Foundations of Quantum Mechanics in the Light of New Technology

The following sections are included:

• PREFACE

• COMMITTEES

• CONTENTS

• OBITUARY: PROFESSOR SADAO NAKAJIMA

No abstract received.

No abstract received.

More than fifty years ago, the dilute hard sphere problem (in 3 dimensions) was studied by Bogoliubov¹, and Lee, Huang, Yang². In particular, the latter arrived at the ground state energy (per particle) of the system in an asymptotic expansion…

Bose-Einstein condensates offer an ideal testing ground for symmetry breaking. I will explain the reasons why and briefly overview our recent work on the related subject.

We discuss theoretically several important topics of atomic Bose-Einstein condensates. We address the dynamics of vortex lattice formation in a rotating condensate, and various types of vortex patterns in rotating two-component Bose condensates. Finally we discuss quantum turbulence in trapped condensates, followed by proposing how to make turbulence and showing that the energy spectra obey the well-known Kolmogorov law.

We have performed high-resolution spectroscopy of quantum degenerate gases of bosonic and fermionic ytterbium atoms using ultra-narrow intercombination transitions to probe quantum properties of the gases. The mean field interaction of the Bose-Einstein condensation and the energy distribution characteristic of the Fermi degeneracy were observed in the spectra.

We investigate superfluid properties of a two-component Fermi gas in the BCS-BEC crossover region. Including strong-coupling effects within a Gaussian fluctuation approximation, we self-consistently determine the superfluid order parameter and chemical potential at finite temperatures. Using these self-consistent solutions, we calculate single-particle excitations, as well as collective mode, over the entire BCS-BEC crossover region. As one increases the strength of a pairing interaction, we show that, while the single-particle excitation gap becomes large, the velocity of collective Goldstone mode becomes small. As a result, the origin of temperature dependence of a physical quantity continuously changes from single-particle excitations to collective excitations, as one passes through the BCS-BEC crossover region. To see this, we examine the superfluid density.

The following sections are included:

• Bosonic vs Fermionic Superfluids

• The BEC-BCS Crossover

• The Unitarity Limit

• High-Temperature Superfluidity

• Realization in Ultracold Atomic Fermi Gases

• Observation of High-Temperature Superfluidity

• Fermionic Superfluidity with Imbalanced Spin Populations

• Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms

• Outlook

• References

Spontaneous magnetization of a spin-1 Bose-Einstein condensate with a ferromagnetic interaction is investigated. Polar-core vortices are shown to be created through the Kibble-Zurek mechanism. The number of created spin vortices obeys the system-size scaling and quench-time scaling predicted by Zurek.

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find that the quasiparticle current of Bogoliubov phonon is remarkably enhanced near the potential barrier at low energies. This leads to the increase of the transmission probability in the low energy region. We show that the quasiparticle current twists the relative phase of the condensate wavefunctions and induces a Josephson supercurrent through the barrier. The Josephson critical current is estimated by calculating the induced supercurrent and phase difference.

Critical velocities of superfluid Fermi gases in optical lattices are theoretically investigated across the BCS-BEC crossover. We calculate the excitation spectra in the presence of a superfluid flow in one- and two-dimensional optical lattices. It is found that the spectrum of low-lying Anderson-Bogoliubov (AB) mode exhibits a roton-like structure in the short-wavelength region due to the strong charge density wave fluctuations, and with increasing the superfluid velocity one of the roton-like minima reaches zero before the single-particle spectrum does. This means that superfluid Fermi gases in optical lattices are destabilized due to spontaneous emission of the roton-like AB mode instead of due to Cooper pair breaking.

When a two-dimensional harmonic potential is applied to bosonic cold atoms, the atoms become a cigar shape and the radial energy level is discretized. If the strength of the potential or the total number of atoms is tuned, the atoms can occupy not only the lowest radial band but also higher bands. This paper shows that when both the lowest band and the second lowest doubly-degenerate bands are filled, the repulsive interaction among atoms induces a ground state circulating along the trap axis, in which the Z₂ reflection symmetry is spontaneously broken.

Recent advances in the studies of anomalous Hall effect (AHE) and the spin Hall effect (SHE) are described with the emphasis on the (i) universal scaling of AHE, (ii) the metallic SHE, and (iii) the noncentro-symmetric superconductors as an analogue of quantum spin Hall system.

