From Flux Quantization to Superconducting Quantum Bits

One of the important predictions of the early phenomenological theories of superconductivity such as the London and Ginzburg–Landau (GL) theory is the quantization of magnetic flux in multiply connected superconductors, which is one of the first demonstrations of a quantum effect on a macroscopic scale. In this paper, which is devoted to Vitalij Lazarevich Ginzburg on the occasion of his 90th birthday, we analyze a superconducting cylinder acting as a flux box as well as a superconducting disk acting as a Cooper pair box in the framework of GL theory. We extend this analysis to leaky flux and Cooper pair boxes which are obtained by introducing weak links allowing for the entry and exit of flux quanta and Cooper pairs from the respective boxes at finite rates. Flux and Cooper pair slippage processes by coherent quantum tunneling result in effective two-level quantum systems forming the basis for flux and charge quantum bits presently considered for the solid-state implementation of quantum information processing. We show that the corresponding Hamiltonians describing the leaky flux and Cooper pair box can be transformed into each other by a canonical transformation. PACS numbers: 74.20.De, 74.25.Bt, 74.25.Ha, 74.50.+r     

Intrinsic Quantum Dissipation in Superconducting Weak Links

We evaluate the frequency spectrum for an intrinsic quantum dissipative environment for the Josephson phase φ formed by subgap Andreev bound states in superconducting weak links. We also analyze the problem of macroscopic quantum tunneling (MQT) of φ in highly transparent weak links and identify several MQT regimes. Within the exponential accuracy, we derive the expression for the supercurrent quantum decay rate both at low temperatures and in the vicinity of the quantum-to-classical crossover.     

Materials Physics and Quantum Coherence in Superconducting Qubits

Nonidealities of real superconductors in the form of residual rf losses and conductance below the superconducting energy gap in tunneling Josephson junctions are discussed as possible sources of decoherence in Josehphson junction flux qubits. The purpose of the paper is pedagogical and aimed at taking a first step in a unified view of the quantum behavior of superconducting qubits and the realities of the materials out of which they are constructed. The need for a dialog between the quantum physicist and the materials physicist is stressed. Residual rf losses and localized states in junction barriers are identified as important subjects for this dialog.     

Recent Progress in Quantum Simulation Using Superconducting Circuits

Quantum systems are notoriously difficult to simulate with classical means. Recently, the idea of using another quantum system—which is experimentally more controllable—as a simulator for the original problem has gained significant momentum. Amongst the experimental platforms studied as quantum simulators, superconducting qubits are one of the most promising, due to relative straightforward scalability, easy design, and integration with standard electronics. Here I review the recent state-of-the art in the field and the prospects for simulating systems ranging from relativistic quantum fields to quantum many-body systems.     

Quantum Entanglement and Correlations in Superconducting Flux Qubits

We report on the quantum correlations dissipative dynamics followed by coupled superconducting flux qubits. The coupling between the superconducting quantum register and the reservoir is described by two different mechanisms: collective and independent decoherence. By means of the Bloch?CRedfield formalism, we solve the quantum master equation and show that coupling under collective quantum noise is more robust to decoherence. This result is demonstrated for different flux qubit initial preparations, taking into account the influence due to external fields and temperature. Furthermore, we compute the entanglement and the quantum discord dissipative dynamics as controlled by external parameters. We show that the discord is more robust against decoherence effects. This fact could be harnessed in the realization of quantum computing tasks that do not need to invoke entanglement in their implementation.     

Four-Wave Mixing-Induced Maximal Entanglement in Superconducting Phase Quantum Circuits

We are interested in studying the entanglement of an array of superconducting phase quantum circuits and external magnetic fluxes. It is shown that in a four-level cascade type quantum system, the degree of entanglement increases by generation of fourth microwave pulse, in multi-photon resonance condition. We achieve the maximal entanglement induced via four-wave mixing in our model. Moreover, it is demonstrated that the population distribution of the dressed states approaches to be uniform as the degree of entanglement becomes maximum. We can control the entanglement of the composite system by changing amplitudes of the applied magnetic fluxes. Our results can be used in quantum information processing via superconducting quantum circuits.     

