arXiv subjects

J. C. Li

Publications and source records attributed to J. C. Li.

At least 19 recordsLinked to original sources

arXiv2024

Single-Atom Verification of the Optimal Trade-Off between Speed and Cost in Shortcuts to Adiabaticity

The approach of shortcuts to adiabaticity enables the effective execution of adiabatic dynamics in quantum information processing with enhanced speed. Owing to the inherent trade-off between dynamical speed and the cost associated with the transitionless driving field, executing arbitrarily fast operations becomes impractical. To understand the accurate interplay between speed and energetic cost in this process, we propose theoretically and verify experimentally a new trade-off, which is characterized by a tightly optimized bound within $s$-parameterized phase spaces. Our experiment is carried out in a single ultracold $^{40}$Ca$^{+}$ ion trapped in a harmonic potential. By exactly operating the quantum states of the ion, we execute the Landau-Zener model as an example, where the quantum speed limit as well as the cost are governed by the spectral gap. We witness that our proposed trade-off is indeed tight in scenarios involving both initially eigenstates and initially thermal equilibrium states. Our work helps understanding the fundamental constraints in shortcuts to adiabaticity and illuminates the potential of under-utilized phase spaces that have been traditionally overlooked.

quant-ph↗

arXiv2023

Experimental investigations of quasi-coherent micro-instabilities in Ohmic plasmas

The ITG and TEM instabilities with quasi-coherent spectra have been identified experimentally, by the newly developed far-forward collective scattering measurements in J-TEXT tokamak Ohmical plasmas. The ITG mode has characteristic frequencies in the range of 30-100kHz and wavenumber of k_θρ_s<0.3. After the plasma density exceeds at critical value, the ITG mode shows a bifurcation behavior, featured by frequency decrease and amplitude enhancement. Meanwhile, the ion energy loss enhancement and confinement degradation are also observed. It gives the direct experimental evidence for ion thermal transport caused by ITG instability.

physics.plasm-ph↗

arXiv2023

Global gyrokinetic simulations of the impact of magnetic island on ion temperature gradient driven turbulence

The effect of island width on the multi-scale interactions between magnetic island (MI) and ion temperature gradient (ITG) turbulence has been investigated based on the global gyrokinetic approach. It is found that the coupling between the island and turbulence is enhanced when the MI width (w) becomes larger. A vortex flow that is highly sensitive to the width of the magnetic island can be triggered, ultimately resulting in a potent shear flow and a consequent reduction in turbulent transport. The shearing rate induced by the vortex flow is minimum at the O-point while it is maximum at both of the two reconnection points of the island, i.e., the X-points, regardless of the island width. There exists a nonmonotonic relationship between zonal flow (ZF) amplitude and island width, showing that the ZF is partially suppressed by medium-sized MIs whereas enhanced in the case of large island. A larger MI can tremendously damage the ITG mode structure, resulting in higher turbulent transport at the X-point whereas a lower one at the O-point, respectively. Such phenomenon will be less distinct at very small island widths below w/a =8% (a is the minor radius), where it shows that turbulence near the X-point is hardly affected although it is still suppressed inside the island. Furthermore, the influence of different island sizes on turbulence transport level is also discussed.

physics.plasm-ph↗

arXiv2022

Performance assessment of helicon wave heating and current drive in EXL-50 spherical torus plasmas

Analysis of helicon wave heating and current drive capability in EXL-50 spherical torus plasmas has been conducted. It is found that the driven current increases with the launched parallel refractive index $n_{||}$ and peaks around $n_{||} = 4.0$ when the frequency of the helicon wave is between 300~MHz and 380~MHz. The helicon wave current drive efficiency shows a relatively stable upward trend with increasing plasma temperature. Moreover, the driven current decreases as the plasma density increases. We also analyzed the current drive with helicon waves of 150~MHz and 170~MHz and found that the driven current at a lower frequency was lower than that at a higher frequency. A positive proportional relationship exists between the driven current and $n_{||}$. Besides, as $n_{||}$ increases, the profile of the driven current becomes wider. Finally, the effect of the scrape-off layer (SOL) region on the helicon wave current drive was also investigated.

physics.plasm-ph↗

arXiv2021

Single-atom verification of the noise-resilient and fast characteristics of universal nonadiabatic noncyclic geometric quantum gates

