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Journal Articles

DC transport in a dissipative superconducting quantum point contact

Visuri, A.-M.*; Mohan, J.*; Uchino, Shun; Huang, M.-Z.*; Esslinger, T.*; Giamarchi, T.*

Physical Review Research (Internet), 5(3), p.033095_1 - 033095_11, 2023/08

We study the current-voltage characteristics of a superconducting junction with particle losses at the contacts. We adopt the Keldysh formalism to compute the steady-state current for varying transmission of the contact. In the low transmission regime, the dissipation leads to an enhancement of the current at low bias, a nonmonotonic dependence of current on dissipation, and the emergence of new structures in the current-voltage curves. The effect of dissipation by particle loss is found to be qualitatively different from that of a finite temperature and a finite inelastic scattering rate in the reservoirs.

Journal Articles

Superfluid signatures in a dissipative quantum point contact

Huang, M.-Z.*; Mohan, J.*; Visuri, A.-M.*; Fabritius, P.*; Talebi, M.*; Wili, S.*; Uchino, Shun; Giamarchi, T.*; Esslinger, T.*

Physical Review Letters, 130(20), p.200404_1 - 200404_8, 2023/05

 Times Cited Count:1 Percentile:83.24(Physics, Multidisciplinary)

We measure superfluid transport of strongly-interacting fermionic lithium atoms through a quantum point contact with local, spin-dependent particle loss. We observe that the characteristic non-Ohmic superfluid transport enabled by high-order multiple Andreev reflections transitions into an excess Ohmic current as the dissipation strength exceeds the superfluid gap. We develop a model with mean-field reservoirs connected via tunneling to a dissipative site. Our calculations in the Keldysh formalism reproduce the observed non-equilibrium particle current, yet do not fully explain the observed loss rate or spin current.

Journal Articles

Dominant Andreev reflection through nonlinear radio-frequency transport

Zhang, T.*; Tajima, Hiroyuki*; Sekino, Yuta*; Uchino, Shun; Liang, H.*

Communications Physics (Internet), 6, p.86_1 - 86_7, 2023/04

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

We theoretically propose the laser-induced Andreev reflection between two-component Fermi superfluid and normal states via spatially-uniform Rabi couplings. By analyzing the tunneling current between the superfluid and normal states up to the fourth order in the Rabi couplings, we find that the Andreev current exhibits unconventional non-Ohmic transport at zero temperature. Remarkably, the Andreev current gives the only contribution in the synthetic junction system at zero detunings regardless of the ratio of the chemical potential bias to the superfluid gap, which is in sharp contrast to that in the conventional superconductor-normal metal junction. Our result may also pave a way for understanding the black hole information paradox through the Andreev reflection as a quantum-information mirror.

Journal Articles

Spin conductivity spectrum and spin superfluidity in a binary Bose mixture

Sekino, Yuta*; Tajima, Hiroyuki*; Uchino, Shun

Physical Review Research (Internet), 5(2), p.023058_1 - 023058_12, 2023/04

We investigate the spectrum of spin conductivity for a miscible two-component Bose-Einstein condensate (BEC) that exhibits spin superfluidity. By using the Bogoliubov theory, the regular part being the spin conductivity at finite ac frequency and the spin Drude weight characterizingthe delta-function peak at zero frequency are analytically computed. We demonstrate that the spectrum exhibits a power-law behavior at low frequency, reflecting gapless density and spin modes specific to the binary BEC. At the phase transition points into immiscible and quantum-dropletstates, the change in quasiparticle dispersion relations modifies the power law. In addition, the spin Drude weight becomes finite, indicating zero spin resistivity due to spin superfluidity. Our results also suggest that the Andreev-Bashkin drag density is accessible by measuring the spin conductivity spectrum.

Journal Articles

Comparative study for two-terminal transport through a lossy one-dimensional quantum wire

Uchino, Shun

Physical Review A, 106(5), p.053320_1 - 053320_14, 2022/11

 Times Cited Count:2 Percentile:70.1(Optics)

Motivated by realization of the dissipative quantum point contact in ultracold atomic gases, we investigate a two-terminal mesoscopic transport system in which a single-particle loss is locally present in a one-dimensional chain. By means of the Dyson equation approach in the Keldysh formalism that can incorporate dissipative effects, we reveal analytic structures of the particle and energy currents whose formal expressions correspond to ones in certain three-terminal systems where the particle loss is absent. The obtained formulas are also consistent with non-hermitian and three-terminal Landauer-Buttiiker analyses. The universality on the current expressions holds regardless of quantum statistics and may be useful for understanding lossy two-terminal transport in terms of three-terminal transport and vice versa.

