Impact of avalanche type of transport on internal transport barrier formation in tokamak plasmas.

In magnetic fusion plasmas, a transport barrier is essential to improve the plasma confinement. The key physics behind the formation of a transport barrier is the suppression of the micro-scale turbulent transport. On the other hand, long-range transport events, such as avalanches, has been recognized to play significant roles for global profile formations. In this study, we observed the impact of the avalanche-type of transport on the formation of a transport barrier for the first time. The avalanches are found to inhibit the formation of the internal transport barrier (ITB) observed in JT-60U tokamak. We found that (1) ITBs do not form in the presence of avalanches but form under the disappearance of avalanches, (2) the surface integral of avalanche-driven heat fluxe is comparable to the time rate change of stored energy retained at the ITB onset, (3) the mean E × B flow shear is accelerated via the ion temperature gradient that is not sustained under the existence of avalanches, and (4) after the ITB formation, avalanches are damped inside the ITB, while they remain outside the ITB.

Coherent spin qubit transport in silicon.

A fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.

Atomic-level characterization of free volume in the structure of Cu67Zr33amorphous alloy.

The structure of Cu67Zr33amorphous alloy was investigated in terms of packing density and free volume by using neutron, x-ray diffraction and reverse Monte Carlo (RMC) modelling. The RMC model was analysed by a method of decomposing the three-dimensional atomic configuration into fundamental polyhedral units (termed as 'holes' referencing the Bernal's works) of which faces are all triangles consisting of chemical bonds. Not only tetrahedral and octahedral holes but also other larger holes were identified. Moreover, the atomic packing fractions and free volumes in the respective polyhedral holes were evaluated with reference to those for the corresponding crystal structures. The results show that the distribution of free volumes for the larger holes can be described by the exponential function assuming that there are no energetic interactions between each other. On the other hand, the local structural fluctuations due to densely and loosely packed tetrahedral holes were observed, leading to the negative free volume spaces.

Operation of a silicon quantum processor unit cell above one kelvin.

Quantum computers are expected to outperform conventional computers in several important applications, from molecular simulation to search algorithms, once they can be scaled up to large numbers-typically millions-of quantum bits (qubits)1-3. For most solid-state qubit technologies-for example, those using superconducting circuits or semiconductor spins-scaling poses a considerable challenge because every additional qubit increases the heat generated, whereas the cooling power of dilution refrigerators is severely limited at their operating temperature (less than 100 millikelvin)4-6. Here we demonstrate the operation of a scalable silicon quantum processor unit cell comprising two qubits confined to quantum dots at about 1.5 kelvin. We achieve this by isolating the quantum dots from the electron reservoir, and then initializing and reading the qubits solely via tunnelling of electrons between the two quantum dots7-9. We coherently control the qubits using electrically driven spin resonance10,11 in isotopically enriched silicon12 28Si, attaining single-qubit gate fidelities of 98.6 per cent and a coherence time of 2 microseconds during 'hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures13-16. Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 tesla, corresponding to a qubit control frequency of 3.5 gigahertz, where the qubit energy is well below the thermal energy. The unit cell constitutes the core building block of a full-scale silicon quantum computer and satisfies layout constraints required by error-correction architectures8,17. Our work indicates that a spin-based quantum computer could be operated at increased temperatures in a simple pumped 4He system (which provides cooling power orders of magnitude higher than that of dilution refrigerators), thus potentially enabling the integration of classical control electronics with the qubit array18,19.

Coherent spin control of s-, p-, d- and f-electrons in a silicon quantum dot.

Once the periodic properties of elements were unveiled, chemical behaviour could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in semiconductor materials disrupt this analogy, so real devices seldom display a systematic many-electron arrangement. We demonstrate here an electrostatically confined quantum dot that reveals a well defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. Various fillings containing a single valence electron-namely 1, 5, 13 and 25 electrons-are found to be potential qubits. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR), leading to faster Rabi rotations and higher fidelity single qubit gates at higher shell states. We investigate the impact of orbital excitations on single qubits as a function of the dot deformation and exploit it for faster qubit control.

Single-spin qubits in isotopically enriched silicon at low magnetic field.

Single-electron spin qubits employ magnetic fields on the order of 1 Tesla or above to enable quantum state readout via spin-dependent-tunnelling. This requires demanding microwave engineering for coherent spin resonance control, which limits the prospects for large scale multi-qubit systems. Alternatively, singlet-triplet readout enables high-fidelity spin-state measurements in much lower magnetic fields, without the need for reservoirs. Here, we demonstrate low-field operation of metal-oxide-silicon quantum dot qubits by combining coherent single-spin control with high-fidelity, single-shot, Pauli-spin-blockade-based ST readout. We discover that the qubits decohere faster at low magnetic fields with [Formula: see text] µs and [Formula: see text] µs at 150 mT. Their coherence is limited by spin flips of residual 29Si nuclei in the isotopically enriched 28Si host material, which occur more frequently at lower fields. Our finding indicates that new trade-offs will be required to ensure the frequency stabilization of spin qubits, and highlights the importance of isotopic enrichment of device substrates for the realization of a scalable silicon-based quantum processor.

