Search

The engineering of quantum devices has reached the stage where we now have small-scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway. However, as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to, but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses. This has important implications in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling process. Our approach suggests another way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors. A method for switching on/off the coupling between quantum elements could assist in the scaling up of quantum circuits. Demonstrations of small-scale quantum processes made using multiple interacting quantum bits (qubits) is an important step towards the development of quantum computers. Scaling up the number of qubits to enable large-scale quantum processes is challenging, however, as it becomes difficult to control the couplings between them whilst maintaining their desirable properties. An international collaboration led by Xiaobo Zhu from the University of Science and Technology of China now demonstrate a simple approach for switching on and off the coupling between quantum elements. Such a protocol should enable qubits to be kept at their optimal conditions during (de)coupling processes, which could help in the development of large-scale quantum circuits.

A central task towards building a practical quantum computer is to protect individual qubits from decoherence while retaining the ability to perform high-fidelity entangling gates involving arbitrary two qubits. Here we propose and demonstrate a dephasing-insensitive procedure for storing and processing quantum information in an all-to-all connected superconducting circuit involving multiple frequency-tunable qubits, each of which can be controllably coupled to any other through a central bus resonator. Although it is generally believed that the extra frequency tunability enhances the control freedom but induces more dephasing impact for superconducting qubits, our results show that any individual qubit can be dynamically decoupled from dephasing noise by applying a weak continuous and resonant driving field whose phase is reversed in the middle of the pulse. More importantly, we demonstrate a new method for realizing two-qubit phase gate with inherent dynamical decoupling via the combination of continuous driving and qubit-qubit swapping coupling. We find that the weak continuous driving fields not only enable the conditional dynamics essential for quantum information processing, but also protect both qubits from dephasing during the gate operation., Comment: 10 pages, 7 figures, and 2 tables including Supplementary

Anyons are quasiparticles occurring in two dimensions, whose topological properties are believed to be robust against local perturbations and may hold promise for fault tolerant quantum computing. Here we present an experiment of demonstrating the path independent nature of anyonic braiding statistics with a superconducting quantum circuit, which represents a 7-qubit version of the toric code model. We dynamically create the ground state of the model, achieving a state fidelity of $0.688\ifmmode\pm\else\textpm\fi{}0.015$ as verified by quantum state tomography. Anyonic excitations and braiding operations are subsequently implemented with single-qubit rotations. The braiding robustness is witnessed by looping an anyonic excitation around another one along two distinct, but topologically equivalent paths: Both reveal the nontrivial $\ensuremath{\pi}$-phase shift, the hallmark of Abelian $1/2$ anyons, with a phase accuracy of $\ensuremath{\sim}99%$ in the Ramsey-type interference measurement.

The law of statistical physics dictates that generic closed quantum many-body systems initialized in nonequilibrium will thermalize under their own dynamics. However, the emergence of many-body localization (MBL) owing to the interplay between interaction and disorder, which is in stark contrast to Anderson localization that only addresses noninteracting particles in the presence of disorder, greatly challenges this concept because it prevents the systems from evolving to the ergodic thermalized state. One critical evidence of MBL is the long-time logarithmic growth of entanglement entropy, and a direct observation of it is still elusive due to the experimental challenges in multiqubit single-shot measurement and quantum state tomography. Here we present an experiment of fully emulating the MBL dynamics with a 10-qubit superconducting quantum processor, which represents a spin-1/2 XY model featuring programmable disorder and long-range spin-spin interactions. We provide essential signatures of MBL, such as the imbalance due to the initial nonequilibrium, the violation of eigenstate thermalization hypothesis, and, more importantly, the direct evidence of the long-time logarithmic growth of entanglement entropy. Our results lay solid foundations for precisely simulating the intriguing physics of quantum many-body systems on the platform of large-scale multiqubit superconducting quantum processors., 6 pages+5 pages supplementary

