arXiv subjects

Yukihiro Ota

Publications and source records attributed to Yukihiro Ota.

At least 37 records · Page 2Linked to original sources

arXiv2012

Confidence and Backaction in the Quantum Filter Equation

We study the confidence and backaction of state reconstruction based on a continuous weak measurement and the quantum filter equation. As a physical example we use the traditional model of a double quantum dot being continuously monitored by a quantum point contact. We examine the confidence of the estimate of a state constructed from the measurement record, and the effect of backaction of that measurement on that state. Finally, in the case of general measurements we show that using the relative entropy as a measure of confidence allows us to define the lower bound on the confidence as a type of quantum discord.

quant-ph↗

arXiv2012

Nontrivial Haldane phase of an atomic two-component Fermi gas trapped in a 1d optical lattice

We propose how to create a non-trivial Haldane phase in atomic two-component Fermi-gas loaded on one-dimensional (1-D) optical lattice with trap potential. The Haldane phase is naturally formed on $p$-band Mott core in a wide range of the strong on-site repulsive interaction. The present proposal is composed of two steps, one of which is theoretical derivation of an effective 1-D S=1 interacting-chain model from the original tight-binding Hamiltonian handling the two $p$-orbitals, and the other of which is numerical demonstration employing the density-matrix renormalization-group for the formation of the Haldane phase on $p$-band Mott core and its associated features in the original tight-binding model with the harmonic trap potential.

cond-mat.quant-gas↗

arXiv2012

Entanglement amplification via local weak measurements

We propose a measurement-based method to produce a maximally-entangled state from a partially-entangled pure state. Our goal can be thought of as entanglement distillation from a single copy of a partially-entangled state. The present approach involves local two-outcome weak measurements. We show that application of these local weak measurements leads to a probabilistic amplification of entanglement. In addition, we examine how the probability to find the maximally-entangled state is related to the entanglement of the input state. We also study the application of our method to a mixed initial state. We show that the protocol is successful if the separable part of the mixed initial state fulfils certain conditions.

quant-ph↗

arXiv2012

Implementing general measurements on linear optical and solid-state qubits

We show a systematic construction for implementing general measurements on a single qubit, including both strong (or projection) and weak measurements. We mainly focus on linear optical qubits. The present approach is composed of simple and feasible elements, i.e., beam splitters, wave plates, and polarizing beam splitters. We show how the parameters characterizing the measurement operators are controlled by the linear optical elements. We also propose a method for the implementation of general measurements in solid-state qubits.

quant-ph↗

arXiv2012

Efficient Numerical Self-consistent Mean-field Approach for Fermionic Many-body Systems by Polynomial Expansion on Spectral Density

We propose an efficient numerical algorithm to solve Bogoliubov de Gennes equations self-consistently for inhomogeneous superconducting systems with a reformulated polynomial expansion scheme. This proposed method is applied to typical issues such as a vortex under randomly distributed impurities and a normal conducting junction sandwiched between superconductors. With various technical remarks, we show that its efficiency becomes remarkable in large-scale parallel performance.

cond-mat.supr-con↗

arXiv2011

Quantum Oracles in Terms of Universal Gate Set

We present a systematic construction of quantum circuits implementing Grover's database search algorithm for arbitrary number of targets. We introduce a new operator which flips the sign of the targets and evaluate its circuit complexity. We find the condition under which the circuit complexity of the database search algorithm based on this operator is less than that of the conventional one.

quant-ph↗

arXiv2011

Variety of c-axis collective excitations in layered multigap superconductors

We present a dynamical theory for the phase differences along a stacked direction of intrinsic Josephson junctions (IJJ's) in layered multigap superconductors, motivated by the discovery of highly-anisotropic iron-based superconductors with thick perovskite-type blocking layers. The dynamical equations describing AC and DC intrinsic Josephson effects peculiar to multigap IJJ's are derived, and collective Leggett mode excitations in addition to the Josephson plasma established in single-gap IJJ's are predicted. The dispersion relations of their collective modes are explicitly displayed, and the remarkable peculiarity of the Leggett mode is demonstrated.