Fidelity benchmarks for two-qubit gates in silicon.

Universal quantum computation will require qubit technology based on a scalable platform1, together with quantum error correction protocols that place strict limits on the maximum infidelities for one- and two-qubit gate operations2,3. Although various qubit systems have shown high fidelities at the one-qubit level4-10, the only solid-state qubits manufactured using standard lithographic techniques that have demonstrated two-qubit fidelities near the fault-tolerance threshold6 have been in superconductor systems. Silicon-based quantum dot qubits are also amenable to large-scale fabrication and can achieve high single-qubit gate fidelities (exceeding 99.9 per cent) using isotopically enriched silicon11,12. Two-qubit gates have now been demonstrated in a number of systems13-15, but as yet an accurate assessment of their fidelities using Clifford-based randomized benchmarking, which uses sequences of randomly chosen gates to measure the error, has not been achieved. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80 to 89 per cent, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7 per cent and an average controlled-rotation fidelity of 98 per cent. These fidelities are found to be limited by the relatively long gate times used here compared with the decoherence times of the qubits. Silicon qubit designs employing fast gate operations with high Rabi frequencies16,17, together with advanced pulsing techniques18, should therefore enable much higher fidelities in the near future.

Increased Sylvian fissure angle as early sonographic sign of malformation of cortical development.

OBJECTIVE: To evaluate Sylvian fissure development by assessing Sylvian fissure angles in fetuses with malformation of cortical development (MCD). METHODS: This was a retrospective study of 22 fetuses with MCD. Cases with a stored three-dimensional (3D) brain volume acquired at 18 + 0 to 30 + 6 weeks of gestation at an ultrasound-based research clinic between January 2010 and December 2017 were identified through a database. Of the 22 fetuses, seven had an extracranial abnormality, such as cardiac, renal, gastrointestinal and/or digital anomalies, and five had a minor abnormality such as micrognathia, low-set ears and/or single umbilical artery. To confirm the final clinical diagnosis of brain abnormality, postmortem histological findings or prenatal or postnatal magnetic resonance images were used. For measurement of Sylvian fissure angle, an anterior coronal plane of the fetal brain on transvaginal 3D volume multiplanar imaging was visualized as a single image from the three orthogonal views. The right and left Sylvian fissure angles were measured between a horizontal reference line (0°) and a line drawn along the upper side of the respective Sylvian fissure. The Sylvian fissure angle on both sides was plotted on the graphs of the reference ranges for gestational age in weeks. RESULTS: In 21 (95.5%; 95% CI, 86.8-100.0%) of 22 fetuses with MCD, the Sylvian fissure angle on one or both sides was larger than the 90th percentile of the normal reference. There was one case with apparent focal MCD in the parietal lobe, but the Sylvian fissure angles were normal. A case with apparent unilateral cortical dysplasia and one with apparent unilateral schizencephaly had conspicuous discrepancies between the left and right Sylvian fissure angles. Abnormal genetic test results were obtained in six cases, including four cases with a mutation in a single gene. CONCLUSIONS: This study has shown that the Sylvian fissures, as defined by the Sylvian fissure angle, have delayed development in most MCD cases prior to the diagnosis of the condition. The Sylvian fissure angle may potentially be a strong indicator for the subsequent development of cortical malformation, before the time point at which the gyri and sulci become obvious on the fetal brain surface. Further research is required to validate these findings. © 2018 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of the International Society of Ultrasound in Obstetrics and Gynecology.

Three-dimensional localization spectroscopy of individual nuclear spins with sub-Angstrom resolution.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful method for analyzing the chemical composition and molecular structure of materials. At the nanometer scale, NMR has the prospect of mapping the atomic-scale structure of individual molecules, provided a method that can sensitively detect single nuclei and measure inter-atomic distances. Here, we report on precise localization spectroscopy experiments of individual 13C nuclear spins near the central electronic sensor spin of a nitrogen-vacancy (NV) center in a diamond chip. By detecting the nuclear free precession signals in rapidly switchable external magnetic fields, we retrieve the three-dimensional spatial coordinates of the nuclear spins with sub-Angstrom resolution and for distances beyond 10 Å. We further show that the Fermi contact contribution can be constrained by measuring the nuclear g-factor enhancement. The presented method will be useful for mapping atomic positions in single molecules, an ambitious yet important goal of nanoscale nuclear magnetic resonance spectroscopy.