A topological insulator is a material with a bulk excitation gap generated by the spin orbit interaction that is topologically distinct from an ordinary insulator. This distinction, characterized by a Z₂ topological invariant, necessitates the existence of gapless electronic states on the sample boundary, which have important implications for electronic transport. In two dimensions, the topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. Here we will outline the theory of this phase and describe two recent experiments in which its signatures have been observed. (1) Transport experiments on HgTe/HgCdTe quantum wells have demonstrated the existence of the edge states predicted for the quantum spin Hall insulator. (2) Photoemission experiments on the semiconducting alloy Bi_1-xSb_x have observed the gapless surface states predicted for a three dimensional topological insulator. We will close by arguing that the proximity effect between an ordinary superconductor and a topological insulator leads to a non-trivial interface state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation.

The spin-Hall effect (SHE) and the inverse spin-Hall effect (ISHE) coupled with magnetization dynamics were investigated using a simple Ni₈₁Fe₁₉/Pt film. A spin current generated by magnetization dynamics was detected electrically using ISHE. The observed magnetic field angle dependence of the ISHE signal is well reproduced by a model calculation based on the dc spin pumping and ISHE. In the same system, we found that spin relaxation in the Ni₈₁Fe₁₉ layer is manipulated electrically using SHE. An electric current applied to the Pt layer exerts the spin torque on the entire magnetization of the Ni₈₁Fe₁₉ layer via the macroscopic spin transfer induced by SHE, which modulates spin relaxation in the Ni₈₁Fe₁₉ layer. This spin-relaxation modulation enables quantitative measurements of spin currents without assuming any microscopic parameters.

We show that a charge current and a spin current are pumped from the spin dynamics in the presence of the spin-orbit interaction. We are particularly interested the inverse spin Hall effect which converts spin currents into charge ones. This conversion mechanism is demonstrated based on the conservation laws of charge and spin. The result is compared with the microscopic calculation.

We theoretically study quantum phase transitions between the quantum spin Hall and insulator phases, both of which are insulating in the bulk. This gives us an alternative view to the topological order in the quantum spin Hall phase. We also explain our theoretical proposal of the ultrathin bismuth film as a candidate to the 2D quantum spin Hall system.

We theoretically study the local anomalous Hall effect due to vector spin chirality carried by the local spins in the s-d model. We will show that the vector spin chirality contribution appears locally in the present of spin polarization, The global Hall effect vanishes if electron transport is homogeneous.

We study the spatial distribution of spin/charge current in the nonlocal geometry of the Py/Cu/Pt nanostructure, where the Py/Cu and Cu/Pt structures are employed as a spin injection/accumulation and a spin/charge current converter, respectively, by applying a finite element method in three-dimensions. Calculating two types of the device operations, we have found that the inhomogeneous current distribution appears in Cu/Pt area, suggesting the possibility of the spin-signal enhancement by controlling the shape of the Cu/Pt structure.

We investigate bulk and boundary multifractality at the metal-quantum spin Hall (QSH) insulator transition driven by disorder in two-dimensions. Recently, we have shown that boundary multifractality at this transition is different from that of the symplectic class by using the network model with random spin-orbit interactions. To give the another evidence for this result, we investigate multifractality at the metal-QSH insulator transition by using a different model, namely, the network model with a constant spin-orbit interaction. It is found that bulk multifractality at the metal-QSH insulator transition is that of the symplectic class while boundary multifractality is different, confirming identical with the previous result.

Roles of scattering events on the anomalous Hall effect in disordered ferromagnets at zero temperature are studied theoretically with a particular emphasis on scaling relations between the longitudinal conductivity σ_xx and the anomalous Hall conductivity σ_xy. The coherent potential approximation, which remains efficient even for the poorly conducting regime, to the two-dimensional ferromagnetic Rashba model has revealed three regimes and the associated scaling behaviors. In particular, it establishes the robustness of the moderately dirty intrinsic regime with σ_xy being insensitive to σ_xx as well as the relation with φ ≈ 1.6 in the poorly conducting regime which has been experimentally observed in a wide class of disordered ferromagnets.

Multiferroics, the materials in which both (anti)ferromagnetism and ferroelectricity can coexist, are the prospective host of the gigantic magnetoelectric (ME) effect. The multiferroics based on the spin-current (or inverse Dzyaloshinskiy-Moriya interaction) mechanism have recently been proved to realize in many cycloidal and conical spin states of frustrated magnets, in which the clamping between the magnetic and ferroelectric domains can show up. Recent advance in the research on the dynamical ME effects toward the electrical control of magnetism is presented.