cond-mat.supr-con↗

arXiv2011

Collective modes in multiband superfluids and superconductors: Multiple dynamical classes

One important way to characterize the states having a gauge symmetry spontaneously broken over multibands should be to look at their collective excitation modes. We find that a three-band system has multiple Leggett modes with significantly different masses, which can be classified into different dynamical classes according to whether multiple inter-band Josephson currents add or cancel. This provides a way to dynamically characterize multiband superconductivity while the pairing symmetry is a static property.

cond-mat.supr-con↗

arXiv2011

Macroscopic quantum tunneling in multigap superconducting Josephson junctions: Escape rate enhancement via quantum fluctuations of Josephson-Leggett mode

We theoretically study the macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multi-gap one such as ${MgB}_{2}$ and iron-based superconductors. In such a Josephson junction more than one phase differences are defined. We clarify the quantum dynamics of the phase differences and construct a theory for the MQT in the multi-gap Josephson junctions. The dynamics of the phase differences are strongly affected by the Josephson-Leggett mode, which is the out-of-phase oscillation mode of the phase differences. The escape rate is calculated in terms of the effective action renormalized by the Josephson-Leggett mode. It is shown that the escape rate is drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.

quant-ph↗

arXiv2010

Shapiro Steps as a Direct Probe of $\pm s$-wave Symmetry in Multi-gap Superconducting Josephson Junctions

We theoretically study the Shapiro steps in a hetero Josephson junction made of a single-gap superconductor and a two-gap one. We find that an anomalous DC Josephson current is induced by tuning the frequency of an applied microwave to the Josephson-Leggett mode frequency, which creates an extra step structure in the $I$-$V$ characteristics besides the conventional Shapiro steps. The step heights at the resonance voltages exhibit an alternate structure of a large and small value reflecting the gap symmetry of the two-gap superconductor. In the $\pm s-wave case in which the two gaps have opposite signs in the two-gap superconductor the steps with odd index are enhanced, whereas in the s-wave case the ones with even index have larger values. The existence of the fractional Shapiro steps is also predicted.

cond-mat.supr-con↗

arXiv2010

Ambegaokar-Baratoff relations of Josephson critical current in heterojunctions with multi-gap superconductors

An extension of the Ambegaokar-Baratoff relation to a superconductor-insulator-superconductor (SIS) Josephson junction with multiple tunneling channels is derived. Appling the resultant relation to a SIS Josephson junction formed by an iron-based (five-band) and a single-band Bardeen-Cooper-Schrieffer (BCS) type superconductors, a theoretical bound of the Josephson critical current ($I_{\rm c}$) multiplied by the resistance of the junction ($R_{\rm n}$) is given. We reveal that such a bound is useful for identifying the pairing symmetry of iron-pnictide superconductors. One finds that if a measured value of $I_{\rm c}R_{\rm n}$ is smaller than the bound then the symmetry is $\pm s$-wave, and otherwise $s$-wave without any sign changes. In addition, we stress that temperature dependence of $I_{\rm c}R_{\rm n}$ is sensitive to the difference of the gap functions from the BCS type gap formula in the above heterojunction.

cond-mat.supr-con↗

arXiv2010

Theory of vortex structure in Josephson junctions with multiple tunneling channels: Vortex enlargement as a probe of $\pm s$-wave superconductivity

We theoretically study Josephson vortex structures in Josephson junctions which have multiple tunneling channels caused by multiple superconducting gaps. Deriving "coupled sine-Gordon equations" from the free-energy taking account of the multiple tunneling channels, we examine two typical situations, a heterotic junction composed of multigap-superconductor, insulator, and single-gap superconductor, and a grain-boundary junction formed by two identical multigap superconductors. Then, we reveal in both situations that the magnetic field distribution of the Josephson vortex for $\pm s$-wave superconductivity is more enlarged than that for s-wave without sign change between the order parameters. Its mechanism is ascribed to a cancellation of the multiple Josephson currents. We display such an anomalous Josephson vortex and suggest how to evaluate the enlargement.