Shielded Superconducting Linear Motor for Towed-Grid Studies of Quantum Turbulence

The purpose of ongoing low-temperature quantum turbulence research is to produce turbulence similar to that studied in classical fluids to compare the experimental data and theories. Specifically, in the absence of viscosity, through what path does the turbulence decay? Homogeneous isotropic turbulence (HIT) is the best characterized classical situation. To produce HIT in a quantum fluid, we must tow a grid through a channel of superfluid helium at 20 mK. A grid motion of 1 cm at a nearly constant speed up to 1 μS is required. To avoid Joule and eddy current heating of the liquid helium, a magnetically shielded superconducting linear motor has been built. The grid is attached to the end of a light insulating armature rod which has two hollow cylindrical niobium cans fixed to it about 26 mm apart. This part of the rod is inside a superconducting solenoid which, when driven with the properly shaped current pulse, produces a magnetic field that accelerates the rod (and grid) in 1 mm, moves the rod and grid at constant speed for 10 mm, and then decelerates it in 1 mm. The motor was built guided by simulations that demonstrated the design and current pulses required are quite reasonable. The simulation and control program is written in LabView with an embedded C compiler. Using the simulator, various designs of solenoid (with and without shielding) and armature were investigated. We compare the simulation and the experimental results. By writing a pulse-generating program in LabView, we can apply virtually any pulse shape required to produce the desired motion. This is necessary because of the almost purely inductive (zero resistance) load of the motor circuit. The simulations, design process, and the experimental data demonstrating the functioning motor will be presented.     

Cross-Correlated Dynamic Resistance of a Direct Current Superconducting Quantum Interference Device

This paper presents experimental data on a comparative study of a dc SQUID with voltage and current bias. We introduce a cross-correlated dynamic resistance of the device defined as a ratio R _dCV = _VC/_IV, where _VC = (V/) _A is the slope of the voltage-to-flux characteristics measured in the current bias mode and _IV = (I/) _A is the slope of the current-to-flux characteristics measured with voltage bias. It has been found that R _dCV may deviate strongly from the dynamic resistance observed in the current bias mode of operation. The intrinsic energy resolution of the SQUID remains unchanged for both modes of operation, but the current noise of the voltage biased device scales with the cross-correlated dynamic resistance. In our SQUID with the loop inductance L = 105 pH, is equal to 37 h in the white noise region at a temperature of 4.2 K.     

Macroscopic Quantum Phenomena in High Critical Temperature Superconducting Josephson Junctions

YBa₂Cu₃O_7?δ grain boundary bi-epitaxial Josepshon junctions (JJs) allow a very clear demonstration of Josephson current variation with the misorientation angle, consistent with the d-wave symmetry of the superconducting order parameter in cuprate, high temperature superconductors. Our bi-epitaxial junctions show a strong suppression of the first harmonic, I ₁ sin ø, of the current phase relation when tunneling from a lobe into a node of the superconducting gap function. In these configurations, the contribution of the second harmonic, I ₂ sin 2ø, becomes of the same magnitude as the first one, giving rise to a characteristic two-well Josephson potential as a function of phase ø instead of the usual single well. This characteristic intrinsic property has suggested proposals of a new class of qu-bit named “quiet” because of the existence a spontaneously degenerate fundamental state without the need of applying an external field. Our experiments probe the macroscopic quantum properties in a d-wave Josephson junction by measuring macroscopic quantum tunneling and energy level quantization. The switching current out of the zero voltage state is measured as a function of temperature down to 20 mK. The temperature variation of the width of an ensemble of switching events goes over from one, which is characteristic of a thermal activation of phase fluctuations to a temperature independent width which is a token of quantum tunneling of the phase. The transition regime is affected by the two-well potential in a 45^° misorientation junction as the second harmonic term gives rise to additional thermal transitions. The difference between quantized energy levels in the harmonic potential was determined by microwave spectroscopy. From the broadening of energy levels, it was possible to extract a Q-value of about 40 for the phase oscillations. The relatively high Q indicates quantum coherence over a sizeable time in d-wave junctions and gives hopes for a realization of a “quiet” high-T _c qu-bit. The contributions of V. L. Ginzburg to several different fields of physics are impressive and long standing. In superconductivity the Ginzburg–Landau theory, for instance, still represents a very powerful approach to model a huge number of different physical systems. High Temperature Superconductors (HTS) have strongly influenced research of the last 20 years and their d-wave order parameter symmetry represents one of the most intriguing features from both the fundamental point of view and some types of innovative long-term applications.     