Quantum gates induced by geometric phases are intrinsically robust against noise due to their global properties of the evolution paths. Compared to conventional nonadiabatic geometric quantum computation (NGQC), the recently proposed nonadiabatic noncyclic geometric quantum computation (NNGQC) works in a faster fashion, while still remaining the robust feature of the geometric operations. Here, we experimentally implement the NNGQC in a single trapped ultracold $^{40}$Ca$^{+}$ ion for verifying the noise-resilient and fast feature. By performing unitary operations under imperfect conditions, we witness the advantages of the NNGQC with measured fidelities by quantum process tomography in comparison with other two quantum gates by conventional NGQC and by straightforwardly dynamical evolution. Our results provide the first evidence confirming the possibility of accelerated quantum information processing with limited systematic errors even in the imperfect situation.

quant-ph↗

arXiv2020

Single-Atom Verification of the Information-Theoretical Bound of Irreversibility at the Quantum Level

Quantitative measure of disorder or randomness based on the entropy production characterizes thermodynamical irreversibility, which is relevant to the conventional second law of thermodynamics. Here we report, in a quantum mechanical fashion, the first theoretical prediction and experimental exploration of an information-theoretical bound on the entropy production. Our theoretical model consists of a simplest two-level dissipative system driven by a purely classical field, and under the Markovian dissipation, we find that such an information-theoretical bound, not fully validating quantum relaxation processes, strongly depends on the drive-to-decay ratio and the initial state. Furthermore, we carry out experimental verification of this information-theoretical bound by means of a single spin embedded in an ultracold trapped $^{40}$Ca$^{+}$ ion. Our finding, based on a two-level model, is fundamental to any quantum thermodynamical process and indicates much difference and complexity in quantum thermodynamics with respect to the conventionally classical counterpart.

quant-ph↗

arXiv2019

Single-qubit measurement of Heisenberg uncertainty lower bounds for three incompatible observables

Being one of the centroidal concepts in quantum theory, the fundamental constraint imposed by Heisenberg uncertainty relations has always been a subject of immense attention and challenging in the context of joint measurements of general quantum mechanical observables. In particular, the recent extension of the original uncertainty relations has grabbed a distinct research focus and set a new ascendent target in quantum mechanics and quantum information processing. In the present work we explore the joint measurements of three incompatible observables, following the basic idea of a newly proposed error trade-off relation. In comparison to the counterpart of two incompatible observables, the joint measurements of three incompatible observables are more complex and of more primal interest in understanding quantum mechanical measurements. Attributed to the pristine idea proposed by Heisenberg in 1927, we develop the error trade-off relations for compatible observables to categorically approximate the three incompatible observables. Implementing these relations we demonstrate the first experimental witness of the joint measurements for three incompatible observables using a single ultracold $^{40}Ca^{+}$ ion in a harmonic potential. We anticipate that our inquisition would be of vital importance for quantum precision measurement and other allied quantum information technologies.

quant-ph↗

arXiv2018

Tracing the Pathway from Drift-Wave Turbulence with Broken Symmetry to the Generation of Sheared Axial Mean Flow

This study traces the emergence of sheared axial flow from collisional drift wave turbulence with broken symmetry in a linear plasma device---CSDX. As the density profile steepens, the axial Reynolds stress develops and drives a radially sheared axial flow that is parallel to the magnetic field. Results show that the non-diffusive piece of the Reynolds stress is driven by the density gradient and results from the spectral asymmetry of the turbulence and thus is dynamical in origin. Taken together, these findings constitute the first simultaneous demonstration of the causal link between the density gradient, turbulence and stress with broken spectral symmetry, and the mean axial flow.

physics.plasm-ph↗

arXiv2018

Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

Detailed measurements of intrinsic axial flow generation parallel to the magnetic field in the controlled shear decorrelation experiment linear plasma device with no axial momentum input are presented and compared to theory. The results show a causal link from the density gradient to drift-wave turbulence with broken spectral symmetry and development of the axial mean parallel flow. As the density gradient steepens, the axial and azimuthal Reynolds stresses increase and radially sheared azimuthal and axial mean flows develop. A turbulent axial momentum balance analysis shows that the axial Reynolds stress drives the radially sheared axial mean flow. The turbulent drive (Reynolds power) for the azimuthal flow is an order of magnitude greater than that for axial flow, suggesting that the turbulence fluctuation levels are set by azimuthal flow shear regulation. The direct energy exchange between axial and azimuthal mean flows is shown to be insignificant. Therefore, the axial flow is parasitic to the turbulence-zonal flow system and is driven primarily by the axial turbulent stress generated by that system. The non-diffusive, residual part of the axial Reynolds stress is found to be proportional to the density gradient and is formed due to dynamical asymmetry in the drift-wave turbulence.