Integrated silicon qubit platform with single-spin addressability, exchange control and single-shot singlet-triplet readout.

Silicon quantum dot spin qubits provide a promising platform for large-scale quantum computation because of their compatibility with conventional CMOS manufacturing and the long coherence times accessible using 28Si enriched material. A scalable error-corrected quantum processor, however, will require control of many qubits in parallel, while performing error detection across the constituent qubits. Spin resonance techniques are a convenient path to parallel two-axis control, while Pauli spin blockade can be used to realize local parity measurements for error detection. Despite this, silicon qubit implementations have so far focused on either single-spin resonance control, or control and measurement via voltage-pulse detuning in the two-spin singlet-triplet basis, but not both simultaneously. Here, we demonstrate an integrated device platform incorporating a silicon metal-oxide-semiconductor double quantum dot that is capable of single-spin addressing and control via electron spin resonance, combined with high-fidelity spin readout in the singlet-triplet basis.

Fabrication of arrays of tapered silicon micro-/nano-pillars by metal-assisted chemical etching and anisotropic wet etching.

Fabrication of a 2D square lattice array of intentionally tapered micro-/nano-silicon pillars by metal-assisted chemical etching (MACE) of silicon wafers is reported. The pillars are square rod shaped with the cross-sections in the range 0.2 × 0.2-0.9 × 0.9 µm2 and heights 3-7 µm. The spacing between pillars in the 2D square lattice was controlled between 0.5 and 3.0 µm. While the pillars after MACE had the high aspect ratio ∼1:5, subsequent anisotropic wet etching in potassium hydroxide solution led to 80°-89.5° tapers with smooth sidewalls. The resulting taper angle showed the relation with geometry of pillar structures; the spacing 0.5-3.0 µm led to the tapering angle 89.5°-80° for 3 and 5 µm tall pillars but 7 µm tall pillars showed no dependency between the tapering angle and the inter-pillar spacing. Such an array of silicon tapered-rods with smooth sidewalls is expected to be applicable as a mold in nanoimprinting applications.

Trigger mechanism for the abrupt loss of energetic ions in magnetically confined plasmas.

Interaction between a quasi-stable stationary MHD mode and a tongue-shaped deformation is observed in the toroidal plasma with energetic particle driven MHD bursts. The quasi-stable stationary 1/1 MHD mode with interchange parity appears near the resonant rational surface of q = 1 between MHD bursts. The tongue-shaped deformation rapidly appears at the non-resonant non-rational surface as a localized large plasma displacement and then collapses (tongue event). It curbs the stationary 1/1 MHD mode and then triggers the collapse of energetic particle and magnetic field reconnection. The rotating 1/1 MHD mode with tearing parity at the q = 1 resonant surface, namely, the MHD burst, is excited after the tongue event.

Quantification of Turbulent Driving Forces for the Geodesic Acoustic Mode in the JFT-2M Tokamak.

We investigate spatial structures of turbulence and turbulent transport modulated by the geodesic acoustic mode (GAM), from which the excitation mechanism of the GAM is discussed. The GAM is found to be predominantly excited through a localized Reynolds stress force, rather than the dynamic shearing force. The evaluated growth rate is larger than the linear damping coefficients and is on the same order of magnitude as the effective growth rate evaluated from time evolution in the GAM kinetic energy.

Enhancement and suppression of turbulence by energetic-particle-driven geodesic acoustic modes.

We propose a novel mechanism of enhancement of turbulence by energetic-particle-driven geodesic acoustic modes (EGAMs). The dynamics of drift-wave-type turbulence in the phase space is investigated by wave-kinetic equation. Spatially inhomogeneous turbulence in the presence of a transport barrier is considered. We discovered that trapping of turbulence clumps by the EGAMs is the key parameter that determines either suppress or enhance turbulence. In regions where turbulence is unstable, EGAM suppresses the turbulence. In contrast, in the stable region, EGAM traps clumps of turbulence and carries them across the transport barrier, so that the turbulence can be enhanced. The turbulence trapped by EGAMs can propagate independent of the gradients of density and temperature, which leads to non-Fickian transport. Hence, there appear a new global characteristic velocity, the phase velocity of GAMs, for turbulence dynamics, in addition to the local group velocity and that of the turbulence spreading. With these effect, EGAMs can deteriorate transport barriers and affect turbulence substantially. This manuscript provides a basis to consider whether a coherent wave breaks or strengthen transport barriers.

Turbulent transport reduction induced by transition on radial electric field shear and curvature through amplitude and cross-phase in torus plasma.