cond-mat.supr-con↗

arXiv2009

Geometric quantum gates in liquid-state NMR based on a cancellation of dynamical phases

A proposal for applying non-adiabatic geometric phases to quantum computing, called the double-loop method [S.-L. Zhu and Z. D. Wang, Phys. Rev. A {\bf 67}, 022319 (2003)], is demonstrated in a liquid state NMR quantum computer. Using a spin echo technique, the original method is modified so that quantum gates are implemented in a standard high-precision NMR system for chemical analysis. We show that the dynamical phase is successfully eliminated and a one-qubit quantum gate is realized, although the fidelity is not so high.

quant-ph↗

arXiv2009

Intergrain Josephson Currents in Multigap Superconductors: Microscopic Origin of Low Intergrain Critical Current and Its Recovery Potential in Iron-Pnictide Materials

We microscopically examine the intergrain Josephson current in iron-pnictide superconductors in order to solve the puzzle of why the intergrain current is much lower than the intragrain one. The theory predicts that the intergrain Josephson current is significantly reduced by the $\pm$s-wave symmetry when the incoherent tunneling becomes predominant and the density of states and the gap amplitude between two bands are identical. We find in such a situation that the temperature dependence of the intergrain Josephson current shows an anomalously flat curve over a wide temperature range. Finally, we suggest important points for increasing the intergrain current.

cond-mat.supr-con↗

arXiv2009

Composite pulses in NMR as non-adiabatic geometric quantum gates

We show that some composite pulses widely employed in NMR experiments are regarded as non-adiabatic geometric quantum gates with Aharanov-Anandan phases. Thus, we reveal the presence of a fundamental issue on quantum mechanics behind the traditional technique of the composite pulses. To examine the robustness of such composite pulses against fluctuations, we present a practical noise model in a two-level system. Then, we find that the composite pulses possesses purely geometrical nature even under a specific type of fluctuations.

quant-ph↗

arXiv2009

Theory of heterotic SIS Josephson junctions between single- and multi-gap superconductors

Using the functional integral method, we construct a theory of heterotic SIS Josephson junctions between single- and two-gap superconductors. The theory predicts the presence of in-phase and out-of-phase collective oscillation modes of superconducting phases. The former corresponds to the Josephson plasma mode whose frequency is drastically reduced for $\pm$ s-wave symmetry, and the latter is a counterpart of Leggett's mode in Josephson junctions. We also reveal that the critical current and the Fraunhofer pattern strongly depend on the symmetry type of the two-gap superconductor.

cond-mat.supr-con↗

arXiv2008

Implementation of holonomic quantum gates by an isospectral deformation of an Ising dimer chain

We exactly construct one- and two-qubit holonomic quantum gates in terms of isospectral deformations of an Ising model Hamiltonian. A single logical qubit is constructed out of two spin-1/2 particles; the qubit is a dimer. We find that the holonomic gates obtained are discrete but dense in the unitary group. Therefore an approximate gate for a desired one can be constructed with arbitrary accuracy.

quant-ph↗

arXiv2007

Decrease of entanglement by local operations in the Dür-Cirac method

One cannot always obtain information about entanglement by the Dür-Cirac (DC) method. The impracticality is attributed to the decrease of entanglement by local operations in the DC method. We show that, even in 2--qubit systems, there exist states whose entangled property the DC method never evaluates. The class of such states in 2--qubit systems is completely characterized by the value of the fully entangled fraction. Actually, a state whose fully entangled fraction is less than or equal to 1/2 is always transformed into a separable state by local operations in the DC method, even if it has negative partial transposition.

quant-ph↗