physics.plasm-ph↗

arXiv2014

Observation of $χ_{c1}$ decays into vector meson pairs $ϕϕ$, $ωω$, and $ωϕ$

Decays of $χ_{c1}$ to vector meson pairs $ϕϕ$, $ωω$ and $ωϕ$ are observed for the first time using $(106\pm4)\times 10^6$ $\psip$ events accumulated at the BESIII detector at the BEPCII $e^+e^-$ collider. The branching fractions are measured to be $(4.4\pm 0.3\pm 0.5)\times 10^{-4}$, $(6.0\pm 0.3\pm 0.7)\times 10^{-4}$, and $(2.2\pm 0.6\pm 0.2)\times 10^{-5}$, for $χ_{c1}\to ϕϕ$, $ωω$, and $ωϕ$, respectively. The observation of $χ_{c1}$ decays into a pair of vector mesons $ϕϕ$, $ωω$ and $ωϕ$ indicates that the hadron helicity selection rule is significantly violated in $χ_{cJ}$ decays. In addition, the measurement of $χ_{cJ}\to ωϕ$ gives the rate of doubly OZI-suppressed decay. Branching fractions for $χ_{c0}$ and $χ_{c2}$ decays into other vector meson pairs are also measured with improved precision.

hep-ex↗

arXiv2014

Thermoelectric properties of Sr0.61Ba0.39Nb2O6-δ ceramics annealed in different oxygen-reduction conditions

The thermoelectric properties of Sr0.61Ba0.39Nb2O6 ceramics, reduced in different conditions, were investigated in the temperature region from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping, the Fermi glass behavior, and the Anderson localized behavior from low temperatures to high temperatures, respectively. The lattice thermal conductivity presents a plateau at high temperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increasing degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value (~0.19 at 1073 K) is obtained in the heaviest oxygen reduced sample.

cond-mat.mtrl-sci↗

arXiv2009

Organic molecular thin films for nanoscale information memory applications

According to Moore law, the silicon semiconductor transistor based information system is facing its physical limitations due to fluctuations of random charge and leakage current. Molecular electronics is becoming more and more attractive owing to the advantages of easy molecular structure variability, flexibility, low-cost and compatibility with bioelectronics. In this handbook chapter, we reviewed the recent research progress of molecular electronics, especially the studies on nanoscale information memories, from the viewpoints of structure-property relationship. Two kinds of molecular systems including redox dendrimeric thin films and self-assembled molecular monolayers are discussed in detail. The investigation and application of other molecular thin films such as polymers, charge transfer salts and Langmuir-Blodgett layers are also briefly introduced. We suggest that two promising molecular systems have the most potentials for using as building blocks in nanoscale information storage. One is single-molecule-based memory device with sub-10 nm characteristics built on self-assembled monolayer. Multimode information storage is the other powerful way to make breakthrough in the challenging area of nanoscale data storage. This relies on further experimental and theoretical advances. Moreover, a big foreseeable obstacle is how to bridge the big gap between such novel system and the current bit world.

cond-mat.mtrl-sci↗

arXiv2007

Nanoscale data storage

The object of this article is to review the development of ultrahigh-density, nanoscale data storage, i.e., nanostorage. As a fundamentally new type of storage system, the recording mechanisms of nanostorage may be completely different to those of the traditional devices. Currently, two types of molecules are being studied for potential application in nanostorage. One is molecular electronic elements including molecular wires, rectifiers, switches, and transistors. The other approach employs nanostructured materials such as nanotubes, nanowires, and nanoparticles. The challenges for nanostorage are not only the materials, ultrahigh data-densities, fabrication-costs, device operating temperatures and large-scale integration, but also the development of the physical principles and models. There are already some breakthroughs obtained, but it is still unclear what kind of nanostorage systems can ultimately replace the current silicon based transistors. A promising candidate may be a molecular-nanostructure hybrid device with sub-5 nm dimensions.