Journal Articles

Optical spin conductivity in ultracold quantum gases

Sekino, Yuta*; Tajima, Hiroyuki*; Uchino, Shun

Physical Review Research (Internet), 4(4), p.043014_1 - 043014_16, 2022/10

We show that the optical spin conductivity being a small AC response of a bulk spin current and elusive in condensed matter systems can be measured in ultracold atoms. We demonstrate that this conductivity contains rich information on quantum states by analyzing experimentally achievable systems. The obtained conductivity spectra being absent in the Drude conductivity reflect quasiparticle excitations and non-Fermi liquid properties. Unlike its mass transport counterpart, the spin conductivity serves as a probe applicable to clean atomic gases without disorder and lattice potentials. Our formalism can be generalized to various systems such as spin-orbit coupled and nonequilibrium systems.

Journal Articles

Optomechanical response of a strongly interacting Fermi gas

Helson, V.*; Zwettler, T.*; Roux, K.*; Konishi, Hideki*; Uchino, Shun; Brantut, J.-P.*

Physical Review Research (Internet), 4(3), p.033199_1 - 033199_10, 2022/09

We study a Fermi gas with strong, tunable interactions dispersively coupled to a high-finesse cavity. Upon probing the system along the cavity axis, we observe a strong optomechanical Kerr nonlinearity originating from the density response of the gas to the intracavity field. Measuring the non-linearity as a function of interaction strength, we extract the zero-frequency density response function of the Fermi gas, and find an increase by a factor of two from the Bardeen-Cooper-Schrieffer to the Bose-Einstein condensate regime. The results are in quantitative agreement with a theory based on operator-product expansion, expressing the density response in terms of universal functions of the interactions, the contact and the internal energy of the gas.

Journal Articles

Asymmetry and nonlinearity of current-bias characteristics in superfluid-normal-state junctions of weakly interacting Bose gases

Uchino, Shun

Physical Review A, 106(1), p.L011303_1 - L011303_5, 2022/07

 Times Cited Count:2 Percentile:45.37(Optics)

We uncover current-bias characteristics of superfluid-normal-state junctions with weakly interacting Bose gases. It is shown that in the presence of a chemical potential bias the characteristics can strongly be asymmetric for origin. The salient feature that is absent in the fermionic counterpart arises from a tunneling process associated with a condensate and a bosonic Andreev reflection process. It turns out that such processes are intrinsically nonlinear and therefore do not obey Ohm's law even at a low bias.

Journal Articles

Optical spin transport theory of spin-$$frac{1}{2}$$ topological Fermi superfluids

Tajima, Hiroyuki*; Sekino, Yuta*; Uchino, Shun

Physical Review B, 105(6), p.064508_1 - 064508_9, 2022/02

 Times Cited Count:4 Percentile:61.85(Materials Science, Multidisciplinary)

We theoretically investigate optical bulk spin transport properties in a spin-$$frac{1}{2}$$ topological Fermi superfluid. We specifically consider a one-dimensional system with an inter-spin $$p$$-wave interaction, which can be realized in ultracold-atom experiments. Developing the BCS-Leggett theory to describe the BCS to Bose-Einstein condensate evolution and the Z$$_{2}$$ topological phase transition in this system, we show how the spin transport reflects these many-body aspects. We find that the optical spin conductivity, which is a small AC response of a spin current, shows the spin-gapped spectrum in the wide parameter region and the gap closes at the Z$$_{2}$$ topological phase transition point. Moreover, the validity of the low-energy effective model of the Majorana zero mode is discussed along the BCS-BEC evolution in connection with the scale invariance at $$p$$-wave unitarity.

Journal Articles

Observation of spin-space quantum transport induced by an atomic quantum point contact

Ono, Koki*; Higomoto, Toshiya*; Saito, Yugo*; Uchino, Shun; Nishida, Yusuke*; Takahashi, Yoshiro*

Nature Communications (Internet), 12, p.6724_1 - 6724_8, 2021/11

 Times Cited Count:7 Percentile:70.49(Multidisciplinary Sciences)

Quantum transport is ubiquitous in physics. So far, quantum transport between terminals has been extensively studied in solid state systems from the fundamental point of views such as the quantized conductance to the applications to quantum devices. Recent works have demonstrated a cold-atom analog of a mesoscopic conductor by engineering a narrow conducting channel with optical potentials, which opens the door for a wealth of research of atomtronics emulating mesoscopic electronic devices and beyond. Here we realize an alternative scheme of the quantum transport experiment with ytterbium atoms in a two-orbital optical lattice system. Our system consists of a multi-component Fermi gas and a localized impurity, where the current can be created in the spin space by introducing the spin-dependent interaction with the impurity. We demonstrate a rich variety of localized-impurity-induced quantum transports, which paves the way for atomtronics exploiting spin degrees of freedom.

