Superconducting circuits are one of the leading technologies for the realization of quantum computers. However, achieving practical, fault-tolerant quantum computers requires solutions to many technological challenges. Metrology for qubits and their peripherical devices could accelerate this development and help gaining more confidence in scientific achievements. The Quantum Electronics Department of PTB is – together with national and international partners – actively pursuing this metrological support and this poster will highlight some of the corresponding activities at PTB: (i) Concepts of voltage metrology, such as variants of the Josephson arbitrary waveform synthesizer (JAWS), are adopted at PTB for the control of superconducting qubits in order to realize quantum accurate synthesizers with a minimum of amplitude and phase noise. First waveforms at GHz frequencies, that are generated from a JAWS circuit, have been realized. Such devices could help improving qubit operation. (ii) Parametric amplifiers, that do not contain dissipative elements, enable amplification at the quantum limit of added noise. These devices are ideally suited to amplify read out signals of superconducting qubits. PTB is working on travelling-wave parametric amplifiers (TWPAs) and resonant amplifiers. The latest devices of PTB show 20 dB gain over a bandwidth of 4 GHz and, thus, are extremely promising for qubit readout. (iii) Using the properties of a two-level system, superconducting qubits can be employed as quantum-accurate and extremely sensitive power sensors at cryogenic temperatures. So far PTB has fabricated Transmon qubits and employed qubit spectroscopy and the AC Stark shift for the demonstration of power sensing. The ultimate goal will be to have dedicated circuit elements, that can be placed everywhere in a superconducting circuit, enabling quantum-accurate power sensing at this position. (iv) Opto-electronic techniques based on femtosecond lasers allow for quasi non-invasive measurements of high-frequency signals with an unprecedented bandwidth. Based upon these techniques, PTB has constructed a cryogenic sampling oscilloscope with a theoretical bandwidth exceeding 1 THz. This oscilloscope has been used for the characterization of ultrafast photodiodes at 4 K and will be further enhanced for in-situ waveform measurements for the optimization of superconducting circuits. The poster will summarize these topics, showing the latest results and future prospects.

We combine a cryogenic BiCMOS integrated circuit, which generates high-speed return-to-zero (RTZ) pulses, with a superconducting Josephson junction array. The BiCMOS circuit acts as a cryogenic pulse pattern generator, delivering data rates of 30 Gb/s, while consuming 302 mW at 4 K. Each electrical pulse of the serializer effectively transfers one magnetic flux quantum through every Josephson junction, so that the average output voltage of the array produces well-defined plateaus (Shapiro steps) in its current-to-voltage characteristic. To the best of our knowledge, this is the first integration of a Josephson junction array with a cryogenic BiCMOS chip. The presented results pave the way toward a hybrid and fully integrated Josephson arbitrary waveform synthesizer (JAWS) that can generate ultra-low-noise signals for quantum voltage metrology and quantum information systems.

With increasing applications of quantum systems operating at high frequencies, the role of integrated microwave components for quantum circuits becomes more and more important. In this article, we present low-temperature superconducting bandpass-bandstop diplexers for a new generation of quantum AC-voltage standards operating at 1 GHz. Based on stringent requirements, the diplexers are designed to provide low passband loss, good stopband rejection, and otherwise a broad ideally matched spectrum with excellent spurious-free performance. A second-order 1 GHz superconducting bandpass-bandstop diplexer is fabricated and characterized for verification. To this end, a cryogenic setup and on-chip calibration standards are implemented for S-parameter measurements. The experimental results exhibit low insertion loss as low as 0.03 dB in the passband section and high rejection in the bandstop branch exceeding 43 dB. Furthermore, the diplexer features wide spurious-free response extending to more than $14 \cdot \mathrm{f}_{0}$ and occupies a very compact area of only $1.8 \text{mm} \times 1.2 \text{mm}$.

We demonstrate a cryogenic electro-optic sampling (EOS) setup that allows for the measurement of microwave signals at arbitrary positions on a cryogenic chip-scale device. We use a Josephson Arbitrary Waveform Synthesizer (JAWS) to generate quantum-accurate voltage signals and measure them with the EOS setup, allowing for the calibration of its response, yielding traceability of the microwave measurements to a quantum standard. We use the EOS setup to determine the time-domain response of ultrafast cryogenic photodiodes and the electrical reflection coefficient, i.e., the S11 scattering parameter, in a superconducting transmission line. Finally, we introduce an optical femtosecond pulse source which can be used to study the fidelity of superconducting transmission lines and terminations, as well as reflections from elements like Josephson junction arrays imbedded in them.

