具有Washer型输入线圈的超导量子干涉放大器的制备与表征

超导量子干涉仪(SQUID)放大器具有低输入阻抗、低噪声、低功耗等优点, 目前被广泛用于微弱信号的检测领域. 与其他工艺相比, Nb/Al-AlO_x/Nb结构的约瑟夫森结具有相对较高的转变温度(T_c)、高的磁通电压调制系数以及良好的热循环能力、较宽的临界电流范围, 因此是制备SQUID放大器的很好选择. 设计并制作了欠阻尼、过阻尼约瑟夫森结以及具有Washer型输入线圈的单SQUID放大器, 通过在He³制冷机3 K温区下对器件电流-电压特性进行测量, 得到良好的结I-V特性曲线、SQUID调制特性, 初步实现利用SQUID进行放大作用, 并计算了SQUID的电流分辨率. 此项工作对于超导转变边沿传感器读出电路的实现具有重要的意义.     

超导量子干涉器件

超导现象是一种宏观量子现象.磁通量子化和约瑟夫森效应是两个最能体现这种宏观量子特性的物理现象.超导量子干涉器件(superconducting quantum interference device,SQUID)是利用这两个特性而形成的超导器件.SQUID器件在磁信号灵敏探测方面具有广泛的应用.本文简要介绍低温超导和高温超导SQUID器件的相关背景和发展现状以及应用领域.     

Design optimization for microwave coupling structures of a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions

A new coupling scheme of the external microwave signal for a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions has been proposed. It is suggested to couple the meander-like array inductively to a parallel microwave transmission line. Within the resistively shunted junction model different types of microwave coupling structures were analyzed with the goal to reduce the mutual phase locking and to achieve high output power and phase-locking stability. A harmonic balance technique was used to derive an analytic expression connecting the amplitudes of microwave voltages and currents in the array and the frequency of the external signal. Numerical simulations were performed to determine the boundaries of stable locking as a function of circuit parameters.     

Self-radiation from arrays of niobium Josephson junctions embedded in the open resonator

This paper focuses on self-radiation from arrays of Josephson junctions embedded in a quasi-optical resonator. The mechanism of coupling this radiation to the principal mode of the open resonator is illustrated using experiments and simulations with CST microwave studio software. Comparing the microstrips and dielectric resonators used as the antennas of the series arrays of discrete Josephson junctions, we demonstrate that the dielectric resonator antennas are more effective than microstrips.     

基于双晶退火衬底的高温超导纳米量子干涉仪在液氦液氮温区下的输运特性

超导纳米量子干涉器件是近年来发展起来的一种新型超导器件, 通过现代微纳加工技术将其超导环缩小到纳米尺寸, 构成高度灵敏的微观自旋探测器, 理论上可以达到测量单电子自旋的灵敏度. 同时, 高温超导由于具有较高的临界温度和上临界场也备受关注. 然而, 相比于低温超导量子干涉器件, 高温超导量子干涉器件的1/f 噪声更加显著, 这限制了其在低频下的应用. 本文基于双晶衬底技术, 通过聚焦离子束加工制备高温超导纳米量子干涉仪, 并对其在液氦和液氮温区下的电学性能、4.2 K 下的磁通噪声进行表征. 实验发现, 通过预先退火处理钛酸锶双晶衬底, 可以有效改善衬底表面的粗糙度, 进而优化高温超导纳米量子干涉仪的1/f 磁通噪声. 最终得到低频下(1 Hz)的磁通噪声灵敏度为4.9 μΦ0/Hz1/2 , 比未事先预处理的小一到两个数量级, 这对于推进高温超导纳米量子干涉器件在低频下的应用意义重大.     

Modulation depth of series SQUIDs modified by Josephson junction area

The superconducting quantum interference device(SQUID) amplifier is widely used in the field of weak signal detection for its low input impedance, low noise, and low power consumption. In this paper, the SQUIDs with identical junctions and the series SQUIDs with different junctions were successfully fabricated. The Nb/Al-AlO_x/Nb trilayer and input Nb coils were prepared by asputtering equipment. The SQUID devices were prepared by a sputtering and the lift-off method.Investigations by AFM, OM and SEM revealed the morphology and roughness of the Nb films and Nb/Al-AlO_x/Nb trilayer.In addition, the current–voltage characteristics of the SQUID devices with identical junction and different junction areas were measured at 2.5 K in the He~3 refrigerator. The results show that the SQUID modulation depth is obviously affected by the junction area. The modulation depth obviously increases with the increase of the junction area in a certain range. It is found that the series SQUID with identical junction area has a transimpedance gain of 58 ? approximately.     