Journal Articles

Tunneling Hamiltonian analysis of DC Josephson currents in a weakly-interacting Bose-Einstein condensate

Uchino, Shun

Physical Review Research (Internet), 3(4), p.043058_1 - 043058_13, 2021/10

Atomtronics experiments with ultracold atomic gases allow us to explore quantum transport phenomena of a weakly-interacting Bose-Einstein condensate (BEC). Here, we focus on two-terminal transport of such a BEC in the vicinity of zero temperature. By using the tunnel Hamiltonian and Bogoliubov theory, we obtain a DC Josephson current expression in the BEC and apply it to experimentally relevant situations such as quantum point contact and planar junction. Due to the absence of Andreev bound states but the presence of couplings associated with condensation elements, a current-phase relation in the BEC is found to be different from one in an $$s$$-wave superconductor.

Journal Articles

Atomtronics

Uchino, Shun

Nihon Butsuri Gakkai-Shi, 76(1), p.4 - 12, 2021/01

Ultracold atomic gases allow us to simulate bare essentials of complicated quantum phenomena. Recently, atomtronics devices to simulate mesoscopic transport with ultracold atomics gases have been realized in experiments. In this article, we review recent progress of two-terminal transport in ultracold atomic gases, with a special focus on point contact transport in two-component Fermi gases.

Oral presentation

Mesoscopic transport with Bose-Einstein condensates

Uchino, Shun

no journal, , 

We discuss mesoscopic transport systems with Bose-Einstein condensates. We demonstrate its transport characteristics under several setups and reveal differences from mesoscopic superconductors.

Oral presentation

Quantum transport with ultracold atomic gases

Uchino, Shun

no journal, , 

Utracold atomic gases play crucial roles in revealing nontrivial quantum transport phenomena. In this presentation, I will discuss recent progresses on mesoscopic transport realized with ultracold atomic gases.

Oral presentation

Mesoscopic transport with weakly-interacting Bose gases

Uchino, Shun

no journal, , 

Utracold atomic gases play crucial roles in revealing nontrivial quantum transport phenomena. Especially, quantum transport of weakly-interacting Bose gases would be interesting in that the realizations with condensed-matter systems are difficult. In this talk, I will demonstrate that two-terminal transport systems with Bose-Einstein condensates show nontrivial transport properties. In spite of the formal similarity with fermionic superconducting systems, the current-bias characteristics in Bose-Einstein condensates is quite different due to the presence of a unique transport process existing in the condensates.

Oral presentation

Mesoscopic transport with a weakly-interacting Bose gas

Uchino, Shun

no journal, , 

Recently, mesoscopic transport with ultracold atomic gases has attracted attention. Based on such a progress, I have engaged in research on mesoscopic transport with a weakly-interacting Bose gas. In this presentation, I will report on recent achievements on it.

Oral presentation

Quantum transport in ultracold atomic gases

Uchino, Shun

no journal, , 

Quantum Transport that allow one to characterize quantum many-body systems plays a crucial role in condensed matter physics. In this presentation, I will explain what happens if the concept of quantum transport is applied to ultracold atomic gases.

Oral presentation

Mesoscopic transport with ultracold atomic gases

Uchino, Shun

no journal, , 

Ultracold atomic gases allow us to simulate bare essentials of complicated quantum phenomena. Recently, atomtronics, which is the cold-atom analog of electronics, has attracted attention due to experimental realizations of mesoscopic and circuit systems with ultracold atomic gases. In this talk, I will focus on mesoscopic transport phenomena realized by two-terminal transport experiments with ultracold atomic gases, and demonstrate that novel transport phenomena including mesoscopic transport with Bose-Einstein condensates and with a lossy quantum-point contact can be revealed with atomtronic devices.

Oral presentation

Quantum point contact transport in the presence of a particle loss

Uchino, Shun

no journal, , 

In mesoscopic systems with ultracold atomic gases, one can explore an effect of particle loss in the conduction channel region. In this talk, I will discuss transport characteristics in the presence of particle loss that can be obtained from an analysis based on the Keldysh and Lindblad formalisms.

Oral presentation

Quantum transport with cold atoms

Uchino, Shun

no journal, , 

I will give a presentation on quantum transport with cold atoms. In particular, I will discuss the recent progresses of quantum transport studies that harness advantages of cold atoms such as transport of bosons, dissipation control, and synthetic dimensions.

23 (Records 1-20 displayed on this page)