arXiv subjects

Shuta Nakajima

Publications and source records attributed to Shuta Nakajima.

49 records · Page 3Linked to original sources

arXiv2019

Passage time of the frog model has a sublinear variance

In this paper, we show that the first passage time in the frog model on $\Z^d$ with $d\geq 2$ has a sublinear variance. This implies that the central limit theorem does not holds at least with the standard diffusive scaling. The proof is based on the method introduced in \cite{BRo, DHS} combining with a control of the maximal weight of paths in locally dependent site-percolation. We also apply this method to get the linearity of the lengths of optimal paths..

math.PR↗

arXiv2019

Current-feedback-stabilized laser system for quantum simulation experiments using Yb clock transition at 578 nm

We developed a laser system for the spectroscopy of the clock transition in ytterbium (Yb) atoms at 578 nm based on an interference-filter stabilized external-cavity diode laser (IFDL) emitting at 1156 nm. Owing to the improved frequency-to-current response of the laser-diode chip and the less sensitivity of the IFDL to mechanical perturbations, we succeeded in stabilizing the frequency to a high-finesse ultra-low-expansion glass cavity with a simple current feedback system. Using this laser system, we performed high-resolution clock spectroscopy of Yb and found that the linewidth of the stabilized laser was less than 320 Hz.

physics.atom-ph↗

arXiv2018

Dissipative Bose-Hubbard system with intrinsic two-body loss

We report an experimental study of dynamics of the metastable $^3P_2$ state of bosonic ytterbium atoms in an optical lattice. The dissipative Bose-Hubbard system with on-site two-body atom loss is realized via its intrinsic strong inelastic collision of the metastable $^3P_2$ atoms. We investigate the atom loss behavior with the unit-filling Mott insulator as the initial state and find that the atom loss is suppressed by the strong correlation between atoms. Also, as we decrease the potential depth of the lattice, we observe the growth of the phase coherence and find its suppression owing to the dissipation.

cond-mat.quant-gas↗

arXiv2018

On properties of optimal paths in first passage percolation

In this paper, we study some properties of optimal paths in the first passage percolation on $\Z^d$ and show the followings: (1) the number of optimal paths has an exponential growth if the distribution has an atom; (2) the means of intersection and union of optimal paths are linear in the distance. For the proofs, we use the configuration--flipping argument introduced in [J. van den Berg and H. Kesten. Inequalities for the time constant in first-passage percolation. Ann. Appl. Probab. 56-80, 1993] with suitable adaptions.

math.PR↗

arXiv2018

Ergodicity of the number of infinite geodesics originating from zero

First-passage percolation is a random growth model which has a metric structure. An infinite geodesic is an infinite sequence whose all sub-sequences are shortest paths. One of the important quantity is the number of infinite geodesics originating from the origin. When $d=2$ and an edge distribution is continuous, it is proved to be almost surely constant [D. Ahlberg, C. Hoffman. Random coalescing geodesics in first-passage percolation]. In this paper, we will prove the same result for higher dimensions and general distributions.

math.PR↗

arXiv2017

Observation of the Mott Insulator to Superfluid Crossover of a Driven-Dissipative Bose-Hubbard System

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 preparation and manipulation of quantum states. Here 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 photo-association 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 non-equilibrium dynamics.

cond-mat.quant-gas↗

arXiv2017

A scanning gate microscope for cold atomic gases

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel, and a position sensitivity below 10nm. Our measurements agree well with an analytical model and ab-initio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.

cond-mat.quant-gas↗

arXiv2016

Concentration results for directed polymer with unbounded jumps

We study the free energy and its relevant quantity for the directed polymer in random environment. The polymer is allowed to make unbounded jumps and the environment is given by the Bernoulli variables. We first establish the concentration of the ground state energy of polymer at zero temperature. Secondly, we also prove the same property of the free energy at finite temperature. In the proof, we use the fact that the maximum jump of any polymer nearly minimizing energy is not too large with high probability. This is an interesting property itself from the first passage percolation viewpoint.

math.PR↗

arXiv2015

Coherent driving and freezing of bosonic matter wave in an optical Lieb lattice

While kinetic energy of a massive particle generally has quadratic dependence on its momentum, a flat, dispersionless energy band is realized in crystals with specific lattice structures. Such macroscopic degeneracy causes the emergence of localized eigenstates and has been a key concept in the context of itinerant ferromagnetism. Here we report the realization of a "Lieb lattice" configuration with an optical lattice, which has a flat energy band as the first excited state. Our optical lattice potential possesses various degrees of freedom about its manipulation, which enables coherent transfer of a Bose-Einstein condensate into the flat band. In addition to measuring lifetime of the flat band population for different tight-binding parameters, we investigate the inter-sublattice dynamics of the system by projecting the sublattice population onto the band population. This measurement clearly shows the formation of the localized state with the specific sublattice decoupled in the flat band, and even detects the presence of flat-band breaking perturbations, resulting in the delocalization. Our results will open up the possibilities of exploring physics of flat band with a highly controllable quantum system.

cond-mat.quant-gas↗

arXiv2013

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

We develop an optical lattice system for an ultracold atomic gas mixture of ytterbium (174Yb) and lithium (6Li), which is an ideal system to study disorder and impurity problems. We load a Bose-Einstein condensate of 174Yb into a three-dimensional optical lattice and observe the interference patterns in time-of-flight (TOF) images. Furthermore, we perform a laser spectroscopy of 174Yb in an optical lattice using the ultra-narrow optical transition 1S0-3P2 in both cases with and without 6Li. Due to the weak interspecies interaction, we do not observe the clear influences of 6Li in the obtained interference patterns and the excitation spectra. However, this is an important first step of 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 positive, zero, or the same as the ground state, which are useful for certain applications.

cond-mat.quant-gas↗

arXiv2010

Measurement of an Efimov trimer binding energy in a three-component mixture of 6Li

The binding energy of an Efimov trimer state was precisely determined via radio-frequency association. It is found that the measurement results significantly shift with temperature, but that the shift can be made negligible at the lowest temperature in our experiment. The obtained trimer binding energy reveals a significant deviation from the nonuniversal theory prediction based on a three-body parameter with a monotonic energy dependence.

cond-mat.quant-gas↗

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↗