A dynamical quantum phase transition can occur during time evolution of sudden quenched quantum systems across a phase transition. It corresponds to the nonanalytic behavior at a critical time of the rate function of the quantum state return amplitude, analogous to nonanalyticity of the free energy density at the critical temperature in macroscopic systems. A variety of many-body systems can be represented in momentum space as a spin-1/2 state evolving on the Bloch sphere, where each momentum mode is decoupled and thus can be simulated independently by a single qubit. Here, we report the observation of a dynamical quantum phase transition in a superconducting qubit simulation of the quantum quench dynamics of many-body systems. We take the Ising model with a transverse field as an example for demonstration. In our experiment, the spin state, which is initially polarized longitudinally, evolves based on a Hamiltonian with adjustable parameters depending on the momentum and strength of the transverse magnetic field. The time evolving quantum state is read out by state tomography. Evidence of dynamical quantum phase transitions, such as paths of time evolution states on the Bloch sphere, non-analytic behavior of the dynamical free energy and the emergence of Skyrmion lattice in momentum-time space, is observed. The experimental data agrees well with theoretical and numerical calculations. The experiment demonstrates for the first time explicitly the topological invariant, both topologically trivial and non-trivial, for dynamical quantum phase transitions. Our results show that the quantum phase transitions of this class of many-body systems can be simulated successfully with a single qubit by varying certain control parameters over the corresponding momentum range., Comment: 10 pages, 7 figures, typos corrected

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 J c were measured on the NIL-made Nb nanowires. The results are compared with those of EBL-made nanowires.

In flux qubits, qubit states are commonly read out by a dc superconducting quantum interference device (SQUID), which is inductively coupled to the qubit and works as a persistent current detector. However, neither the bottom nor the top regions of the critical current modulation curve of the SQUID can be used for the readout. In this paper, we show that a flux-trap loop inserted to the readout SQUID provides the ability to adjust the working point, which helps to avoid this constraint. This improvement is useful for some complicated flux qubit experiments.

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.

Here we report on the production and tomography of genuinely entangled Greenberger-Horne-Zeilinger states with up to 10 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 unambiguously probed, with a fidelity of $0.668 \pm 0.025$. Our results demonstrate the largest entanglement created so far in solid-state architectures, and pave the way to large-scale quantum computation., Comment: Revised version with 16 pages, 13 figures, and 2 tables

Realization of a flexible and tunable coupling scheme among qubits is critical for scalable quantum information processing. Here, we design and characterize a tunable coupling element based on Josephson junction, which can be adapted to an all-to-all connected circuit architecture where multiple Xmon qubits couple to a common coplanar waveguide resonator. The coupling strength is experimentally verified to be adjustable from 0 MHz to about 40 MHz, and the qubit lifetime can still be up to 12 μ s in the presence of the coupling element.

We study the fringe visibility and the distinguishability of a general Mach-Zehnder interferometer with an asymmetric beam splitter. Both the fringe visibility V and the distinguishability D are affected by the input state of the particle characterized by the Bloch vector S=(Sx,Sy,Sz) and the second asymmetric beam splitter characterized by paramter /beta. For the total system is initially in a pure state, it is found that the fringe visibility reaches the upper bound and the distinguishability reaches the lower bound when cos(/beta) = -Sx. The fringe visibility obtain the maximum only if Sx = 0 and /beta = /pi/2 when the input particle is initially in a mixed state. The complementary relationship V2 + D2, 5 pages, 3 figures

