken into account to derive nonequilibrium equalities. We discuss a model of a feedback control on a qubit system to illustrate the obtained equalities. By introducing $N$ heat baths each composed of a qubit and letting them interact with the system, we show how the entropy reduction via feedback control can be converted into work. An explicit form of extractable work in the presence of absolute irreversibility is given.

cond-mat.stat-mech↗

arXiv2015

Classicality condition on a system's observable in a quantum measurement and relative-entropy conservation law

We consider the information flow on a system's observable $X$ corresponding to a positive-operator valued measure under a quantum measurement process $Y$ described by a completely positive instrument from the viewpoint of the relative entropy. We establish a sufficient condition for the relative-entropy conservation law which states that the averaged decrease in the relative entropy of the system's observable $X$ equals the relative entropy of the measurement outcome of $Y$, i.e. the information gain due to measurement. This sufficient condition is interpreted as an assumption of classicality in the sense that there exists a sufficient statistic in a joint successive measurement of $Y$ followed by $X$ such that the probability distribution of the statistic coincides with that of a single measurement of $X$ for the pre-measurement state. We show that in the case when $X$ is a discrete projection-valued measure and $Y$ is discrete, the classicality condition is equivalent to the relative-entropy conservation for arbitrary states. The general theory on the relative-entropy conservation is applied to typical quantum measurement models, namely quantum non-demolition measurement, destructive sharp measurements on two-level systems, a photon counting, a quantum counting, homodyne and heterodyne measurements. These examples except for the non-demolition and photon-counting measurements do not satisfy the known Shannon-entropy conservation law proposed by Ban~(M. Ban, J. Phys. A: Math. Gen. \textbf{32}, 1643 (1999)), implying that our approach based on the relative entropy is applicable to a wider class of quantum measurements.

quant-ph↗

arXiv2014

Quantum Mass Acquisition in Spinor Bose-Einstein Condensates

Quantum mass acquisition, in which a massless (quasi)particle becomes massive due to quantum corrections, is predicted to occur in several subfields of physics. However, its experimental observation remains elusive since the emergent energy gap is too small. We show that a spinor Bose-Einstein condensate is an excellent candidate for the observation of such a peculiar phenomenon as the energy gap turns out to be 2 orders of magnitude larger than the zero-point energy. This extraordinarily large energy gap is a consequence of the dynamical instability. The propagation velocity of the resultant massive excitation mode is found to be decreased by the quantum corrections as opposed to phonons.

cond-mat.quant-gas↗

arXiv2014

Entanglement prethermalization: Locally thermal but non-locally non-thermal states in a one-dimensional Bose gas

A well-isolated system often shows relaxation to a quasi-stationary state before reaching thermal equilibrium. Such a prethermalization has attracted considerable interest recently in association with closely related fundamental problems of relaxation and thermalization of isolated quantum systems. Motivated by the recent experiment in ultracold atoms, we study the dynamics of a one-dimensional Bose gas which is split into two subsystems, and find that individual subsystems relax to Gibbs states, yet the entire system does not due to quantum entanglement. In view of recent experimental realization on a small well-defined number of ultracold atoms, our prediction based on exact few-body calculations is amenable to experimental test.

cond-mat.quant-gas↗