Development and Investigation of SNS Josephson Arrays for the Josephson Arbitrary Waveform Synthesizer

Superconductor–normalmetal–superconductor(SNS) Josephson junction series arrays are developed and investigated for applications in the Josephson arbitrary waveform synthesizer (JAWS). The series arrays contain up to about 10 000 junctions arranged in a meanderlike design. AC voltages and arbitrary waveforms are synthesized by operation of the Josephson junctions by short current pulses. Higher harmonics are at least 110 dB below the fundamental. To verify the generated waveforms, a spectrum analyzer that has initially been calibrated using a binary-divided programmable Josephson voltage standard is used.      

Operating Margins for a Pulse-Driven Josephson Arbitrary Waveform Synthesizer Using a Ternary Bit-Stream Generator

We describe measurements with a pulse-driven Josephson arbitrary waveform synthesizer (JAWS) that uses a ternary arbitrary bit-stream generator as the bias source. This system is designed to be used as an ac Josephson voltage standard. From these measurements, we conclude that the system is operational for root-mean-square voltages of up to 220 mV. We measured the operating margins for different voltages synthesized with different numbers of junctions, waveforms, and bias conditions. We also present results on the influence of the critical current on the operating margins.      

Error and Transient Analysis of Stepwise-Approximated Sine Waves Generated by Programmable Josephson Voltage Standards

We are developing a quantum-based 60 Hz power standard that exploits the precision sinusoidal reference voltages synthesized by a programmable Josephson voltage standard (PJVS). PJVS systems use series arrays of Josephson junctions as a multibit digital-to-analog converter to produce accurate quantum-based dc voltages. Using stepwise-approximation synthesis, the system can also generate arbitrary ac waveforms [i.e., an ac programmable Josephson voltage standard (ACPJVS)] and, in this application, produces sine waves with calculable root mean square (rms) voltage and spectral content. The primary drawback to this ACPJVS synthesis technique is the uncertainty that results from switching between the discrete voltages due to finite rise times and transient signals. In this paper, we present measurements and simulations that elucidate some of the error sources that are intrinsic to the ACPJVS when used for rms measurements. In particular, we consider sine waves synthesized at frequencies up to the audio range, where the effect of these errors is more easily measured because the fixed transition time becomes a greater fraction of the time in each quantized voltage state. Our goal for the power standard is to reduce all error sources and uncertainty contributions from the PJVS-synthesized waveforms at 60 Hz to a few parts in 10⁷ so that the overall uncertainty in an ac power standard will be a few parts in 10⁶.     

10 V SINIS Josephson junction series arrays for programmablevoltage standards

A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al₂O₃ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed     

交流约瑟夫森电压合成装置的研究

设计制作了一套交流约瑟夫森电压合成装置(JAWS),能够驱动1 V SINIS型可编程约瑟夫森结阵合成峰值1.2 V、 200 Hz以下频率的交流量子电压。实验结果表明,该装置能够合成200 Hz以下频率的交流量子电压,且合成60 Hz交流电压的不确定度优于5×10^-6,为进一步开展我国首个交流量子电压基准的研究工作奠定了基础。     

Development of a 60 Hz Power Standard Using SNS Programmable Josephson Voltage Standards

We are implementing a new standard for 60 Hz power measurements based on precision sinusoidal reference voltages from two independent programmable Josephson voltage standards (PJVS): one for voltage and one for current. The National Institute of Standards and Technology PJVS systems use series arrays of Josephson junctions to produce accurate quantum-based DC voltages. Using stepwise-approximation synthesis, the PJVS systems produce sinewaves with precisely calculable RMS voltage and spectral content. We present measurements and calculations that elucidate the sources of error in the RMS voltage that are intrinsic to the digital-synthesis technique and that are due to the finite rise times and transients that occur when switching between the discrete voltages. Our goal is to reduce all error sources and uncertainty contributions from the PJVS synthesized waveforms to a few parts in 10 ⁷ so that the overall uncertainty in the AC-power standard is a few parts in 10⁶     

Near-Zero Bias Arrays of Josephson Tunnel Junctions Providing Standard Voltages up to 1 V

Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.     

AC and DC voltages from a Josephson arbitrary waveform synthesizer

We have synthesized and measured ac and dc voltages using a Josephson arbitrary waveform synthesizer. On-chip filtering has enabled the first practical operating margins for ac and arbitrary waveforms. Using a digital voltmeter, we demonstrate the operating margins and linearity of 101 synthesized dc voltages and confirm the flatness of a single voltage step. We present the first ac-dc and ac-ac voltage measurements of the synthesizer at 3.65 mV using an ac-dc thermal transfer standard. This quantum-based standard source can be used to measure the ac-dc difference of thermal transfer standards at small voltages     

SIS junctions with frequency dependent damping for a programmableJosephson voltage standard

