A high-Tc 4-bit periodic threshold analog-to-digital converter

Using ramp-type Josephson junctions a 4-bit periodic threshold ADC has been designed, fabricated and tested. Practical design constraints will be discussed in terms of noise immunity, flux flow, available technology, switching speed etc. In a period of four years we fabricated about 100 chips in order to bring the technology to an acceptable level and to test various designs and circuit layouts. This resulted in a basic comparator that is rather insensitive to the stray field generated by the analog input signal or variations in mask alignment during fabrication. The input signal is fed into the comparators using a resistive divider network. Full functionality at low frequencies has been demonstrate

Bridging the gap between nanowires and Josephson junctions: a superconducting device based on controlled fluxon transfer across nanowires

The basis for superconducting electronics can broadly be divided between two technologies: the Josephson junction and the superconducting nanowire. While the Josephson junction (JJ) remains the dominant technology due to its high speed and low power dissipation, recently proposed nanowire devices offer improvements such as gain, high fanout, and compatibility with CMOS circuits. Despite these benefits, nanowire-based electronics have largely been limited to binary operations, with devices switching between the superconducting state and a high-impedance resistive state dominated by uncontrolled hotspot dynamics. Unlike the JJ, they cannot increment an output through successive switching, and their operation speeds are limited by their slow thermal reset times. Thus, there is a need for an intermediate device with the interfacing capabilities of a nanowire but a faster, moderated response allowing for modulation of the output. Here, we present a nanowire device based on controlled fluxon transport. We show that the device is capable of responding proportionally to the strength of its input, unlike other nanowire technologies. The device can be operated to produce a multilevel output with distinguishable states, which can be tuned by circuit parameters. Agreement between experimental results and electrothermal circuit simulations demonstrates that the device is classical and may be readily engineered for applications including use as a multilevel memory

SQUID BASED CRYOGENIC CURRENT COMPARATOR FOR MEASURING LOW-INTENSITY ANTIPROTON BEAMS

In the low-energy Antiproton Decelerator (AD) and the Extra Low ENergy Antiproton (ELENA) rings at CERN, an absolute measurement of the beam intensity is essential to commission and troubleshoot the different accelerator systems, to measure the operational efficiency, and to provide calibration information for the different experiments using the antiproton (p) beam. Both the AD and ELENA are synchrotron decelerators, operating with both bunched and debunched - Direct Current (DC) - beams. The beam currents can be smaller than 100 nA, and the total number of circulating particles is of the order of 10^7. Non-intercepting measurements of low-intensity charged particle beams are particularly challenging due to the low amplitude of the induced electromagnetic fields. This is even more difficult for DC beams. The most common diagnostics that are able to measure DC beams are the DC Current Transformers (DCCTs), but these present considerable limitations when used to measure low-intensity beams, since these are limited in current resolution to 1 µA. In the AD a longitudinal-Schottky (Schottky) monitor is currently used for intensity measurements but this presents several limitations, including accuracy errors above 10 %. Several laboratories have shown in the past the potential of Superconducting QUantum Interference Device (SQUID)-based Cryogenic Current Comparator (CCC), using Low-Temperature Superconductors (LTS) technology, to measure beam current intensities of slowly varying beams in the nano-ampere range. However, previous CCC beam monitors suffered from a strong susceptibility to mechanical vibrations, and ElectroMagnetic Interference (EMI) perturbations, and also presented limited availability which limited their operational use. Additionally, these were never able to cope with short bunched beams in a synchrotron accelerator. In the present work a CCC system was developed for the AD machine. This monitor was optimised in terms of its current resolution, ability to cope with short bunched beams and overall system stability. Also, a dedicated cryostat was designed and fabricated to house the CCC, and to be installed in the AD beam line, aiming at decoupling external mechanical vibrations from the monitor, and to have a reduced heat in-leak, allowing for a standalone operation with an external liquid helium reliquefier. The new monitor was characterized in laboratory and different measurements are presented. Measurements with real beam were also performed in the AD, and the resolution and accuracy of beam current and beam intensity measurements were assessed. Optimal beam current resolutions of 2.5 nA, and beam intensity resolutions of 1.2 × 10^4 charges (at the highest beam energy) were obtained. However, the limiting factor in the obtained absolute measurement accuracy was the observed drift of the zero beam baseline, which could amount to 25 nA. These are the first CCC beam current and intensity measurements ever performed in a synchrotron machine with both coasting and short bunched beams. Future improvements could be obtained by studying the origin and effect of the external perturbations causing the observed drift, leading to the implementation of mitigation and compensation techniques