We have developed a cryogenic characterization platform for ultrafast photodiodes, whose time domain responses are extracted by electro-optic sampling using femtosecond laser pulses in a pump-probe configuration. The excitation of the photodiodes with the pump beam and the electro-optic sampling crystals with the probe beam are realized in a fully fiber-coupled manner. This allows us to use the characterization platform at different temperatures, ranging from cryogenic to room temperature. As an application example, we characterize the time-domain response of commercial p-i-n photodiodes with a nominal bandwidth of 20 GHz and 60 GHz at temperatures of 4 K and 300 K and in a large parameter range of photocurrent and reverse bias. For these photodiodes, we detect frequency components up to approximately 250 GHz, while the theoretical bandwidth of our sampling method exceeds 1 THz. Our measurements demonstrate a significant excitation power and temperature dependence of the photodiodes’ ultrafast time responses, reflecting, most likely, changes in carrier mobilities and electric field screening. Since our system is an ideal tool to characterize and optimize the response of fast photodiodes at cryogenic temperatures, it has a direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators.

We discuss the flip-chip mounting process of photodiodes and fiber sleeves on silicon substrates to meet the increasing demand for fabrication of highly integrated and hybrid quantum circuits for operation at cryogenic temperatures. To further increase the yield and success rate of the flip-chip procedure, the size of the gold stud bumps, and flip-chip parameters were optimized. Moreover, to connect optical fibers to the photodiodes in an optimal position, the fiber sleeves were aligned with specially fabricated alignment circles before applying thermocompression with the flip-chip machine. The mounted photodiodes were tested at both room temperature and cryogenic temperature, and we find that mechanical imperfections of the sleeve-ferrule combination limit the overall alignment accuracy. The experimental results show that our flip-chip process is very reliable and promising for various optical and electrical applications and, thus, paves the way for fabrication of hybrid chips, multi-chip modules and chip-on-chip solutions, which are operated at cryogenic temperatures.

We report on electro-optic sampling (EOS) of microwave signals under cryogenic conditions employing a fully fiber-coupled measurement scheme. We use this setup to characterize the time response of ultrafast photodiodes at a temperature of 4 K. We additionally integrate a Josephson Arbitrary Waveform Synthesizer (JAWS) to the EOS platform, which allows us to calibrate the EOS response, yielding traceability of the microwave measurements to a quantum standard. Finally, we determine the electrical reflection coefficient, i.e., the S11 scattering parameter, in a Niobium transmission line using EOS at different positions. Our work denotes an important step for traceable in-situ microwave measurements in superconducting circuits.

We discuss the flip-chip mounting process of photodiodes and fiber sleeves on silicon substrates to meet the increasing demand for fabrication of highly integrated and hybrid quantum circuits for operation at cryogenic temperatures. To further increase the yield and success rate of the flip-chip procedure, the size of the gold stud bumps, and flip-chip parameters were optimized. Moreover, to connect optical fibers to the photodiodes in an optimal position, the fiber sleeves were aligned with specially fabricated alignment circles before applying thermocompression with the flip-chip machine. The mounted photodiodes were tested at both room temperature and cryogenic temperature, and we find that mechanical imperfections of the sleeve-ferrule combination limit the overall alignment accuracy. The experimental results show that our flip-chip process is very reliable and promising for various optical and electrical applications and, thus, paves the way for fabrication of hybrid chips, multi-chip modules and chip-on-chip solutions, which are operated at cryogenic temperatures.

This paper describes recent developments for increasing the output voltage of the Josephson Arbitrary Waveform Synthesizer (JAWS) at PTB. For this purpose, we developed modified broadband Wilkinson power dividers, which enable the operation of two or four Josephson junction series arrays per RF channel of the pulse pattern generator and thus the generation of increased output voltages. In detail, we designed and investigated two different Wilkinson power dividers: the one-stage three-section Wilkinson power divider with extended bandwidth and the two-stage single-section Wilkinson divider with more outputs to combine four parallel series arrays. They were both integrated with triple-stacked Josephson junction series arrays. Additionally, a new modulator pulse amplifier was introduced into the JAWS set-up to provide sufficient power for all the junction arrays. Our measurement results show that spectrally pure sinusoidal waveforms were successfully generated with both types of power dividers. With the one-stage three-section Wilkinson power divider, we obtained RMS voltages of nearly 53 mV at a clock-frequency of 15 GHz combined with a test array of 3000 Josephson junctions. As for the two-stage single-section Wilkinson power divider combined with a test array of 6000 Josephson junctions, we synthesized RMS output voltages of about 105 mV. These test results show a promising prospect for quantum voltage standard applications.