检测/校准用计算机及自动检测系统的质量控制

本文论述了在检测和校准实验室质量管理体系中,用计算机或自动检测系统对检测/校准数据进行自动采集、处理、记录、报告、存贮或检索等工作时进行质量控制的方法.     

SC200超导质子回旋加速器束流传输真空系统及控制设计

束流传输系统真空是合肥超导质子医疗设备(SC200)的重要技术保障。研究束流传输系统真空对有效保证束流传输环境和束流最终品质,使束流在合理的偏差范围内到达治疗终端有着重要作用。SC200输运线上二极铁、四级铁、校正铁、束流阻断器和束测等部件分布密集,局部机械空间紧张。在此设计输入下,束流传输系统的真空部件和真空管结构既要有利于束流传输的品质,又要确保机械安装空间和后期设备维护的便利性。SC200超导质子回旋加速器束流传输真空系统总长约65 m,动态真空要求优于5×10^-4 Pa(局部可降低为5×10^-3 Pa)。研究结果表明,SC200超导质子回旋加速器束流传输系统真空设计满足设计输入要求。     

Superfluid, Superconducting, and Magnetic Ordering in Mesoscopic Quantum Dots

The nature of superfluid, superconducting, and magnetic ordering is elucidated for mesoscopic systems in which the single-particle level spacing is much larger than both the temperature and the critical temperature of ordering. Ordering is defined as a spontaneous breaking of symmetry, the gauge invariance and time reversal being by definition symmetries broken in superfluidity (superconductivity) and magnetism contexts, respectively. Superfluidity and superconductivity are realized in thermodynamic equilibrium states with a nonintegral average number of particles and are accompanied by the spontaneous breaking of time homogeneity. In Fermi systems, two types of superfluidity and superconductivity are possible which are characterized by the presence of pair or single-particle condensates. The latter is remarkable in that spontaneous breaking of fundamental symmetries such as spatial 2 rotation and double time reversal takes place. Possible experiments on metallic nanoparticles and ultracold atomic gases in magnetic traps are discussed.     

The Quantum Phase Slip Phenomenon in Superconducting Nanowires with High-Impedance Environment

Quantum phase slip (QPS) is the particular manifestation of quantum fluctuations of the order parameter of a current-biased quasi-1D superconductor. The QPS event(s) can be considered a dynamic equivalent of tunneling through conventional Josephson junction containing static in space and time weak link(s). At low temperatures T<<T _c, the QPS effect leads to finite resistivity of narrow superconducting channels and suppresses persistent currents in tiny nanorings. Here, we demonstrate that the quantum tunneling of phase may result in Coulomb blockade: superconducting nanowire, imbedded in high-Ohmic environment, below a certain bias voltage behaves as an insulator.     

Operation of a Wideband Terahertz Superconducting Bolometer Responding to Quantum Cascade Laser Pulses

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2–7?K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100?Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1?Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450?ns.     

高温超导磁悬浮测试系统

介绍一种高温超导磁悬浮测试系统,该系统主要包括液氮低温容器,永磁体轨道,数据采集和处理,机械传动和自动控制四部分,采用了能放量高温超导体块材的薄底液氮低温容器（杜瓦容器）,解决了将高温超导体块材置于永久磁体之上的磁悬浮测量问题,能实时检测多块或单块高温超导体块材的各种高温超导磁悬浮性能。     

Driving Mechanism of a Superconducting Magnetic Bearing System

Driving the rotor of a superconducting magnetic bearing without mechanical contact in the optimum conditions is an important task for high operational speeds. For this reason, an alternative eddy current mechanism is proposed to drive the rotor by means of magnetism with a speed higher than that of the driver. The designed driving system provides strong and stable magnetic coupling between the relatively rotating parts. The drag and lift forces between the rotor and driver disc are discussed by considering various conditions, such as the rotor configurations and the saturation of the magnetic field within the conducting material. The overall results indicate that the designed electromechanical driving system has potential solutions for the various applications for magnetic bearings from the effective driving mechanism point of view.     