Spin accumulation is a crucial but imprecise concept in spintronics., the science that strives to harness the electron spin for the benefit of information and communication technology. In metal-based spintronics it is characterized in terms of semiclassical distribution functions. In semiconductors with a strong spin-orbit coupling the spin accumulation is instead described as superpositions of coherent eigenstates. Here we present theoretical arguments that both views of the spin accumulation can be reconciled by taking into account the electron-electron interaction: a sufficiently strong self-consistent exchange field reduces a spin accumulation to a chemical potential difference between the two spin bands even in the presence of spin-orbit coupling. We demonstrate the idea on a clean two-dimensional electron gas (2DEG) by showing how the exchange field protects a spin accumulation from dephasing.

We demonstrate a spin interference effect in a small array of InGaAs mesoscopic rings. The spin interference is based on the time reversal Aharonov-Casher (AC) effect. The AC interference oscillations are controlled over several periods. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs two-dimensional gas channel. We observe a reproducible half-oscillation, which may be attributed to the competition between the Rashba SOI and the Dresselhaus SOI.

Interplay between magnetization dynamics and electric current in a conducting ferromagnet is theoretically studied based on a microscopic model calculation. First, the effects of the current on magnetization dynamics (spin torques) are studied with special attention to the "dissipative" torques arising from spin-relaxation processes of conduction electrons. Next, an analysis is given of the "spin motive force", namely, a spin-dependent 'voltage' generation due to magnetization dynamics, which is the reaction to spin torques. Finally, an attempt is presented of a unified description of these effects.

We study the transport properties of a p-InMnAs/n-InAs/Nb junction, where a p-InMnAs can be regarded as a spin injector. We fabricate the junctions at different distances between InMnAs and Nb electrodes and measure the differential conductance of the n-InAs channel. The conductance varies significantly depending on the direction of the injection current and the distance between the spin injector and the superconductor. We also calculate the conductance in the n-InAs channel by taking into account the exchange field in the InAs channel that is induced by the ferromagnetic semiconductor InMnAs. The difference between the conductance behaviors for different injection current directions can be explained by the inverse proximity effect in which the exchange field is also induced in the superconductor. From the experimental and theoretical results, it can be said that we successively observed the phenomena due to the inverse proximity effect that depends on a spin-polarized carrier injection.

Controlling the magnetization by low-voltage charge depletion in field effect transistors has been a formidable challenge due to the typically large carrier concentrations in ferromagnets compared to semiconductors. Here we demonstrate that this concept is viable in an all-semiconductor, p-n junction transistor utilizing a thin-film ferromagnetic (Ga,Mn)As channel. We report gate-dependent Curie temperature and magnetoresitance, and persistent magnetization switchings induced by short electrical pulses of a few volts.

We study the single-shot response of magnetic tunnel junctions induced by spin transfer torque, using ultra-short pulse measurement. After the pulse is applied, the switching proceeded by ns-scale incubation delay, followed by 400 ps transition, terminated by a ringing with the frequency of 1.4 GHz. While, the incubation delay has large distribution, the other resistance traces are reproducible.

Magnetic domain wall motion induced by magnetic fields and spin-polarized electrical currents is experimentally well established. A full understanding of the underlying mechanisms, however, remains elusive. We reported a detail comparison of such motions in the thermally activated subthreshold, or "creep," regime, where the wall velocity obeys an Arrhenius law. Experiment shows the velocity scales in an evidently different way with a driving current and field, proving the nonequivalence of two drives. Scaling theory explains the important features of experiment.

We have investigated the spin transport in superconducting Nb wire by using a lateral multi-terminal superconductive/normal/ferromagnetic metal junction. The spin polarized current exhibits a longer Cooper-pair recombination time than the unpolarized one because of the spin-flipping. A pure spin current injection is also found to induce nonlocally the quasi-particle and supercurrents in the superconductor.

Toward the manipulation of a single atomic spin, we theoretically study the switching of a localized quantum spin S = (S_x, S_y, S_z) induced by spin injection in electrode/S/electrode junctions. This S has a uniaxial anisotropy energy, , which shows the bistable potential between S_z = -S and S, with D being an anisotropy constant. Furthermore, S interacts with the atomic vibration. For the initial state of S_z = -S, we consider a situation in which up-spin electrons exhibit the spin-flip tunneling from the left electrode to the right one through an exchange interaction between the electron spin and S. Using the master equation approach, we investigate the time t dependence of the current I, the expectation value of S_z, 〈S_z〉, and that of the vibration quantum number, 〈n〉, of an S = 2 system, which corresponds to an Fe atom on CuN surface. The systems exhibit the switching or nonswitching depending on the transition probability due to spin-atomic vibration interaction within a period of 10 ns. In addition, the t dependence of I and 〈n〉 is explained on the basis of 〈S_z〉 and the probability distribution.