Interface and surface physics is an important sub-discipline within condensed matter physics in recent decades. Novel concepts like oxide-electronic device are prompted, and their performance and lifetime are highly dependent on the flatness and abruptness of the layer surfaces and interfaces. Reflection high-energy electron diffraction (RHEED), which is extremely sensitive to surface morphology, has proven to be a versatile technique for the growth study of oxide thin films. A differential pumping unit enables an implementation of RHEED to pulsed laser deposition (PLD) systems, ensuring an in situ monitoring of the film growth process in a conventional PLD working oxygen pressure up to 30 Pa. By optimizing the deposition conditions and analyzing the RHEED intensity oscillations, layer-by-layer growth mode can be attained. Thus atomic control of the film surface and unit-cell control of the film thickness become reality. This may lead to an advanced miniaturization in the oxide electronics, and more importantly the discovery of a range of emergent physical properties at the interfaces. Herein we will briefly introduce the principle of high-pressure RHEED and summarize our main results relevant to the effort toward this objective, including the growth and characterization of twinned La2/3Ca1/3MnO3 thin films and ReTiO3+δ/2 (Re = La, Nd; δ = 0 ∼ 1) A n B n O3n+2 structures, on YSZ-buffered ‘Silicon on Insulator’ and LaAlO3 substrates, respectively, as well as the study of the initial structure and growth dynamics of YBa2Cu3O7−δ thin films on SrTiO3 substrate. Presently we have realized in situ monitoring and growth mode control during oxide thin film deposition process.

Herein we develop an Al/AlO x /Al trilayer process, feasible to fabricate complex circuits with wiring crossovers, for the preparation of Al junctions and phase qubits. The AlO x layer is obtained by in situ thermal oxidation, which provides high-quality junction tunnel barriers. The Al junctions show a considerably low leakage current and the Josephson critical current density can be conveniently controlled in the range of a few to above 100 A/cm2, which is favorable in the phase qubit application. Macroscopic quantum tunneling, energy spectrum, energy relaxation time, Rabi oscillation, and Ramsey interference of the Al phase qubits are measured, demonstrating clearly quantum coherent dynamics with a timescale of 10 ns. Further improvements of the coherent dynamic properties of the device are discussed.

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., 4 pages, 6 figures, IEEE ASC2016

We report the results of electrical resistivity and magnetoresistance (MR) measurements carried out on La 2/3 Ca 1/3 MnO 3 /BaZrO 3 composites prepared by solid state reaction method. The metal-to-insulator transition and the magnitude of magnetoresistance are significantly influenced by the content of BaZrO 3 , suggesting the presence of ionic diffusion between La 2/3 Ca 1/3 MnO 3 and BaZrO 3 , although the dependence of the electrical resistivity as a function of La 2/3 Ca 1/3 MnO 3 volume fraction can be roughly described by classical percolation conduction model. The conductive behavior above the metal-to-insulator transition temperature can be well explained by Mott's variable range hopping model.

We have carried out NMR experiments in the microtesla range using a high-Tc DC-SQUID sensor. The measurements were carried out in a simple magnetically shielded room. Resonance spectra of 1H nuclei from tap water and other substance samples were obtained in the field range 7-70 μT. The effect of frequency, strength, direction and phase of a low frequency external disturbance field on the NMR spectra was investigated. A sinusoidal field was applied along or perpendicular to the measurement field to simulate the external disturbance. The results were compared with the numerical calculations based on the Bloch equation and good agreement was obtained. In all cases, the field disturbance less than a few Hz are proved to influence the NMR spectra more severely, suppressing the resonance peak and driving it into split bands. On the other hand, the influence of the disturbance field at the frequency range around 50 Hz on the resonance peak position and profile is small, even though some additional small peaks appear in the spectra due to frequency mixing. The effect is more significant when the disturbance field is along the direction of the measurement field. The phase difference between the disturbance field and the free-induction-decay signal could change the NMR peak position and profile drastically.

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1−x Sb x (0⩽x⩽0.5). The X-ray diffraction, transport and magnetic measurements were performed on these samples to investigate the structure, the superconducting properties and the normal state transport and magnetic properties. Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase, the transport data showed observable changes of superconductivity, normal state resistivity and magnetoresistance. This was represented by the increase in the superconducting transition temperature and the upper critical field. Also, for the samples with a low level of Sb content, a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed. Furthermore, the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler’s rule. The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.

