Dongning Zheng

Resistive switching properties in oxygen-deficient Pr0.7Ca0.3MnO3 junctions with active Al top electrodes

Current-voltage characteristics, conduction mechanisms, and resistive switching properties are investigated in Al/Pr0.7Ca0.3MnO3 (PCMO)/Pt junctions. The junction resistance exhibits an irreversible increase from 2 to 90 MΩ in the forming process, the first several repeated bias sweeps. In contrast to the PCMO junctions involving inert top electrode (TE), the active Al-TE-based junctions show very large junction resistance and opposite cycling directions. It is found that the junction resistance sequence is qualitatively consistent with the standard Gibbs energies ΔG0 for the formation of corresponding TE oxides, rather than the Schottky barrier heights. Current-voltage fits indicate that the conduction processes in high and low resistance states are controlled by Poole–Frenkel emission and space-charge-limited conduction, respectively. The junctions show asymmetric switching thresholds with the minimal switching voltages are +1 V at the positive and −4 V at the negative side. Resistance retention tests i...

10-Qubit Entanglement and Parallel Logic Operations with a Superconducting Circuit.

Here we report on the production and tomography of genuinely entangled Greenberger-Horne-Zeilinger states with up to ten qubits connecting to a bus resonator in a superconducting circuit, where the resonator-mediated qubit-qubit interactions are used to controllably entangle multiple qubits and to operate on different pairs of qubits in parallel. The resulting 10-qubit density matrix is probed by quantum state tomography, with a fidelity of 0.668±0.025. Our results demonstrate the largest entanglement created so far in solid-state architectures and pave the way to large-scale quantum computation.

Coherent population transfer between uncoupled or weakly coupled states in ladder-type superconducting qutrits

Stimulated Raman adiabatic passage offers significant advantages for coherent population transfer between uncoupled or weakly coupled states and has the potential of realizing efficient quantum gate, qubit entanglement and quantum information transfer. Here we report on the realization of the process in the superconducting Xmon and phase qutrits—two ladder-type three-level systems in which the ground state population is coherently transferred to the second excited state via the dark state subspace. We demonstrate that the population transfer efficiency is no less than 96% and 67% for the two devices, which agree well with the numerical simulation of the master equation. Population transfer via stimulated Raman adiabatic passage is significantly more robust against variations of the experimental parameters compared with that via the conventional resonant π pulse method. Our work opens up a new venue for exploring the process for quantum information processing using the superconducting artificial atoms.

A Novel Quasi-Elliptic HTS Filter With Group-Delay Equalization Using Compact Quasi-Lumped Element Resonators in VHF Band

To produce a filter small enough to fit a 2-in wafer in the very high frequency (VHF) band while avoiding parasitic cross coupling among nonadjacent resonators, a novel quasi-lumped element resonator with interdigital capacitor, double-spiral inductor, and pad capacitor have been introduced. This resonator has not only a highly miniature structure but also a very weak far-field radiation. A ten-pole quasi-elliptic filter with group-delay equalization, which has a 7.1-MHz 1-dB passband and a center frequency of 257.5 MHz, is designed and fabricated on a 37.3 x mm x 30.1 x mm x 0.5 x mm LaAlO3 substrate. The measured results showed a 0.24-dB insertion loss, a 15-dB return loss, and a 70-dB out-of-band rejection. Moreover, the group delay variation is less than 50 ns over 70% of the 1-dB passband and the band where the phase error ripple is within plusmn5deg is more than 80% of the 1-dB passband. The overall experimental performance showed excellent agreement with the theory and simulation, which is a good proof of the advantage of our weak far-field radiation resonator. The result showed that the novel resonator is very suitable to fabricate narrowband ultrahigh frequency, VHF, or even lower frequency high-temperature superconductor filters on a 2-in wafer.

Understanding glass-forming ability through sluggish crystallization of atomically thin metallic glassy films

The glass-forming ability (GFA) of an alloy, closely related to its ability to resist crystallization, is a crucial issue in condensed matter physics. So far, the studies on GFA are mostly statistical and empirical guides. Benefiting from the ultrahigh thermal stability of ultrathin metallic glassy film and high resolution spherical aberration-corrected transmission electron microscope, the crystallization of atomically thin ZrCu and its microalloyed ZrCuAl glasses with markedly different GFA was investigated at the atomic scale. We find the Zr diffusivity estimated from the density of nuclei is dramatically decreased by adding of Al, which is the major reason for the much better GFA of the ZrCuAl metallic glass.