Experimental and computational results on programmable Josephson junction array (JJA) chips based on superconductor-insulator-superconductor (SIS) junctions are presented. Implications of circuit design and fabrication process on the performance are discussed. We introduce a method to decrease the attenuation of the pump microwave. Different designs are compared, suggesting that 1 V chips operating at the third constant voltage step with 70 GHz pump frequency can be produced with our process     

1V脉冲驱动型交流量子电压源研究

脉冲驱动型交流量子电压标准ACJVS通过高速脉冲序列驱动约瑟夫森结阵芯片的方式实现宽频带交流量子电压的合成,相比于可编程型交流量子电压标准PJVS,具有免台阶切换、频谱纯净、频带宽等优点。搭建的系统主要包括8位高速脉冲码型发生器、微波放大器、直流阻断、约瑟夫森结阵芯片等。通过驱动包含4个子阵列,每个子阵列含12810个约瑟夫森结的结阵芯片,并结合4通道联合低频补偿的方式,成功产生了1V有效值的脉冲驱动型交流量子电压,为进一步建立交流量子电压基准打下了坚实的基础。最后,展望了脉冲驱动型交流量子电压在量子阻抗桥、交流量子功率源、交流量子功率表方面的应用价值。     

Precision Differential Sampling Measurements of Low-Frequency Synthesized Sine Waves With an AC Programmable Josephson Voltage Standard

We have developed a precision technique to measure sine-wave sources with the use of a quantum-accurate ac programmable Josephson voltage standard. This paper describes a differential method that uses an integrating sampling voltmeter to precisely determine the amplitude and phase of high-purity and low-frequency (a few hundred hertz or less) sine-wave voltages. We have performed a variety of measurements to evaluate this differential technique. After averaging, the uncertainty obtained in the determination of the amplitude of a 1.2 V sine wave at 50 Hz is 0.3 $muhbox{V/V}$ (type A). Finally, we propose a dual-waveform approach for measuring two precision sine waves with the use of a single Josephson system. Currently, the National Institute of Standards and Technology (NIST) is developing a new calibration system for electrical power measurements based on this technique.      

First Direct Comparison of a Cryocooler-Based Josephson Voltage Standard System at 10 V

A commercially available and fully automated 10-V Josephson voltage standard system with a liquid helium free cooling has been developed as a result of the cooperation between the Institute of Photonic Technology and Supracon AG, both in Jena, Germany. The system operates with an array of 19 700 superconductor–insulator–superconductor Josephson tunnel junctions installed in a pulse tube cooler. A stable operation is achieved by the proper integration of the voltage standard circuit to the cold stage of the cryocooler. Different operation setups are discussed. A direct comparison of a cryocooler-based Josephson voltage standard system versus a liquid-helium-based system was performed at a voltage level of 10 V. We obtained a voltage difference of 1.3 nV with a total combined uncertainty of 2 nV. This corresponds to a relative uncertainty of $2 times 10^{-10}$.      

Progress Toward a 1 V Pulse-Driven AC Josephson Voltage Standard

We present a new record root mean square (rms) output voltage of 275 mV, which is a 25% improvement over the maximum that is achieved with previous ac Josephson voltage standard (ACJVS) circuits. We demonstrate the operating margins for these circuits and use them to measure the harmonic distortion of a commercial digitizer. Having exceeded the threshold of 125 mV rms for a single array of Josephson junctions, we propose and discuss the features of an eight-array circuit that is capable of achieving 1 V rms. We investigate the use of a resistive divider to extend the ACJVS voltage accuracy to higher voltages. By the use of a switched-input measurement technique, an integrating sampling digital voltmeter, a resistive voltage divider, and ACJVS synthesized sine waves as reference voltages, we characterize the stability of a commercial calibration source for a few voltages up to 2.7 V.      

Comparison of a Multichip 10-V Programmable Josephson Voltage Standard System With a Superconductor–Insulator–Superconductor-Based Conventional System

We developed a 10-V dc programmable Josephson voltage standard (PJVS) using a multichip technique. The PJVS was based on $hbox{NbN/TiN}_{x}/hbox{NbN}$ junctions and operated using a 10-K compact cryocooler. We carried out an indirect comparison with a superconductor–insulator–superconductor-based conventional Josephson voltage standard (JVS) by measuring the voltage of a 10-V zener diode reference standard. The combined standard uncertainty of the comparison was $u_{c} = 0.03 muhbox{V}(k = 1)$, and the relative combined standard uncertainty was $3 times 10^{-9}$.      