Development of highly integrated RSFQ circuits on the basis of intrinsically shunted Josephson junctions

At PTB, for application in rapid single flux quantum (RSFQ) and voltage standard circuits, the development of highly integrated SDE circuits is focused on devices based on intrinsically shunted Josephson junctions in the SINIS and SNS technologies. In SINIS technology, the fabrication process has been optimized to values of the critical current density of j_C=500 A/cm² and the characteristic voltage of V_C=190 μV. To raise the circuit integration level, successive steps of development are shown by the example of the layout of an elementary RSFQ cell designed for higher values of j_C. In SNS technology, a fabrication process has been developed to produce small ramp-type junctions with contact areas smaller than 0.4 μm² and with values for j_C and V_C of about j_C=200 kA/cm² and V_C=20 μV. The design allows the SNS junction size to be further reduced down to the deep sub-micron range.     

Fast and Robust Quantum Information Transfer in Annular and Radial Superconducting Networks

In this paper, we propose a protocol to achieve fast and robustness quantum information transfer (QIT) in annular and radial superconducting networks, where each quantum node is composed of a superconducting quantum interference device (SQUID) inside a coplanar waveguide resonator (CPWR). The process is based on reversely constructing time‐dependent control Hamiltonian by designing evolution operator. With the protocol, the maximal population of lossy intermediate states and the amplitudes of pulses can be easily controlled by two corresponding control parameters. Therefore, one can design feasible pulses for QIT with great flexibility. Besides, the speed of the QIT here is much faster compared with that with adiabatic QIT. Moreover, numerical simulations show that the protocol still possesses high fidelity when lossy factors and imperfect operations are taken into account. Therefore, the protocol may provide a useful way to manipulate quantum information networks.     

An experimental scheme to verify the dynamics of the Aharonov--Bohm effect

There are two different viewpoints on the Aharonov--Bohm (A--B) effect. One asserts that the A--B effect is due to the existence of the vector potential A. The other asserts that the A--B effect is due to the interaction energy between the magnetic field produced by the moving charges and the magnetic field in the solenoid. The difference of these two viewpoints is analyzed in this paper. To judge which viewpoint is right, this paper suggests a new experimental method.     

没有薛定谔猫态

新近有个超导量子干涉器件的实验,使澄清量子力学基本解释有了可能.分析此实验和相关实验之后,作出断言,抽象量子态没有信息,且波函数是个尚待观察者要去做的实验的几率幅.故用常用术语将量子态或波函数过分具体化,恐怕要招致误解.因而从本质上讲,薛定谔猫态是不存在的.     

Superconducting quantum interference devices made of Sb-doped Bi2Se3 topological insulator nanoribbons

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) made of Sb-doped Bi₂Se₃ topological insulator (TI) nanoribbon (NR) contacted with PbIn superconducting electrodes. When an external magnetic field was applied along the NR axis, the TI NR exhibited periodic magneto-conductance oscillations, the so-called Aharonov-Bohm oscillations, owing to one-dimensional subbands. Below the superconducting transition temperature of PbIn electrodes, we observed supercurrent flow through TI NR-based SQUID. The critical current periodically modulates with a magnetic field perpendicular to the SQUID loop, revealing that the periodicity corresponds to the superconducting flux quantum. Our experimental observations can be useful to explore Majorana bound states (MBS) in TI NR, promising for developing topological quantum information devices.     

超导量子器件片上皮法电容的制备及测量

片上集成电容是超导量子芯片上的核心器件，其数值一般在百飞法(fF)至皮法(pF)范围.采用常规微纳加工技术在蓝宝石基片上制备了铌-氧化硅-铝(Nb/SiO₂/Al)平行板电容.利用刻蚀工艺制备了平行板电容器的下极板Nb,利用剥离工艺制备平行板电容器的上极板Al和介电层SiO₂.室温下利用锁相放大原理和桥式电路原理测定电容大小，两种方法测定电容值基本一致，表明锁相放大原理测试pF级电容的可靠性.利用该电容与铝基约瑟夫森结组成谐振器，制备了中心频率位于4.35 GHz的约瑟夫森参量放大器.在稀释制冷机中10 mK温度下测定直流偏置谐振器的磁通-频率相位图，拟合数据获得的电容值与室温测定电容值接近，表明在mK、GHz条件下工作的片上集成电容可在室温、kHz条件下测定其数值大小.     