Ultrahigh stability of atomically thin metallic glasses

We report the fabrication and study of thermal stability of atomically thin ZrCu-based metallic glass films. The ultrathin films exhibit striking dynamic properties, ultrahigh thermal stability, and unique crystallization behavior with discrete crystalline nanoparticles sizes. The mechanisms for the remarkable high stability and crystallization behaviors are attributed to the dewetting process of the ultrathin film. We demonstrated a promising avenue for understanding some fundamental issues such as glassy structure, crystallization, deformation, and glass formation through atomic resolution imaging of the two dimensional like metallic glasses.

Solving Systems of Linear Equations with a Superconducting Quantum Processor

Superconducting quantum circuits are a promising candidate for building scalable quantum computers. Here, we use a four-qubit superconducting quantum processor to solve a two-dimensional system of linear equations based on a quantum algorithm proposed by Harrow, Hassidim, and Lloyd [Phys. Rev. Lett. 103, 150502 (2009)PRLTAO0031-900710.1103/PhysRevLett.103.150502], which promises an exponential speedup over classical algorithms under certain circumstances. We benchmark the solver with quantum inputs and outputs, and characterize it by nontrace-preserving quantum process tomography, which yields a process fidelity of 0.837±0.006. Our results highlight the potential of superconducting quantum circuits for applications in solving large-scale linear systems, a ubiquitous task in science and engineering.

Decrease of TC and persistent two gaps upon enhancement of the Ca doping in MgB2 superconductor

Superconducting properties have been investigated in the Ca-doped MgB2 (Mg1−xCaxB2) superconductors, which have been synthesized under high temperature and pressure. With increasing doping level x, a linear drop of the critical temperature Tc is observed, and the gap Δσ keeps nearly fixed and the gap Δπ deceases, giving rise to the persistent two-gap feature in the Mg1−xCaxB2 samples for x up to 0.07. Both the Sommerfeld constant γn and the relative weight γπ∕γn are found to decrease with raising x, indicative of the decrease of Nπ(0).

Direct observation of atomic-level nucleation and growth processes from an ultrathin metallic glass films

Till date, there have been no direct atomic-level experimental observations of the earliest stages of the nucleation and growth processes of nanocrystals formed by thermally induced crystallization in ultrathin metallic glasses (MGs). Here, we present a study of the crystallization process in atomically thin and highly stable MG films using double spherical aberration-corrected scanning transmission electron microscopy (Cs-TEM). Taking advantage of the stability of MG films with a slow crystallization process and the atomic-level high resolution of Cs-TEM, we observe the formation of the nucleus precursor of nanocrystals formed by atom aggregation followed by concomitant coalescence and stepwise evolution of the shape of the nanocrystals with a monodispersed and separated bimodal size distribution. Molecular dynamics simulation of the atomic motion in the glass film on a rigid amorphous substrate confirms the stepwise evolution processes of atom aggregation, cluster formation, cluster movement on the substrate, and cluster coalescence into larger crystalline particles. Our results might provide a better fundamental understanding of the nucleation and growth processes of nanocrystals in thin MG films.

Continuous-variable geometric phase and its manipulation for quantum computation in a superconducting circuit

Geometric phase, associated with holonomy transformation in quantum state space, is an important quantum-mechanical effect. Besides fundamental interest, this effect has practical applications, among which geometric quantum computation is a paradigm, where quantum logic operations are realized through geometric phase manipulation that has some intrinsic noise-resilient advantages and may enable simplified implementation of multi-qubit gates compared to the dynamical approach. Here we report observation of a continuous-variable geometric phase and demonstrate a quantum gate protocol based on this phase in a superconducting circuit, where five qubits are controllably coupled to a resonator. Our geometric approach allows for one-step implementation of n-qubit controlled-phase gates, which represents a remarkable advantage compared to gate decomposition methods, where the number of required steps dramatically increases with n. Following this approach, we realize these gates with n up to 4, verifying the high efficiency of this geometric manipulation for quantum computation.Geometric phase is of fundamental interest and has practical application in quantum computation. Here the authors observe continuous-variable geometric phase in a superconducting circuit and demonstrate a multi-qubit controlled phase gate protocol based on this geometric effect.