Based on the spin-polarized free-electron model, we derive the spin transfer torque in the magnetic tunnel junctions with the synthetic ferrimagnetic layers by analyzing the spin and charge transports in the ballistic regime. In the realistic junctions, the spin torque exerted on the magnetizations of two ferromagnetic layers in the synthetic ferrimagnetic layer shows the unidirectional rotation. It is suggested that, through the antiferromagnetic interlayer coupling in the synthetic ferrimagnetic layer, this unidirectional rotation induces the cooperative reversal of magnetizations in two ferromagnetic layers, and expected that this cooperative rotation reduces the critical current for the magnetization reversal in the synthetic ferrimagnetic layer.

We theoretically study one-dimensional spin-1/2 multiferroic systems. We focus on the chiral ordered phase with gapless excitations that appears under easy-plane anisotropy. The in-plane chirality dynamics contains gapless solitons associated with local flips of the chirality. These excitations contribute to the low-frequency dielectric response through the magnetoelectric coupling.

A brief review is given on exotic electronic properties of monolayer, bilayer, and multilayer graphenes from a theoretical point of view. In multi-layer graphenes, the Hamiltonian is decomposed into those of monolayer and bilayer graphenes.

Zero-gap state with the Dirac cone type energy dispersion has been found in an organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional zero-gap state discovered in bulk crystals with layered structure and exhibits transport phenomena characteristic to electrons on the Dirac cone type energy structure. We find a characteristic transport feature of bulk system in inter-layer resistance. In the zero-gap system, n = 0 Landau level called zero-mode always appear at the contact point of Dirac cone under the magnetic field and the degeneracy increases proportional to the strength of the magnetic field. The effects of zero-mode conduces the remarkable negative inter-layer magnetoresistance at low temperatures.

In molecular solids α-(BEDT-TTF)₂I₃, it has been clarified that the gapless state originates from extremely anisotropic massless Dirac fermions described by the tilted Weyl equation. The inter-band effects of magnetic filed and extremely small amount of electron doping give rise to anomalous transport phenomena, i.e. the sharp and continuous reversal of the sign of the Hall coefficient at low temperatures. Among materials having Dirac fermions, α-(BEDT-TTF)₂I₃ has a unique feature of layered bulk structure, where motions of the fermions are highly two-dimensional but have weak inter-layer hopping. In order to study how this unique feature affects the electronic properties, we examine roles of the weak inter-layer hopping in the conductivity, the Hall conductivity, and the orbital susceptibility. It is shown how the sharpness of the sign reversal of the Hall coefficient is reduced by the inter-layer hopping. The sharp sign-reversal observed in α-(BEDT-TTF)₂I₃ can be explained when the inter-layer hopping is comparable or smaller than the effective spectrum broadening.

Hall effects of bismuth have been studied theoretically with a special reference to the diamagnetic susceptibility. The conductivity, the Hall conductivity, and the orbital susceptibility are calculated on the basis of the Kubo formula for the 4 × 4 Dirac fermions in three dimension. The Hall coefficient exhibits unexpected peaks at the band-edge and sign change at the center of the band gap. Implications of the present results to bismuth alloys are discussed.

We report a theoretical calculation explaining the quantum Nernst effect observed experimentally in a bismuth single crystal. Generalizing the edge-current picture in two dimensions, we show that the peaks of the Nernst coefficient survive in three dimensions due to a van Hove singularity. We also evaluate the phonon-drag effect on the Nernst coefficient numerically. Our result agrees with the experimental result for a bismuth single crystal.

We study the transport properties of a single InAs self-assembled quantum dot contacted with superconducting leads. The charging energy E_c of the quantum dot is much larger than the superconducting gap energy Δ. For the dot whose tunnel coupling Γ to the lead is much larger than Δ but smaller than E_c, we observe enhancement of first-order Andreev reflections by the Kondo effect. We find that the zero-bias conductance measured for various Δ's and Kondo temperature T_K's collapses onto a single curve with Δ/T_K as the only relevant energy scale, providing experimental evidence for universal scaling in this system. On the other hand, for the dot with Γ comparable to E_c we observe a supercurrent flowing through the dot, reflecting the charge fluctuation sufficiently greater that one.

We theoretically discuss coherent charge transport through an Aharonov-Bohm interferometer formed by a triple quantum dot using the nonequilibrium Green's function method. In particular, we introduce the coherent indirect coupling parameter α, which characterizes the indirect coupling strength via the reservoir between two quantum dots. When α = 0, the period of the Aharonov-Bohm oscillations for the linear conductance is h/e. For α ≠ 0, the period of the AB oscillations becomes 3h/e associated with the three small triangular paths of electrons, which are formed by the triple quantum dot and the reservoir.