Digital Readout and Control of a Superconducting Qubit

In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily

Gate-Tunable Critical Current of the Three-Dimensional Niobium Nano-Bridge Josephson Junction

Recent studies have shown that the critical currents of several metallic superconducting nanowires and Dayem bridges can be locally tuned using a gate voltage {V_g}. Here, we report a gate-tunable Josephson junction structure constructed from a three-dimensional (3D) niobium nano-bridge junction (NBJ) with a voltage gate on top. Measurements up to 6 K showed that the critical current of this structure can be tuned to zero by increasing {V_g}. The critical gate voltage Vgc was reduced to 16 V and may possibly be reduced further by reducing the thickness of the insulation layer between the gate and the NBJ. Furthermore, the flux modulation generated by Josephson interference of two parallel 3D NBJs can also be tuned using {V_g} in a similar manner. Therefore, we believe that this gate-tunable Josephson junction structure is promising for superconducting circuit fabrication at high integration levels.Comment: 15 pages, 5 figure

Investigating the feasibility of using nanobridge weak links as the active Josephson element in Rapid Single Flux quantum circuitry

Josephson junctions are used in present day voltage standards. To extend their use to AC voltage standards a high bandwidth, low-noise detector is required. A candidate component for this detector is a superconducting comparator based on Rapid Single Flux Quantum (RSFQ) circuits. The work presented here is a study to determine if nanobridge weak links can be used as the active Josephson element in these circuits. In order to achieve this an understanding of the nanobridge properties and in particular their critical currents is fundamental. We present simulations of a simple comparator using the circuit simulation software JSIM in order to study the effect of varying nanobridge parameters such as width, length, and loop area. These geometrical variables have an affect on the critical currents and loop inductances which subsequently effect device performance. Particular emphasis is given to investigation of how these parameters affect a key figure of merit, the grey zone width

Implementation of the Quantum Hall Effect based precision resistance measurement system

The integer Quantum Hall Effect (QHE) occurs when a two-dimensional electron gas (2DEG) is subjected to a strong perpendicular magnetic field and when the system is cooled to low temperatures. The QHE harbours a wealth of unique phenomena. Of interest is the existence of the Quantum Hall Resistance (QHR) which had found to be related to two fundamental constants of nature via the von Klitzing constant h e 2 , where e is the charge of the electron and h Planck’s constant. This thesis investigates the properties of the QHE in a low dimensional electron gas system. The von Klitzing constant is determined as well as the electron density n2D and mobility µ of the material measured. The results are compared to the accepted value of the von Klitzing constant as determined by the metrological community. The average von Klitzing constant obtained is 25 783.637 Ω within an accuracy of 1.13 × 10−12. Our results are further interpreted using the Landau quantum mechanical model of electron transport in perpendicular magnetic field. The measurement of standard resistances utilising a standard DC resistance measurement system were also undertaken at the National Metrology Institute of South Africa (NMISA). This ties in with the ongoing project of NMISA to develop an in-house quantum Hall measurement system to provide the full traceability for resistance standard measurements in the Republic of South Africa. The device measured utilised a GaAs/AlGaAs heterostructure structure, grown via Molecular Beam Epitaxy (MBE). A micron sized Hall bar with Ohmic contacts was patterned using standard clean room procedures. Magnetotransport measurements at low temperatures, sub 200 mK were carried out on the device. The transverse and longitudinal resistances were obtained and plotted against the perpendicular magnetic field. Quantum Hall plateaus and Shubnikov de-Haas (SdH) oscillations were observed. Properties of the heterostructure such as the electron density (n2D) and mobility (µ) were determined. The n2D obtained was 2.27 × 1011 cm−2 with µ at 3.5 × 105 cm2V−1 s −1 . All results were compared to current literature values

Real-time imaging systems for superconducting nanowire single-photon detector arrays

Superconducting nanowire singe-photon detectors (SNSPD) are promising detectors in the field of applications, where single-photon resolution is required like in quantum optics, spectroscopy or astronomy. These cryogenic detectors gain from a broad spectrum in the optical and infrared range and deliver low dark count rates and low jitter times. This thesis improves the understanding of the detection mechanism of SNSPDs and intodruces new and promising multi-pixel readout concepts