A new 10-V array of Josephson junctions based on low-frequencydesigns scaled up to 94 GHz

A novel approach is described for the design of improved coupling of millimeter-waves in a 10-V array of Josephson junctions. In this approach, microwave structures were fabricated and tested in the range 2.8 GHz to 3.6 GHz, and a scaling calculation was then used to transform them to 94 GHz. By optimizing the trade-off between antenna size and efficiency, it was possible to make more space available on the device chip to increase the number of Josephson junctions. Simulation results showed that the optimal antenna is a short double-fin-line type. This antenna type allowed us to fabricate a Josephson junction array that had 25944 Nb/Al-AlO_x/Nb junctions. For this new array, zero-crossing steps were observed up to 18.5 V. The new array was then successfully used to calibrate a Zener voltage standard that had a 10-V output     

High precision comparison between SNS and SIS Josephson voltagestandards

Recently, a new Josephson voltage standard based on a 1 V programmable chip provided by the National Institute of Standards and Technology (NIST) was implemented at the Swiss Federal Office of Metrology (OFMET). A comparison with a conventional Josephson voltage standard showed an agreement of (1.4±3.4)×10^-10 at 1 V. This result demonstrates the new system is functioning properly and can be used in various types of measurements. In particular, it will be one of the key components of the Watt balance experiment (1999) at OFMET     

High-performance dc SQUIDs with submicrometer niobium Josephson junctions

We report on the fabrication and performance of low-noise, all-niobium, thin-film planar dc SQUIDs with submicrometer Josephson junctions. The junctions are evaporated obliquely through a metal shadow evaporation mask, which is made using optical lithography with 0.5 µm tolerance. The Josephson junction barrier is formed by evaporating a thin silicon film and with a subsequent oxidation in a glow discharge. The junction parameters can be reproduced within a factor of two. Typical critical currents of the SQUIDs are about 3 µA and the resistances are about 100 Ω. With SQUIDs having an inductance of 1 nH the voltage modulation is at least 60 µV. An intrinsic energy resolution of 4×10^?32 J/Hz has been reached. The SQUIDs are coupled to wire-wound input coils or with thin-film input coils. The thin-film input coil consists of a niobium spiral of 20 turns on a separate substrate. In both cases the coil is glued onto a 2-nH SQUID with a coupling efficiency of at least 0.5. Referred to the thin-film input coil, the best coupled energy resolution achieved is 1.2×10^?30 J/Hz measured in a flux-locked loop at frequencies above 10 Hz. As far as we know, this is the best figure achieved with an all-refractory-metal thin-film SQUID. The fabrication technique used is suited for making circuits with SQUID and pickup coil on the same substrate. We describe a compact, planar, first-order gradiometer integrated with a SQUID on a single substrate. The gradient noise of this device is 3×10^?12 T m^?1. The gradiometer has a size of 12 mm×17 mm, is simple to fabricate, and is suitable for biomedical applications.     

Josephson-Voltage-Standard-Locked Sine Wave Synthesizer: Margin Evaluation and Stability

This paper describes a Josephson-voltage-standard-locked synthesizer where a commercial digital-to-analog converter is used as a sine wave generator. The output amplitude of the generator is controlled by the calculable fundamental of the stepwise waveform generated by a SINIS Josephson junction array. Such a system combines the versatility of a commercial source with the stability and accuracy of the Josephson standard. By discarding the measurements performed during the transients, broad voltage steps of 1.7 mA were obtained up to frequencies of 500 Hz.      

Experimental Studies to Realize Josephson Junctions and Qubits in Cuprate and Fe-based Superconductors

Cuprate superconductors are very promising in terms of Josephson junction device because of the large energy scale of high-T _c superconductivity. In particular, fabrication of qubits attracts lots of attention because of its primary importance for future computer technology. We will present some of our recent activities pointing to this purpose. (1) We succeeded in fabricating Josephson junction of cuprate without making any extra oxide barrier layer, rather putting Fe islands on the small strip of cuprate superconductors. This is very promising, since the fabrication of good Josephson junction was unsuccessful so far. Together with the introduction of the data, we discuss the possible mechanism of the weak-link fabrication in this structure. (2) We investigated the switching events in the I–V characteristics of the intrinsic Josephson junctions of Bi-cuprate superconductor, where macroscopic quantum tunneling (MQT) observation is well established recently. In addition to confirm the MQT for the first switching at 1 K in the multiple-branched current–voltage characteristics, we found that the temperature independence of the switching distribution for the second switching up to higher temperatures (10 K) is not due to the trivial Joule heating. We discuss the mechanism of the phenomena, including the possibility of MQT. (3) New Fe-based superconductors are also promising in terms of the application of superconductivity, since the anisotropy looks rather weak, in contrast to cuprates. We will introduce our trial to fabricate epitaxial thin films as the initial step to fabricate Josephson junction of this material.     

基于任意波形发生器的高压短路试验测量系统校准方法和装置的研究

提出了一种将标准短路试验波形注入多通道任意波形发生器,来产生模拟实际的校准波形,进而对测量系统进行校准的方法。校准装置使用现场可编程门阵列(FPGA)、直接数字频率合成器(DDS)等器件。对装置的检定结果表明:在10 Hz^200 kHz频率范围内,输出频率、输出电压最大误差分别为2.1×10^-6、3×10^-3。重复输出10次,输出幅值的最大相对标准偏差为5.7×10^-4,1年内幅值变化的最大相对标准偏差为1.9×10^-4。通过将该装置用于实际高压短路试验测试系统的校准,验证了试验波形的噪声、零漂及带宽均会对测量系统的准确度产生显著影响。