We used a parallel coupled vertical double quantum dot device consisting of two laterally tunnel-coupled dots and a common source and drain electrodes to study the Aharanov-Bohm (AB) effects in the vertically flowing single electron tunneling current. In the charging diagram measured at a finite source-drain voltage a bonding state as a ground state, and an antibonding state as an excited state were distinguished for the one-electron state. We observed periodic magnetic oscillations for the current through the bonding state, due to the AB effect for the two tunnel-coupled dots.

We investigate the electronic properties of a sub-micron vertical resonant tunneling structure containing three self-assembled InAs quantum dots (SADs) surrounded by four gate electrodes. The stability diagram is obtained by measuring Coulomb oscillation peaks as a function of the four gate voltages, which are used to modulate the electro-chemical potential of each SAD differently. We assign the charge states in the diagram by identifying three sets of Coulomb oscillation lines. We observe specific anti-crossing behaviors for the two Coulomb oscillation lines in the different sets, reflecting parallel couplings between three SADs in the conduction channel.

We fabricate superconducting single-electron transistors on a semiconductor heterostructure substrate, and examine the energy levels of their charge states by means of microwave spectroscopy. Light illumination induces the carriers at the heterointerface, which work as a dissipative electromagnetic environment. We observe prohibition of tunneling due to dissipation in a device with smaller tunnel coupling, while the other device with larger tunnel coupling preserves its quantum nature in spite of dissipation.

Resonant tunneling through an open quantum dot in the Coulomb blockade regime T ≪ e²/C₀ is studied numerically using the path-integral Monte Carlo method. The quantum dot is connected to two bulk leads by single-mode point contacts. We consider on-site repulsion between electrons and analyze its influence on the resonant conductance. Our numerical results exhibit power-law dependence of the peak width and the phase transition between perfect conductor and insulator, which is consistent with the analytical results.

We study the full counting statistics for multi-terminal quantum dots. We show that the microscopic reversibility naturally results in a symmetry of the cumulant generating function, which generalizes the fluctuation theorem in the context of the coherent quantum transport. Using this symmetry, we derive the fluctuation-dissipation theorem and the Onsager-Casimir relation in the linear transport regime and the universal relations among nonlinear transport coefficients.

We analyze single-atom lasing in a system composed of an artificial atom coupled to a cavity mode. The population inversion of the artificial atom is modeled as a reversed relaxation process that takes the atom from its ground state to its excited state. In this setup there is no lasing threshold below which lasing does not occur, but lasing can be suppressed if the 'relaxation' rate, i.e. the atom pumping rate, is larger than a certain threshold value. Using transition-rate equations, we derive analytic expressions for the lasing suppression condition and the photon-distribution of the cavity, both in the lasing and suppressed-lasing regimes.

Since ISQM 05, entangled photons have been used in our laboratory for a number of novel experiments, both in the foundations of quantum mechanics and in quantum information. Because of the high quality of entanglement of photons, precision tests of a non-local realistic theory proposed by Leggett were possible. These experiments indicate that the concept to be abandoned in quantum mechanics is most likely that of objectivism or realism. In parallel experiments, many schemes of one-way quantum computation were performed. There, because of active feed-forward, one can realize an efficient quantum computer and eliminate the effect of the randomness of individual events. The most significant properties of all-optical quantum computation are the very short cycle time, which can easily be below 100 ns and the high fidelity of the operation. Finally, experiments on quantum entanglement over distances of up to 144 km not only confirmed that quantum communication with satellites is possible, they also allowed novel tests of Bell's inequality.

Measurements have been performed on superconducting flux qubits using a Josephson bifurcation amplifier (JBA). The qubit states are identified with the two stable dynamic states of a strongly driven non-linear oscillator that contains a SQUID sensor. Readout with high fidelity is achieved. Correlations in repeated measurements demonstrate that this readout is fully projective.

Artificial atoms, making transitions between discrete energy levels, can be made using superconducting circuits. Such circuits can used to conduct atomic-physics experiments on a silicon chip and test quantum mechanics at macroscopic scales.

We have theoretically investigated macroscopic quantum tunneling (MQT) and the influence of nodal quasiparticles and zero energy bound states (ZES) on MQT in s-wave/d-wave hybrid Josephson junctions. In contrast to d-wave/d-wave junctions, the low-energy quasiparticle dissipation resulting from nodal quasiparticles and ZES is suppressed due to a quasiparticle-tunneling blockade effect in an isotropic s-wave superconductor. Therefore, the inherent dissipation in these junctions is found to be weak. This result suggests high potential of s-wave/d-wave hybrid junctions for applications in quantum information devices.