We developed a fabrication process to establish large arrays of up to 5-stacked Josephson junctions for the Josephson Arbitrary Waveform Synthesizer (JAWS). SNS-type Josephson junctions with NbxSi1-x barriers are used for this application. By modifying our standard window process, e.g. to add a CMP (chemical mechanical polishing) and an ALD (atomic layer deposition) step, the yield of this process was increased. An output voltage of 1 V RMS could be achieved by using 4 JAWS arrays in series with a total number of 60,000 5-stacked junctions. The investigation of the current-voltage characteristics (IVC) showed an interesting and potentially useful feature. The required pulse amplitude per junction for getting the maximum Shapiro step width reduces for higher junction stacks. We found that the pulse power linearly reduces with the square of the junction number in the array. The origin of this effect might be a strong neighbour-neighbour interaction of the junctions, which are closely arranged within the stacked configuration. This might generate a strong self-synchronization of the junctions.

To make the experimental setup of the Josephson arbitrary waveform synthesizer less complex by reducing the number of RF cables between room and cryogenic temperature, we developed on-chip RF power dividers. By integration of these components, we can eventually increase the number of Josephson junctions operated by one single pulse-pattern generator channel, and thus reduce the costs of the setup. At Physikalisch-Technische Bundesanstalt, we designed, fabricated, and investigated the performance of two different RF power dividers types: the serial & x2013;parallel and the Wilkinson power divider. Spectrally pure sinusoidal waveforms were successfully synthesized with both types of power dividers. With the Wilkinson power divider, we obtained 17.55 mV (rms) at a clock frequency of 15 GHz combined with a test array of 1000 Josephson junctions. As for the serial & x2013;parallel power divider combined with a test array of 2000 Josephson junctions, we synthesized rms output voltages of 19.0 mV.

The pulse-driven Josephson Voltage Standard, also called Josephson Arbitrary Waveform Synthesizer (JAWS) is already well established for different applications in AC voltage metrology. To further increase the output voltage towards 10 V and to reduce the complexity of the JAWS systems we investigated two different approaches, which finally can be combined. One approach is to integrate an optimized on-chip power splitter to reduce the number of high-frequency (HF) channels from room temperature down to 4 K. A pulse pattern generator with less HF outputs will directly reduce the complexity and costs of a JAWS system. The second approach is to use an optical pulse-drive implementing cold photodiodes close to the JAWS chip. The use of optical fiber will have two main advantages: the optical fibers will reduce the high frequency noise and will enable an easy splitting into parallel optical channels. We will present first results with both approaches.

Series arrays of overdamped Josephson junctions compose the basis of AC Josephson voltage standards. Presently, an advanced technology is based on Josephson junctions consisting of an NbxSi1-x barrier. Using junction stacks containing five Josephson junctions, series arrays of 15,000 junctions have been successfully fabricated at PTB for the pulse-driven Josephson Arbitrary Waveform Synthesizer (JAWS). The operation of eight arrays simultaneously enable spectrally pure waveforms with output voltages above 1 V to be synthesized.