Two-Tone Response of Radiofrequency Signals Using the Voltage Output of a Superconducting Quantum Interference Filter

In the presence of weak time-harmonic electromagnetic fields, superconducting quantum interference filters (SQIFs) show the typical behavior of nonlinear mixers. The SQIFs are manufactured from high-T _c grain boundary Josephson junctions and operated in active microcooler. The dependence of dc voltage output V _dc versus static external magnetic field B is nonperiodic and consists of a well pronounced unique dip at zero field, with marginal side modulations at higher fields. We have successfully exploited the parabolic shape of the voltage dip around B = 0 to mix quadratically two external time-harmonic rf-signals, at frequencies f ₁ and f ₂ below the Josephson frequency f _J, and detect the corresponding mixing signal at |f ₁− f ₂|. When the mixing takes place on the SQIF current–voltage characteristics, the component at 2f ₂−f ₁ is present. The experiments suggest potential applications of a SQIF as a nonlinear mixing device, capable to operate at frequencies from dc to few gigahertz with a large dynamic range.     

基于改进量子粒子群算法的纸浆浓度控制系统

目的为了克服传统PID控制在具有大时滞性、非线性等特点的纸浆浓度控制系统中性能不足和参数调整困难等问题,研究参数在线调整的方法。方法在传统PID控制的基础上,结合量子粒子群仿生算法(QPSO),提出一种量子粒子群算法优化的传统PID控制器参数,并应用于纸浆浓度控制系统;同时对基本量子粒子群算法进行改进,引入交叉算子,并将该控制算法应用到纸浆浓度控制系统中,并与传统控制进行对比。结果与传统PID控制和基本量子粒子群优化的PID相比较,改进的优化算法能够得到更加令人满意的控制效果,具有系统超调量小、响应速度快、鲁棒性高等优良的性能。结论基于改进的量子粒子群优化算法的纸浆浓度控制系统可有效控制纸浆浓度,能够明显提高系统的控制精度等性能指标,更好地满足实际应用的要求。     

Superconducting Gap in the Multi-band System MgB2

Based on the angle-resolved photoemission spectra of single crystals, it is demonstrated that a couple of bands cross the Fermi energy in MgB₂, which is in good agreement with band theory. The superconducting gap in this multiband system is carefully examined by Raman scattering spectroscopy with various polarizations. It has been revealed that the large gap (24k _B T _c) that is typical for a clean limit s-wave superconductor is restricted to the -bands, while the gap on the -bands is much smaller (21.1k _B T _c) and strongly affected by the impurity scattering, which gives a dirty limit behavior. This unusual two-gap behavior might be caused by the lack of interband scattering due to special separation of the - and -bands, as predicted by Mazin et al.     

Structural Investigations of La-2125 Mixed Oxide Superconducting System

Crystallographic studies on the (La_{2 – x} Nd _x )Ca _y Ba₂Cu_{4 + y} O _z [y = 2x; 0.0 x 0.5] samples are carried out using neutron diffraction technique. The analysis of the neutron diffraction patterns for the series of samples with 0.0 x 0.5 was done using Rietveld method. The La₂Ba₂Cu₄Oz (La-224, x = 0.0) system exhibits tetragonal phase and is a nonsuperconductor, which on addition of a CaCuO₂ rock salt like layer becomes superconducting. It is important to note the significant role played by Ca-doping in improvising the superconducting properties. Analysis of the structural data reveals that, as Ca concentration increases, the unit cell volume decreases while T _c increases with a maximum value of 78 K for y = 1.0. The detailed crystal structure for LaNdCaBCO series of samples has been studied in the light of changes in bond lengths with increase in Ca concentration and its role in turning on the superconductivity.