MgB2 was thought as a promising superconductor used at temperatures around 20 K for cryogen-free magnet. Nanostructure materials were often selected as MgB2 additives due to their high chemical reactivity. In this paper, the mixture of nano-C and nano-SiC was doped to MgB2 bulks and tapes. The co-doping effect of C and SiC on the phase formation, microstructure, and critical current density of MgB2 bulks and tapes were systematically investigated. The mechanisms for the superconducting properties improvement in co-doped MgB2 superconductor was analyzed based on the characterization and measuring results.

A series of Ca-doped MgB2 superconducting compounds has been synthesized under high pressure and high temperature with a calcium source of Ca3B2N4. Their structure and superconducting properties have been investigated, revealing that the lattice parameters of a and c for the Ca-doped MgB2 are enlarged and the superconducting transition temperature Tc is suppressed from 37.8 K for pure MgB2 to 36.5 K for Mg0.92Ca0.08B2 by the Ca doping. In addition to the main phase of the Ca-doped MgB2, the MgO and CaB6 phases have been observed with the MgO and CaB6 grains being of the size of several hundred nanometres and being embedded randomly in the MgB2 matrix. In comparison to the undoped MgB2, the rapid decrease in Jc observed in the Ca-doped samples is mainly due to the relatively large grain sizes of the second phases, which result in the poor connectivity between grain boundaries.

Tl 2 Ba 2 CaCu 2 O y (Tl-2212) thin films were fabricated on (001) LaAlO 3 substrates by a two-step technique in a semi-closed aluminate crucible and a sealed steel container, respectively. First, the amorphous precursor films were deposited by the pulsed laser deposition (PLD) method, and then they were thalliated in different containers. Almost pure phase Tl-2212 thin films can be prepared in both of the containers. However, the as-annealed films show obvious differences in superconducting properties and microstructure.

This paper reports the fabrication and test of a high-T-c SQUID planar gradiometer which is patterned from YBCO thin film deposited on a SrTiO3 bicrystal substrate. The measurement of noise spectrum at 77K shows that the white noise at 200 Hz is about 1 x 10(-4)Phi(0)/root Hz. The minimal magnetic gradient is measured and the results suggest that the minimal magnetic gradient is 94 pT/m. The planar gradiometer is used in non-destructive evaluation (NDE) experiments to detect the artifacts in conducting aluminium plates by performing eddy current testing in an unshielded environment. The effect of the exciting coil dimension on the NDE results is investigated. By mapping out the induced field distribution, flaws about 10mm below the plate surface can be clearly identified.

Highly oriented Ba0.5Sr0.5TiO3 (BST) thin films were grown on MgO (001) single-crystal substrates by pulsed-laser deposition at 800 degrees C in oxygen pressure ranging from 1.2 x 10(-3) to 40 Pa. A strong correlation was observed between the growth process, structure and dielectric properties for the BST films. The dielectric properties in the low frequency range 10k-1MHz were measured in the interdigital capacitor configuration. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), surface morphology and dielectric response of the films are strongly dependent on the oxygen deposition pressure. With increase in oxygen pressure, the in-plane strain for the BST films changes from compressive to tensile. The BST film grown at 25 Pa exhibits the best overall dielectric properties. This corresponds to the film with a slight in-plane tensile strain (D = 1.0012). It is believed that a reasonable tensile strain, which increases the ionic displacement and thus promotes the in-plane polarization in the field direction, could enhance the tunability and dielectric constant. Based on the BST film grown at 25 Pa, distributed phase shifters were successfully fabricated with promising performance. The phase shifter shows a relatively low insertion loss of about 3.5 dB at zero bias and 10 GHz, a good return loss better than -15 dB for all phase states from dc to 16 GHz and a differential phase shift of about 43 degrees with 120V dc bias at 10 GHz.

