Keqiang Huang

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.

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.

Determination of the tilt and twist angles of curved GaN layers by high-resolution x-ray diffraction

The full-width at half-maximum (FWHM) of an x-ray rocking curve (XRC) has been used as a parameter to determine the tilt and twist angles of GaN layers. Nevertheless, when the thickness of GaN epilayer reaches several microns, the peak broadening due to curvature becomes non-negligible. In this paper, using the (0 0 l), l = 2, 4, 6, XRC to minimize the effects of wafer curvature was studied systematically. Also the method to determine the tilt angle of a curved GaN layer was proposed while the Williamson-Hall plot was unsuitable. It was found that the (0 0 6) XRC-FWHM had a significant advantage for high-quality GaN layers with the radius curvature of r less than 3.5 m. Furthermore, an extrapolating method of gaining a reliable tilt angle has also been proposed, with which the calculated error can be improved by 10% for r < 2 m crystals compared with the (0 0 6) XRC-FWHM. In skew geometry, we have demonstrated that the twist angles deriving from the (2 0 4) XRC-FWHM are in accord with those from the grazing incidence in-plane diffraction (IP-GID) method for significantly curved samples.

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.

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.

Fabrication of Nb Superconducting Nanowires by Nanoimprint Lithography

Nanoimprint lithography (NIL) is considered to be an attractive nonconventional lithographic technique in the fabrication of nanostructures with many advantages including low cost, high throughput, and high resolution on relatively large areas. In this paper, NIL was used to pattern superconducting nanowires with meander structures based on ultrathin (~4 nm) Nb films deposited by dc-magnetron sputtering at room temperature. A combination of thermal-NIL and UV-NIL was exploited to transfer the meander pattern from the imprint hard mold to Nb films. The hard mold, etched into a Si wafer, was defined by e-beam lithography (EBL), which was nonexpendable due to the application of IPS as a soft mold to transfer the pattern to the imprint resist in the NIL process. Superconducting properties such as transition temperature T c and critical current density Jc were measured on the NIL-made Nb nanowires. The results are compared with those of EBL-made nanowires.

Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.

High-Quality Stepped-Impedance Resonators Suitable for Circuit-QED Measurement of Superconducting Artificial Atoms

High-quality factor coplanar resonators are critical elements in superconducting quantum circuits. We describe the design, fabrication, and measurement of stepped impedance resonators (SIRs), which are more compact in size than commonly used uniform impedance resonators (UIRs). With properly chosen impedance ratio, SIRs can be 27% shorter than UIRs. As a result, the area occupied by SIRs can be reduced. Two kinds of designs containing both SIRs and UIRs are fabricated and measured. The power dependence of the extracted internal quality factors (Qi ) for all the resonators showed that SIRs and UIRs have comparable performance with Qi around half-million under single-photon level excitation. These results indicate that SIRs could be used in superconducting quantum circuits. The reduced size of SIRs may also lead to reduced overall circuit area and increased integration level.