cond-mat.mtrl-sci↗

arXiv2006

Optoelectronic switching of addressable molecular crossbar junctions

This letter reports on the observation of optoelectronic switching in addressable molecular crossbar junctions fabricated using polymer stamp-printing method. The active medium in the junction is a molecular self-assembled monolayer softly sandwiched between gold electrodes. The molecular junctions are investigated through currentvoltage measurements at varied temperature (from 95 to 300 K) in high vacuum condition. The junctions show reversible optoelectronic switching with the highest on/off ratio of 3 orders of magnitude at 95 K. The switching behavior is independent of both optical wavelength and molecular structure, while it strongly depends on the temperature. Initial analysis indicates that the distinct binding nature of the molecule/electrode interfaces play a dominant role in the switching performance.

cond-mat.soft↗

arXiv2006

Water in Carbon Nanotubes Is Not the Same Old Stuff

The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice and water in carbon nanotubes at low temperatures has been measured using deep inelastic neutron scattering. We find that the momentum distribution for the water in the nanotubes is qualitatively unlike that in any other phase of water or ice. The kinetic energy of the protons is 35mev less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond and the stretch mode of the proton in the hydrogen bonds, is not present. We observe a phase transition between 230K and 268K in the nanotube data. The high momentum tail is present in the higher temperature measurement, which resembles that of ice Ih at the same temperature. Molecular dynamics simulations show the phase transition to be associated with the reordering of the hydrogen bonds of the 2-D ice layer, coating the interior of the nanotube at low temperatures, into a 3-D structure at 268K. We conclude that the protons in the hydrogen bonds in the 2-D ice layer are coherently delocalized, and that the 2-D ice layer is a qualitatively new phase of ice.

cond-mat.mtrl-sci↗

arXiv2005

Charge-transfer interfaces between metal and redox arylamine molecular films: As probed with anode interfacial engineering approach in single-layer organic diodes

We investigate the charge-transfer interfaces between metal and redox arylamine molecular films through studying the current-voltage characteristics of single-layer organic diodes with the aid of anode interfacial engineering method. The diode turn-on voltage is shown to be highly sensitive to the arylamine/metal charge-transfer interfaces and thus can serve as a probe in detecting such organic/metal interfaces. We show that the diode electrical performance could be tuned through engineering the arylamine/metal interfaces via controlling the factors of anode work function, arylamine substitute groups, and active layer surface morphology etc. The conduction mechanism of the diodes is shown to be injection limited, which could be well described with Richardson-Schottky thermonic emission model. Our work may provide some insight into the use of single-layer organic diode and interfacial engineering method to rapidly probe the organic/metal and even organic/organic charge-transfer interfaces.

cond-mat.mtrl-sci↗

arXiv2005

Organic nanowires and chiral patterns of tetracyanoquinodimethane (TCNQ) grown by vacuum vapor deposition

Organic nanowires and quasi-two-dimensional chiral patterns of tetracyanoquinodimethane have been successfully generated by vacuum thermal evaporation. The nanowires and patterns were characterized by using atomic force microscopy and transmission electron microscopy. The influence of electric charged clusters, deposition rate, and substrate temperature were experimentally investigated. Contrary to previous reports, charged clusters are found to be unnecessary to the chiral pattern formation. It was shown that the nanowire and pattern formation should be mainly dominated by its special crystallization properties, though the effect of the growth conditions cannot be neglected.

cond-mat.stat-mech↗

arXiv2004

Stochastic Resonance and Nonequilibrium Dynamic Phase Transition of Ising Spin System Driven by a Joint External Field

We studied the dynamic response and stochastic resonance of kinetic Ising spin system (ISS), subject to the joint external field of weak sinusoidal modulation and stochastic white-noise, through solving the mean-field equation of motion based on Glauber dynamics. The periodically driven stochastic ISS shows the occurrence of characteristic stochastic resonance as well as nonequilibrium dynamic phase transition (NDPT) when the frequency and amplitude h0 of driving field, the temperature t of the system and noise intensity D attain a specific accordance in quantity. There exist in the system two typical dynamic phases, referred to as dynamic disordered paramagnetic and ordered ferromagnetic phases respectively, corresponding to zero and unit dynamic order parameter. We also figured out the NDPT boundary surface of the system which separates the dynamic paramagnetic and dynamic ferromagnetic phase in the 3D parameter space of h0~t~D. An intriguing dynamical ferromagnetic phase with an intermediate order parameter at 0.66 was revealed for the first time in the ISS subject to the perturbation of a joint determinant and stochastic field. Our primary result indicates that the intermediate order dynamical ferromagnetic phase is dynamic metastable in nature and owns a peculiar characteristic in its stability and response to external driving field when compared with fully order dynamic ferromagnetic phase.

cond-mat.stat-mech↗