Exciting applications of superconductivity are based on the macroscopic quantum state which exists in a superconductor. In this chapter we investigate the behaviour of junctions consisting of two weakly coupled superconductors. These junctions are nowadays called Josephson junctions1 (Josephson, 1962). The macroscopic quantum state results in an exceptional behaviour of these Josephson junctions. They are the basis for various applications in superconductive electronics (cf. Anders et al, 2010), e.g. in the field of metrology for highprecision measurements. The most significant representative of a metrological application is the Josephson voltage standard. This quantum standard enables the reference of the unit of voltage, the volt, just to physical constants. It is nowadays used in many laboratories worldwide for high-precision voltage measurements. The main component of each modern Josephson voltage standard is the highly integrated series array consisting of tens of thousands of Josephson junctions fabricated in thin-film technology. While Josephson junctions are conceptually simple, nearly 50 years of developments were needed to progress from single junctions delivering a few millivolt at most to highly integrated series arrays containing more than 10,000 or even 100,000 junctions. These large series arrays enable the generation of dc and ac voltages at the 10 V level, which is relevant for most applications. Conventional Josephson voltage standards based on underdamped Josephson junctions are used for dc applications. The increasing interest in highly precise ac voltages has stimulated different attempts to develop measurement tools on the basis of Josephson arrays for ac applications, namely programmable Josephson voltage standards containing binary-divided arrays and pulse-driven Josephson voltage standards both based on overdamped Josephson junctions. This chapter describes the development of these modern dc and ac Josephson voltage standards as well as their fundamentals and applications. The development and use of Josephson voltage standards have also been described recently in several review papers (amongst others: Niemeyer, 1998; Hamilton, 2000; Yoshida, 2000; Behr et al., 2002; Kohlmann et al., 2003; Benz & Hamilton, 2004; Jeanneret & Benz, 2009).

Josephson voltage standards are nowadays well established as an important application of the Josephson effects for voltage measurements at ultimate precision. This chapter describes the development of Josephson voltage standards from conventional DC to modern AC standards focused on refractory Josephson junctions based on the Nb/Al-Al2O3 technology and its enhancements for Josephson voltage standards. The development of conventional DC Josephson voltage standards for output voltages up to 10 V was performed over a period of about 25 years. AC Josephson voltage standards are therefore based on overdamped Josephson junctions showing a nonhysteretic current-voltage characteristic, which remains singlevalued under microwave irradiation. The demands for overdamped Josephson junctions used in AC voltage standards partly differ from that for underdamped junctions used in conventional standards. Pulse-driven and binary-divided series arrays containing up to 300,000 junctions have been demonstrated for output voltages up to 1 V and 10 V or even 20 V, respectively.

Quantum-accurate arbitrary voltage waveforms have been generated using electrical pulse driven Josephson junction arrays (JJAs) by several groups [1, 2]. An alternative method is to use optoelectronic components to convert the electrical pulse drive into an optical signal which is subsequently converted by a photodiode into an electrical drive signal connected directly to the JJA. This optoelectronic pulse drive system has several advantages. Due to the electrical isolation between electrical pulse drive and JJA, it is possible to connect several arrays in series without the requirement for complex electrical compensation circuits. This allows larger output voltages to be realized. An optoelectronic drive also offers the ability to easily tune pulse height, via continuous optical attenuation, allowing the drive signal to be tuned to the middle of the JJA constant voltage step giving optimum operation margins. We here present an optoelectronic pulse drive which forms part of a system incorporating a JJA and delta sigma feedback loop [3] for quantum-accurate digitization of arbitrary voltage waveforms. Results showing the generation of quantum-accurate voltages synchronized to the feedback loop master clock are presented.

Pulse-driven Josephson junctions allow the synthesis of very precise both spectrally pure and arbitrary wave forms with frequencies up to the megahertz range. We investigated the properties relevant for metrological applications of series arrays with 4000 Josephson junctions fabricated at PTB in cryocooler and liquid helium. DC electrical parameters were evaluated and Shapiro steps dependence on operating conditions was studied. Both cooling techniques provided similar results for all relevant parameters. In particular, we were able to observe Shapiro step widths of more than 1 mA in cryocooler. Yet, we found that some specific effects related to the different thermal conditions must be taken into account for proper operation in cryocooler. (C) 2016 Elsevier Ltd. All rights reserved.

AbstractFamous applications of Josephson junctions have successfully been established in electrical metrology for high‐precision measurements. The main important representative of a metrological application is the Josephson voltage standard initially used for dc measurements. Meanwhile, the developments are focused on Josephson voltage standards for ac applications. These quantum voltage standards for output voltages up to 10 V contain series arrays of 100 000 Josephson junctions or even more. This chapter describes the development and present status of modern Josephson voltage standards.

A practical dc voltage standard with output voltages Ł/hts from 0.1 V to 10 V using high temperature superconductor (HTS) arrays of Josephson junctions cooled to 77 K was characterized. HTS arrays with increased current margins and the voltage divider with a high stability of the divider's coefficients enable stable and reproducible automated calibration of the new HTS standard. The direct comparison of U HTS against the voltage generated on an array of niobium Josephson junctions cooled to 4.2 K reveals the agreement of voltages at 1 V level with a Type A uncertainty equal to a few parts in 10 8 .