We present both experimental and theoretical results on silicon isolated double quantum-dots (IDQDs) fabricated through trench isolation of silicon-on-insulator, with near-by single electron transistors (SETs) for charge state sensing. The IDQD can be described as an artificial molecule and is therefore capable of exhibiting quantum superposition states suitable for quantum information processing.

Nanometre scale quantum dot devices in silicon consisting of a single electron transistor and an isolated double quantum dot with multiple control gates have been fabricated. The polarisation of the double dot when biased with the control gates has been observed indirectly through its effect on the conduction in the single electron transistor. Simulation has confirmed the experimental results and aided in determining the mechanism of this effect. This work demonstrates the possibility of using the isolated double quantum dot as a charge qubit.

The finite difference method and anisotropic effective mass density functional theory (DFT) are used to study the dependence of the electronic structure, wavefunctions and charge density on the width of the Silicon neck between the dots in an Si/SiO₂ isolated double quatum dot (IDQD) structure. An optimised algorithm for determining the self-consistent DFT potential was used. We demonstrate that the behaviour changes from strong to weak coupling between 2nm and 5nm width of the neck connecting the 30nm diameter dots.

Semiconductor quantum dots have attracted much interest in implementing solid-state quantum information processing. Using InAs based quantum dots, we demonstrate quantum coupling between two stacked quantum dot molecules in electroluminescence, controlling Stark shifts of single quantum dots in electroluminescence, and 'plug and play' single photon emission at telecommunication wavelengths.

The qubit readout process with a Josephson bifurcation amplifier (JBA) is analyzed quantum-mechanically. We calculate and clarify the dynamics of the density operator of a driven nonlinear oscillator and a qubit coupled system during the measurement. In purely quantum-mechanical JBAs, the bifurcation is impossible. It requires finite decoherence. When the JBA is coupled to a qubit, that is initially in a superposition state, we can observe the entanglement formation between the qubit and the probe (JBA) and the projection into one of two separable states at the moment of the JBA transition begins. To readout the measurement result, however, we must wait until the two JBA states are macroscopically well separated. The waiting time is determined by the strength of the decoherence in the JBA. This would give a trade-off on the strength of the JBA decoherence for a precise readout.

A double-loop four-Josephson-junction (4-JJ) flux qubit has an advantage over a three-Josephson-junction flux qubit in that the energy gap Δ at the degeneracy point can be controlled in situ by varying the magnetic flux in one of the two loops. We report the results of microwave spectroscopy and ground-state measurements of a 4-JJ qubit. The value of Δ is estimated from the resonances due to one-photon and two-photon processes. The appearance of a qubit step on a two-dimensional flux map is in agreement with the theoretical prediction. The dependence of the maximum slope of the qubit step on the control flux indicates the tunability of the barrier height.

We theoretically compare transport properties of Fano-Kondo effect with those of Fano effect. We focus on shot noise characteristics of a triple quantum dot (QD) system in the Fano-Kondo region at zero temperature, and discuss the effect of strong electric correlation in QDs. We found that the modulation of the Fano dip is strongly affected by the on-site Coulomb interaction in QDs.

A universal single qubit operation on an ensemble of cold sodium atoms was accomplished by pure geometric rotations of the Bloch vector around axes 2 and 3 using carefully prepared resonant laser-controlled pulses. The phase shift between two states was measured using geometric atom interferometer and it was shifted double the phase difference of control with an uncertainty of 1%. The fidelity of the universal single qubit operation was evaluated to be 90%.

Dynamical aspects of quantum Brownian motion at low temperatures are investigated. Exact calculations of quantum entanglement among two Brownian oscillators are given without invoking the Born-Markov or Born approximation widely used for the study of open systems. Our approach is applicable for arbitrary time scale, in particular, suitable to probe short time regime at cold temperatures where many experiments on quantum information processing are performed. We study separability criteria based on the uncertainty relation, negativity, and entanglement of formation. We found a crossover behavior in a disentanglement process between quantum and thermal fluctuation dominated regimes. The fluctuation-dissipation relation which holds for one-particle quantum Brownian particle in the long time limit does not hold for two particles interacting with common environment. The deviation from the relation is originated in the interaction mediated by environment which drives two particles into a steady oscillatory state preventing the system to thermalize. Consequently there is a residual entanglement at low temperatures for arbitrary coupling strength. This entanglement is generated from environment nonperturbatively even when two modes are not entangled initially. When a distance of two oscillators are varied, competition between entanglement induced from environment and modified decoherence due to finite separation leads to the characteristic distance where entanglement is minimized. We also discuss the influence of Unruh effect on entanglement dynamics.