Spatiotemporal evolutions of radial electric field and turbulence are measured simultaneously in the H-mode transition, which is a prototypical example of turbulence structure formation in high-temperature plasmas. In the dynamical phase where transport barrier is established abruptly, the time-space-frequency-resolved turbulent particle flux is obtained. Here we report the validation of the mechanism of transport barrier formation quantitatively. It is found that the particle flux is suppressed predominantly by reducing density fluctuation amplitude and cross phase between density fluctuation and potential fluctuation. Both radial electric field shear and curvature are responsible for the amplitude suppression as was predicted by theory. Turbulence amplitude reduction immediately responds to the growth of the radial electric field non-uniformity and saturates, while cross phase continuously approaches zero.

Quantum-disordered state of magnetic and electric dipoles in an organic Mott system.

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. Developing approaches to enhance quantum fluctuations may open paths to realize even more fascinating quantum states. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms, that is, proton fluctuations in a hydrogen-bonded organic Mott insulator provides a different class of quantum spin liquids (QSLs). We find that divergent dielectric behavior associated with the approach to hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a QSL-a quantum-disordered state of magnetic and electric dipoles-through the coupling between the electron and proton degrees of freedom.

Crystallization and vitrification of electrons in a glass-forming charge liquid.

Charge ordering (CO) is a phenomenon in which electrons in solids crystallize into a periodic pattern of charge-rich and charge-poor sites owing to strong electron correlations. This usually results in long-range order. In geometrically frustrated systems, however, a glassy electronic state without long-range CO has been observed. We found that a charge-ordered organic material with an isosceles triangular lattice shows charge dynamics associated with crystallization and vitrification of electrons, which can be understood in the context of an energy landscape arising from the degeneracy of various CO patterns. The dynamics suggest that the same nucleation and growth processes that characterize conventional glass-forming liquids guide the crystallization of electrons. These similarities may provide insight into our understanding of the liquid-glass transition.

Suggestion of the updated IVIS cut-off values for identifying non-ocular irritants in the bovine corneal opacity and permeability (BCOP) assay.

The bovine corneal opacity and permeability (BCOP) assay is an alternative to the Draize test in rabbits. Although it can be used to evaluate ocular non-irritants and severe irritants, it is not used for the assessment of mild-irritating substances. In this assay, a chemical with an in vitro irritancy score (IVIS)≤3 is defined as a "non-irritant" while one with an IVIS>55 is defined as a "corrosive" or "severe irritant." We attempted to evaluate mild eye irritants by assessing the recovery of corneal lesions histopathologically. Our results indicated that chemicals with IVIS≤6 may be defined as "non-irritants," because the rapid recovery of lesions limited to the squamous layer was predicted by the histopathology. In cases where lesions extended to the wing and basal cell layers, when some intact basal cells remained, the lesions were also predicted to be reversible. Thus, chemicals which induce lesions in which basal cells remain intact can be considered to be mild irritants.

A clustering approach to identify and characterize the asthma and chronic obstructive pulmonary disease overlap phenotype.

BACKGROUND: Asthma and chronic obstructive pulmonary disease (COPD) are heterogeneous diseases. The phenotypes that have clinical features of both asthma and COPD are still incompletely understood. OBJECTIVE: To clarify the best discriminators of the asthma-COPD overlap phenotype from asthma and COPD subgroups using a clustering approach. METHODS: This study assessed pathophysiological parameters, including mRNA expression levels of T helper cell-related transcription factors, namely TBX21 (Th1), GATA3 (Th2), RORC (Th17) and FOXP3 (Treg), in peripheral blood mononuclear cells in asthma patients (n=152) and in COPD patients (n=50). Clusters were determined using k-means clustering. Exacerbations of asthma and COPD were recorded during the 1-year follow-up period. RESULTS: The cluster analysis revealed four biological clusters: cluster 1, predominantly patients with COPD; cluster 2, patients with an asthma-COPD overlap phenotype; cluster 3, patients with non-atopic and late-onset asthma; and cluster 4, patients with early-onset atopic asthma. Hazard ratios for exacerbation were 2.5 (95% confidence interval [CI], 1.1-5.6) in cluster 1 and 2.3 (95% CI, 1.0-5.0) in cluster 2 compared with patients in other clusters. Cluster 2 was discriminated from other clusters by total serum IgE level ≥310 IU/mL, blood eosinophil counts ≥280 cells/µL, a higher ratio of TBX21/GATA3, FEV1 /FVC ratio <0.67 and smoking ≥10 pack-years with an area under the curve of 0.94 (95% CI, 0.90-0.98) in the receiver operating characteristic analysis. CONCLUSIONS AND CLINICAL RELEVANCE: The asthma-COPD overlap phenotype was characterized by peripheral blood eosinophilia and higher levels of IgE despite the Th2-low endotype.