Pure and 2 mol% Mn doped Ba0.6Sr0.4TiO3 (BST) thin films have been deposited on La0.67Sr0.33MnO3 (LSMO) coated single-crystal (001) oriented LaA103 substrates using pulsed-laser deposition technique. The bilayer films of BST and LSMO were epitaxially grown in pure single-oriented perovskite phases for both samples, and an enhanced crystallization effect in the BST film was obtained by the addition of Mn, which were confirmed by X-ray diffraction (XRD) and in situ reflective high energy electron diffraction (RHEED) analyses. The dielectric properties of the BST thin films were measured at 100 kHz and 300 K with a parallel-plate capacitor configuration. The results have revealed that an appropriate concentration acceptor doping is very effective to increase dielectric tunability, and to reduce loss tangent and leakage current of BST thin films. The figure-of-merit (FOM) factor value increases from 11 (undoped) to 40 (Mn doped) under an applied electric field of 200 W/cm. The leakage current density of the BST thin films at a negative bias field of 200 kV/cm decreases from 2.5 x 10(-4) A/cm(2) to 1.1 X 10(-6) A/cm(2) by Mn doping. Furthermore, a scanning-tip microwave near-field microscope has been employed to study the local microwave dielectric properties of the BST thin films at 2.48 GHz. The Mn doped BST film is more homogeneous, demonstrating its more potential applications in tunable microwave devices. (c) 2005 Elsevier B.V All rights reserved.

A new structure for the characterization of the permittivity of ferroelectric thin films at microwave frequencies is proposed. This new structure involves a coplanar waveguide (CPW) bandstop filter based on a ferroelectric thin film. Using the resonant frequency and Q value of the CPW bandstop filter, the dielectric constant (er) and the loss tangent (tan δ) of the ferroelectric thin film were determined by comparing the measured responses with simulated results. To demonstrate this new structure, a CPW bandstop filter was fabricated on MgO substrate coated with Ba0.5Sr0.5TiO3 (BST-0.5). The dielectric parameters determined as a function of temperature and external bias are reported.

Barium strontium titanate Ba0.7Sr0.3TiO3 (BST) thin films, with different growth temperatures (Tg) as well as different film thicknesses, have been prepared on Pt/Ti/SiO2/Si substrates by a reactive pulsed laser deposition method. We observed strong dependences of dielectric properties, such as the Curie–Weiss temperature, dielectric constant, loss tangent, dielectric tunability and leakage current, on the Tg and the BST film thickness. With increase of Tg from 630 to 750 °C, the dielectric constant gradually increases due to the increase in the crystallinity and the grain size. However, the dielectric tunability, loss tangent and leakage current characteristics drastically degrade when the Tg increases up to 750 °C, due to the diffuse and rough interface. The BST film grown at 690 °C shows the best overall dielectric properties with a figure-of-merit of 33 (at 400 kV/cm). These results suggest that film growth process could be optimized by systematically investigating the structure-property relationships. Furthermore, as the BST film thickness increases from 250 to 560 nm, the dielectric properties are remarkably enhanced. The film thickness effect is attributed to the interfacial low-dielectric layers (the so-called “dead layer”) between the BST film and both metal electrodes, which is well explained in terms of a series capacitor model. The thickness and the average dielectric constant for the dead layer are experimentally estimated to be 1.9 nm and 20.3, respectively, in Pt/BST/Pt capacitors.

La2/3Ca1/3MnO3 thin films have been grown on SrTiO3, LaAlO3, and yttria-stabilized zirconia buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved on the SrTiO3 and LaAlO3 substrates, a coexistence of the cube-on-cube and cube-on-diagonal epitaxy was observed in the the manganite films on SOI substrates. Besides the intrinsic four-fold magnetocrystalline anisotropy, a uniaxial anisotropy also exists in the films, which is determined by the demagnetization field and the mismatch-induced strain. A tensile strain leads to an easy plane, while a compressive strain favors an easy axis. The different magnetization configurations in the films on different substrates are the reason for their varied transport and magnetic properties. Due to a combined effect of these magnetic anisotropy, the magnetization in the two crystallography domains in the film on SOI tends to lie in the film plane but align in their respective easy axes. There are always large spin angles across the domain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. It shows a resistance change of ∼20% at 50 K in a magnetic field ∼700 Oe, which is promising for real applications.