Exploiting the intrinsic nonlinearity of superconducting Josephson junctions, we propose a scalable circuit with superconducting (SC) qubits which is very similar to the successful one now being used for trapped ions. The SC qubits are coupled to the "vibrational" mode provided by a superconducting LC circuit or its equivalent (e.g., a SQUID). Both single-qubit rotations and qubit-LC-circuit couplings/decouplings can be controlled by varying frequencies of the applied ac magnetic fluxes. The circuit is scalable since the qubit-qubit interactions, mediated by the LC circuit, can be selectively performed, and the information transfer can be realized in a controllable way. We also discuss how to control the couplings between the qubit and the data bus via "dressed" qubit states. This approach can also be reduced to the one used for trapped ions.

We present a simple protocol to purify bipartite entanglement in spin-1/2 particles by utilising only natural spin-spin interactions, i.e. those that can commonly be realised in realistic physical systems, and S_z-measurements on single spins. Even the standard isotropic Heisenberg interaction is shown to be sufficient to purify mixed state entanglement if there are at least three pairs of spins. This approach could be useful for quantum information processing in solid-state-based systems.

We experimentally demonstrate remote state preparation and 9 × 9 density matrix reconstruction for spatial qutrits generated by passing down-converted photons through triple slits.

It is well known that the estimation of a small phase shift ɸ from the associated changes in the photon distribution in the output of a two-path interferometer can maximally achieve a sensitivity 1/δɸ equal to the uncertainty Δ(n₁ - n₂) of the photon number difference between the two paths. In the following, it is shown that the condition for achieving this quantum Cramer-Rao bound at any phase shift Φ₀ is that the state is symmetric with respect to an (unphysical) exchange of the two paths in the interferometer. If this condition is met, the phase sensitivity should be independent of the initial phase shift Φ₀, implying that experimentally observed phase dependences of sensitivity indicate either a non-optimal estimation strategy, or an effect of decoherence.

We show that a class of non-classical superposition states can be generated by interference between down-converted photons and a weak coherent laser light. These interferometric states are characterized by a single parameter η which is determined by the ratio of down-converted amplitude to squared coherent amplitude. For η > 2, the state is a superposition of two states, where all N-photons are either in one or the other of two non-orthogonal modes.

We study the superconducting properties of the β-pyrochlore oxides AOs₂O₆, where A is Cs, Rb and K. The variations in the superconducting transition temperature T_c from 3.3 K for Cs to 6.3 K for Rb and to 9.6 K for K and also in the superconducting character from weak- to strong-coupling with increasing T_c are ascribed to the enhancement in the rattling of the A ions. The rattling is essentially a local, anharmonic mode with low energy and serves as an effective media for the Cooper pairing in the β-pyrochlore oxides.

A parametric electromechanical resonator was fabricated using a GaAs/AlGaAs modulation-doped heterostructure. Three on-chip operations - actuation, detection and mechanical resonance frequency control - were all demonstrated using piezoelectric based strain-voltage transduction. We clearly confirmed that the device can perform bit storage operation in the same way a Parametron computer does. Finally, methods for constructing an electromechanical shift register and AND/OR gates are also proposed.

Electron phase microscopy based on the Aharonov-Bohm (AB) effect principle has been used to illuminate fundamental phenomena concerning superconductivity and magnetism by revealing the microscopic distributions of magnetic flux. These phenomena include the unconventional behaviors of interlayer Josephson vortices in anisotropic layered high-Tc superconducting YBa₂Cu₃O_7-δ (YBCO) thin films, which show various behaviors, especially when the applied magnetic field is greatly tilted to the layer plane, and the magnetization of tiny magnetic heads for perpendicular recording.

Frequency standards (atomic clocks) based on narrow optical transitions in ²⁷Al⁺ and ¹⁹⁹Hg⁺ have been developed over the past several years at NIST. Both types of standards are based on single ions confined in Paul traps, but differ in the methods used to prepare and detect the internal atomic states. Al⁺ lacks a strong, laser-accessible transition for laser-cooling and for state preparation and detection. Coupling with a Be⁺ ion, trapped simultaneously with the Al⁺ ion, enables state manipulation, detection, and cooling of the Al⁺ ion. Both standards have achieved absolute reproducibilities of a few parts in 10¹⁷. Development of femtosecond laser frequency combs makes it possible to directly compare optical frequencies. The present determination of f_Al/f_Hg is 1.052 871 833 148 990 438 (55), where the uncertainty is expressed in units of the least significant digit. Measurements of f_Al/f_Hg made over about one year show a drift rate consistent with zero. This result can be used to place limits on time variations of fundamental constants such as the fine structure constant α.