超导量子干涉器(SQUID)原理及应用

严谨的超导量子干涉器原理的阐述是非常深奥和繁杂的,我们在这里采用简明的物理图像和数学语言论述超导量子干涉器的原理及应用,以推动“大学物理现代化”的进程.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device （SQUID） qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed in two lower flux states, and the excited state [2〉 would not participate in the procedure. The SQUIDs undergo no transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum iogic in SQUID-system.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device (SQUID) qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum logic in SQUID-system.     

基于高温超导量子干涉仪的超低场核磁共振成像研究

对基于高温超导量子干涉仪的低场核磁共振成像进行了较为系统的探索.首先对低场核磁共振系统进行了改进和完善,使得装置能够用于成像实验.在此基础上进行了一维、二维成像实验并取得了成功.二维成像分别采用了直接背投影成像法和傅里叶变换重建法.采用直接背投影方法成功获得了不同水样品分布的图形并与实物符合较好,同时还尝试对生物样品如青椒和芹菜的切片进行了成像,也得到了符合原物的二维投影像.尝试用傅里叶变换法对水样品进行成像,得到的图形能够显示样品轮廓,但信噪比偏低.对两种二维成像方法进行了比较和讨论.     

Analytical results of the Fokker-Planck equation derived for one superconducting nanowire quantum interference device and for DC SQUIDs-asymmetric devices

We consider an asymmetric two-junction superconducting quantum interference device, whose junctions are assumed to be overdamped, and regard Sin Fourier series for their current-phase relations. We take into account the effects of thermal fluctuations by forming a two-dimensional Fokker-Planck equation for the distribution function. We judge a series expansion of first order with respect to the components of the reduced inductance for distribution function and obtain current-voltage relation. We consider the measured resistance of the superconducting nanowire quantum interference device with mesoscopic leads that Hopkins et al. reported in Hopkins et al. [D.S. Hopkins, D. Pekker, P.M. Goldbart, A. Bezryadin, Science 308 (2005) 1762] and analyzed in Pekker et al. [D. Pekker, A. Bezryadin, D.S. Hopkins, P.M. Goldbart, Phys. Rev. B 72 (2005) 104517], by defining loop inductance, and by considering appropriate relations for resistance of nanowires. In fact we extend Chesca formulation [B. Chesca, J. Low Temp. Phys. 112 (1998) 165] simultaneously in three aspects and give a unified theory for nanowire two-junction devices, low T_c and high T_c DC SQUIDs, in restricted conditions.     

SQUID在被动生命信号检测中的应用前景

从生物电磁效应的概念出发,简要介绍了人体及其周围的电磁特性;重点介绍了能探测人体电磁信号的超导量子干涉仪;通过灵敏度分析证实了SQUID可以用来探测微弱的人体电磁信号;提出了将已经广泛应用于医学的SQUID转而通过探测人体电磁信号来进行被动生命信号检测的应用研究;希望能为地震灾害与煤矿瓦斯灾害等搜救工作提供一定的帮助。     

Pertubation analysis of the dynamical behavior of two-junction interferometers

The dynamical properties of symmetric quantum interferometers with equal junctions of negligible capacitance have been studied by means of perturbation analysis in the limit of small values of the parameter β. In this limit, two characteristic time constants arise. These quantities may be linked to two different dynamical processes in the system: the first is related to the time evolution of the average superconducting phase difference across the two junctions; the second defines the time scale for flux motion. The response of the system to constant and time-dependent externally applied magnetic fields is considered and a general perturbed solution for the average superconducting phase difference and the fluxon number variable is derived to first order in β.     

Quantum Entanglement of Two Flux Qubits Induced by an Auxiliary SQUID

We revisit a theoretical scheme to create quantum entanglement of two three-level superconducting quantum interference devices (SQUIDs) with the help of an auxiliary SQUID. In this scenario, two three-level systems are coupled to a quantized cavity field and a classical external field and thus form dark states. The quantum entanglement can be produced by a quantum measurement on the auxiliary SQUID. Our investigation emphasizes the quantum effect of the auxiliary SQUID. For the experimental feasibility and accessibility of the scheme, we calculate the time evolution of the whole system including the auxiliary SQUID. To ensure the efficiency of generating quantum entanglement, relations between the measurement time and dominate parameters of the system are analyzed according to detailed calculations.