We report the observation of quantum stochastic synchronization (QSS) in a dissipative quantum system-a moderately damped current-biased Josephson junction. We show that when the junction is biased subcritically by a combination of a constant current and a small sinusoidal modulation signal, forced phase synchronization between the otherwise random, incoherent quantum tunneling and the signal may arise as a consequence of the interplay between quantum fluctuation and the weak periodic drive. It is found that optimal synchronization occurs at a signal frequency f(QSS) that is comparable to the quantum tunneling rate. We also find a phenomenon called "modulation induced suppression of quantum noise": for signal frequency well below fQSS, the power spectral density of escape time distribution of the junction could be reduced substantially from the floor level of the unmodulated system. Furthermore, we demonstrate that the application of a small signal with proper phase can suppress the average tunneling rate and enhance the stability of the metastable system. Our experimental results agree well with the numerical calculations without the use of adjustable parameters.

The integration of high-temperature superconductors into advanced semiconductor technology is rather intriguing in the microelectronics field. In this paper, we report on YBa2Cu3O7−δ (YBCO) thin films deposited on silicon-on-insulator (SOI) substrates buffered by yttria-stabilized zirconia (YSZ) layers using the in situ pulsed laser deposition (PLD) technique. The strain in the YBCO films and the ageing effect caused by the thermally induced cracks are carefully studied. It is believed that because of the adoption of the SOI substrate, the strain in the YBCO films is greatly relaxed and the quality and stability of YBCO films are rather good, which is advantageous to the superconductive devices.

Barium strontium titanate Ba0.5Sr0.5TiO3 (BST) thin films have been deposited on Pt/Ti/SiO2/Si substrates by a pulsed laser deposition method. The low-frequency dielectric responses of the BST films, grown at different substrate temperatures (Ts), were measured as functions of frequency in the frequency range from 1 kHz to 1 MHz. With increase of Ts, the grain size of BST thin films became larger and the crystallinity was greatly improved, and then the dielectric permittivity increased, while the dielectric dispersion rose drastically. The origin of the large dielectric relaxation is believed to result from the aggravation of oxygen diffusion at the BST/Pt interface for the BST thin films grown at comparatively higher temperatures. This concept could be further explained by considering the influence of post-annealing in oxygen ambient on the dielectric properties of BST thin films. Our results reveal that the dielectric properties are strongly dependent on the processing conditions and the microstructure of thin films.

High quality Ba0.5Sr0.5TiO3/SrRuO3 (BST/SRO) heterostructure films have been successfully fabricated by pulsed-laser deposition on (0 0 1) LaAlO3 (LAO) substrates. The bilayer films were both (0 0 1) oriented normal to the substrate surface confirmed by X-ray diffraction analysis. The SRO layer with atomically smooth surface and good metallic behavior serves as not only an ideal bottom electrode but also an excellent template for epitaxial growth of the BST thin film. A parallel-plate capacitor configuration was used to measure the capacitance-voltage and leakage current characteristics of the BST films. The dielectric constant and maximum loss tangent measured at 1 MHz and 300 K were 1058 and 0.0092, respectively. The FOM-factor value reached 49 under the condition of +/-6 V DC bias and 1 MHz. A leakage current density of less than 6 x 10(-7) A/cm(2) was obtained at an electric field intensity of 2 x 10(5) V/cm. These results show the. potential of the BST/SRO heterostructures for microelectronic device applications. (C) 2004 Elsevier B.V. All rights reserved.