A metallofullerene Ti₂C₂@C₇₈, in which two titanium atoms and C₂ cluster are encapsulated, is studied by scanning tunneling microscopy and spectroscopy. Measurements of Ti₂C₂@C₇₈ on Cu(111) surface reveal that their cage symmetry is C₇₈-D_3h. There is a preferential orientation of Ti₂C₂@C₇₈ resulting from electrostatic interaction same as other metallofullerene.

We describe measurements on a silicon single electron transistor (SET) carried out using a custom cryogenic CMOS measurement circuit (LTCMOS) in close proximity to the device. Quantum mechanical states in the SET were mapped by continuous microwave spectroscopy. The real time evolution of a particularly long lived quantum mechanical state was observed in a single shot measurement, made possible by the much faster measurement rate (50kHz bandwidth). This technique is intended to be applied to the measurement of coherent states in a charge qubit device made of a silicon double dot.

The detection sensitivity (DS) of the commercial single-photon-receiver based on InGaAs gatemode avalanche photodiode is estimated. Instalment of a digital-blanking-system (DBS) to reduce dark current makes the difference between DS and the detection efficiency (DE). By numerical simulations, it is found that the blanked number of light-pulses by DBS is as many as a quarter of all incident pulses for a specific operation condition. DS is estimated at 0.27, which is 35% larger than a given DE.

Optical gains in electroluminescence of ultra-thin Si films sandwiched between SiO₂s are calculated from first principles. The gain of the most efficient film is comparable to that of the bulk GaAs if one-order-of-magnitude higher density of carriers is assumed. The importance of the surface structure of the Si film is also investigated and the interface with quartz crystal is found to be favorable for efficient light-emission.

First principles nonlinear optical spectroscopy, which is a theoretical alternative of the conventional nonlinear optical spectroscopy, has been realized by our new software calculating the linear and nonlinear optical spectra of materials on the basis of the density functional pseudopotential method. The new software can calculate second- and third-order nonlinear optical susceptibilities of insulating solids, accurately, taking into account the frequency dependence of the susceptibilities. The software is assumed to be a promising theoretical tool of design of new nonlinear optical materials, which is needed in opto-electronics and laser optics.

We propose a "field-induced polymorphous disorder" model to explain bias-stress instability in molecular organic thin-film transistors, based on the experimental results showing the strong correlation between the micro-structural change in semiconductor layer composed of penrtacene molecules and the threshold voltage (Vth) shift due to electron trapping in a reversible manner under the successive bias-stress, thermal annealing, and light irradiation.

Recently, quantum dynamics and coherence in Josephson junctions (JJs) made of high Tc superconductors (HTSCs) attract much attention. The HTSCs have relatively large energy gap and higher plasma frequency compared to those of metal superconductors, which suggests their high potential for quantum electron devices. On the other hand, the HTSC JJs have a disadvantage that the pairing symmetry is d-wave, and thus the intrinsic damping effect in the dynamic due to the nodal quasiparticles is inevitable. In addition to this, the coupling effect due to the atomic scale stack structure induces complex switching. Therefore, the peculiarities in quantum dynamics of the HTSC JJs must be clarified in order to realize the HTSCs quantum electronics. Here we report our efforts to observe macroscopic quantum tunneling and quantum coherence in the HTSC JJs.

Motivated by the prospect of the experimental realization of applications like quantum simulation and scalable quantum computation using planar Coulomb crystals, we investigated planar Coulomb crystals with one or multicomponents and analyzed in detail, for the first time, their structure and properties. We also studied the RF-heating of planar crystals and found out in which conditions it can be minimized. These results together with the estimation of the error in the implementation of quantum simulation allowed us to find the experimental requirements for realizing quantum simulation with planar crystals. Trying to fulfill these requirements we designed an RF trap system for the realization of planar Coulomb crystals and quantum simulation.

We give examples of situations where the negative binomial distribution has appeared in quantum physics since its debut in the work of Planck. Several of its properties are reviewed, and Mandel's Q-parameter is shown to play an interesting role. The photon-pair distributions of squeezed vacuum and squeezed single-photon states are identified as negative binomial.

The quantum mechanical system with continuum of modes is considered as model of the decaying atom. The exponential decay and the Lorentian shape of distribution of the emitted photon are deduced as the non-perturbative solution of the time-dependent Schrödinger equation. The possible application of the formalism to the decay of trapped atoms is discussed.

The following sections are included:

• LIST OF PARTICIPANTS

• AUTHOR INDEX