Highly oriented Ba0.5Sr0.5TiO3 (BST) perovskite thin films with different thickness ranging from 120 to 600 nm have been fabricated epitaxially on SrRuO3 (SRO) electrode layer, and (001) LaAlO3 single crystal as substrates. We observed a strong dependence of dielectric properties on the thickness of the BST films: with the increasing of the thickness, the dielectric loss decreases, the dielectric constant increases, and the dielectric tunability increases as well. For the 600-nm-thick BST thin film, the room-temperature values at the frequency of 1 MHz of the relative dielectric constant, epsilon(r) and loss tangent, tandelta, of 1058 and 0.0092 under no bias, and 583 and 0.0068 under 6 V bias, respectively, were achieved. A tunability of 44.8% has been achieved with DC bias as low as 6 V The obtained measurement data renders them very attractive for the tunable microwave device applications. (C) 2004 Elsevier B.V. All rights reserved.

Ba0.1Sr0.9TiO3/YBa2Cu3O7−δ heterostructure films have been fabricated by pulsed-laser deposition on LaAlO3 substrates. During the fabrication, a c-axis oriented YBa2Cu3O7−δ thin film with atomically smooth surfaces was epitaxially grown in the step-flow mode on a vicinal LaAlO3 substrate. The superconducting transition temperature Tc was above 88 K. Then a single-crystalline Ba0.1Sr0.9TiO3 layer was deposited on top of the YBa2Cu3O7−δ layer. A parallel-plate capacitor configuration was used to measure the capacitance–voltage properties of the Ba0.1Sr0.9TiO3 thin films at 300 K and 77 K, respectively. The dielectric constant and loss tangent measured at 1 MHz and 77 K are 1200 and 0.0045, respectively. A tunability of 60% has been achieved under ±30 V DC bias, confirming their usefulness for frequency-agile microwave devices.

We have fabricated two types of lumped-element Josephson parameter amplifiers (JPAs) by using a multilayer micro-fabrication process involving wet etching of Al films. The first type is a narrow band JPA which shows typical gain above 14 dB in a bandwidth around 35 MHz. The second type is a wideband JPA which is coupled to an input 50 Ω transmission line via an impedance transformer that changes the impedance from about 15 Ω on the non-linear resonator side to 50 Ω on the input transmission line side. The wideband JPA could operate in a 200 MHz range with a gain higher than 14 dB. The amplifiers were used for superconducting qubit readout. The results showed that the signal to noise ratio and hence the readout fidelity were improved significantly.

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device (SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as are fabricated for the qubit, in which the number of the decoherence-causing two-level systems (TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T 1 and dephasing time on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.

We have prepared high-quality a-axis-oriented superconducting YBa2Cu3O7-delta (YBCO) films using a self-template method on (100) SrTiO3 (STO) substrates by de sputtering. X-ray diffraction and transmission electron microscopy analysis showed that the films were grown epitaxially on the STO substrates with the a-axis perpendicular to the surface. The full-width at half-maximum value of 0.081 was measured through the rocking curve of the (200) diffraction peak. The effect of the deposition temperature and the thickness of the template layer and the upper layer on the quality of the films was investigated. We found evidence suggesting that structural strain was present at the interface between the template layer and the upper layer. For the film deposited under optimal conditions, the zero-resistance T-c was 85 K and the transition width was less than 2 K. The anisotropy of normal state resistance and critical current density was also studied.

A flip-chip-type gradiometer has been constructed with a 10 mm x 5 mm planar DCSQUID gradiometer fabricated on a SrTiO 3 bicrystal substrate and a flux transformer made from a YBCO * /YBCO/Ce0 2 /YSZ multilayer on a Φ50.8 mm Si wafer. The coupling coefficient between the flux transformer and the planar gradiometer is 0.18. The transformer increases effectively the resolution of the gradiometer. A magnetic field gradient resolution of 73 ff. cm -1 Hz -1/2 in the white region and 596 fT- cm -1 Hz -1/2 at 1 Hz has been obtained. High quality magnetocardiogram signals have been successfully measured by using this flip-chip-type gradiometer in an unshielded environment.

We present a distributed phase shifter with an equal ripple return loss at its operation frequency range. The phase shifter is based on a periodic structure and consists of a coplanar waveguide (CPW) line periodically loaded with voltage-variable barium strontium titanate (BST) interdigitated capacitors. Measurements show that its return loss is better than -15dB at frequencies from direct current to 16GHz, and at 9.4GHz, its phase shift is 41/spl deg/ under 120-V applied bias voltage.

In the preparations of superconducting qubits, circuit design is a vital process because the parameters and layout of the circuit not only determine the way we address the qubits, but also strongly affect the qubit coherence properties. One of the most important circuit parameters, which needs to be carefully designed, is the mutual inductance among different parts of a superconducting circuit. In this paper we demonstrate how to design a gap-tunable flux qubit by layout design and inductance extraction using a fast field solver FastHenry. The energy spectrum of the gap-tunable flux qubit shows that the measured parameters are close to the design values.

Superconducting nanowire single photon detector (SNSPD), as a new type of superconducting single photon detector (SPD), has a broad application prospect in quantum communication and other fields. In order to prepare SNSPD with high performance, it is necessary to fabricate a large area of uniform meander nanowires, which is the core of the SNSPD. In this paper, we demonstrate a process of patterning ultra-thin NbN films into meander-type nanowires by using the nano-imprint technology. In this process, a combination of hot embossing nano-imprint lithography (HE-NIL) and ultraviolet nano-imprint lithography (UV-NIL) is used to transfer the meander nanowire structure from the NIL Si hard mold to the NbN film. We have successfully obtained a NbN nanowire device with uniform line width. The critical temperature (Tc) of the superconducting NbN meander nanowires is about 5 K and the critical current (Ic) is about 3.5 μA at 2.5 K.

YBa2Cu3O7-δ (YBCO) thin films ablated from melt-textured-growth (MTG) YBCO target (containing-Y211 phases) by pulsed laser deposition were studied by transmission electron microscopy. The films grow epitaxially on the SrTiO3 substrates in a c-axis orientation with very few large particles on the film surface: instead rectangular Y2O3 inclusions were embedded in the films. The crystallographic relationship was (100)Y2O3//(001)YBCO, [011]Y2O3//[010]YBCO. The inclusions do not degrade the superconducting property of the YBCO films. MTG YBCO targets result in films with lower level of large impurity particles.

Quantum phase diffusion in a small underdamped Nb/AlO$_x$/Nb junction ($\sim$ 0.4 $\mu$m$^2$) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant., Comment: 4 pages, 4 figures, 1 table

A hyper chaos based encryption algorithm for data of network risk assessment is designed in this paper. The size of plain group is variable and determined by the encryption rounds in the algorithm. In encryption procedure of the algorithm, the plaintext is first divided into groups of size k × 2n byte, then the plaintext is encryption group by group. For every plaintext group of k × 2n bytes, it is divided into 2n blocks of k bytes, after n rounds encryption transformation and one round permutation, the ciphertext group of k × 2n bytes is obtained. To improve the security of the algorithm, Chen's system is iterated for several times before the encryption of every plaintext group. It's from experiments and analysis that the algorithm is secure and valid.

e have developed a simple process to obtain large magnetoresistance (MR) in perovskite manganite thin films by a combination of focused ion beam (FIB) milling and 120 keV H$_{2}^{+}$ ion implantation. Metal slits about 70 nm in width were printed by 30 kV focused Ga ion beam nanolithography on a 4 mm track, and the materials in these slits are then irradiated by the accelerated H$_{2}^{+}$ ions. Using this method, in a magnetic field of 5 T we can get a MR${>}$60% over a 230 K temperature scope, with a maximum value of 95% at around 70 K. This technique is very promising in terms of its simplicity and flexibility of fabrication and has potential for high-density integration., 3 pages, 5 figures