T. Ortlepp

Digital SQUID sensor based on SFQ technique

For high sensitive measurements of small magnetic fields a Digital SQUID is superior to conventional analog SQUIDs in terms of dynamic properties, but recent realizations as single-flux-quantum (SFQ) circuit suffers from high complexity. A new kind of Digital SQUID as a full digital sensor device with the advantage of a small number of Josephson junctions and a large slew rate was developed. The circuit consists of basic SFQ cells and an internal digital feedback loop. The operation with a bidirectional clock signal ensures a decreased effort on superconducting electronics. The SFQ/dc converter and an additional voltage driver provides a processable digital output signal for hybrid systems including semiconductor electronics. The sensor circuit was simulated, optimized and fabricated in niobium technology. From investigation of dynamic properties of the circuit we expect a flux slew rate in the gigahertz range.

Noise induced timing jitter: a general restriction for high speed RSFQ devices

All complex devices in rapid single flux quantum (RSFQ) technique work at much lower clock rates than possible for simple cells. New data driven self timed or asynchronous concepts can reduce this gap, but even in this case still a discrepancy between simulation and experiment is shown. It has been pointed out in previous studies, that the influence of thermal fluctuations in RSFQ circuits is divided into static and dynamic switching bit errors as well as timing jitter induced failures. The bit error rate depends exponentially on the temperature and is not the main issue in the low temperature technique. Our last results show for the first time, that the variance of the switching time is much slower decreased by reducing the temperature and is still important at 4.2 K. We describe a general method for calculating the switching time distribution for RSFQ cells. Furthermore, we present detailed results for a timing analysis of a dc/SFQ-converter. The delay between output signal and the input current ramp shows a variation over 1 ps which is more than 10% of the switching time itself. This new understanding of the background of timing jitter enables a goal-oriented improvement in the design process of RSFQ circuits.

Technology Related Timing Jitter in Superconducting Electronics

Digital superconducting systems based on Josephson junctions make generally use of the synchronous timing strategy. Today, one of its most promising applications is the high speed and high dynamic range signal sensing. A digital signal acquisition system should have a high resolution as well as a short sampling interval with a well known and constant time period. Short term clock fluctuations (clock jitter) induced by thermal noise can significantly disturb the system operation due to hazards of timing constraint violations. This uncertainty in the time period is currently a strong limitation for further improvements of fast signal sensing systems based on superconductive electronics. Recently, several theoretical and experimental studies describe the timing jitter in simple circuits, like Josephson transmission lines. In the presented work, we analyse different fabrication technologies for rapid single flux quantum (RSFQ) electronics and predict their timing jitter by the numerical solution of stochastic differential equations. We obtained a very good agreement between our simulation data and recent experimental results.

Passive Phase Shifter for Superconducting Josephson Circuits

Quantized values of magnetic flux trapped in a superconducting loop enable a new type of passive phase shifting elements. These elements can be incorporated into digital Josephson circuits making their design compact. We have proven the functionality of such phase shifters fabricated in conventional Nb/Al trilayer technology. We report on the successful low speed operation of a rapid single flux quantum toggle flip-flop circuit with the integrated passive pi-phase shifting element.

Comparison of RSFQ Logic Cells With and Without Phase Shifting Elements by Means of BER Measurements

Rapid single flux quantum (RSFQ) electronics is characterized by a very low switching energy. This advantage leads to a noise susceptibility, which becomes a challenge for large-scale circuits as well as for circuits using Josephson junctions with reduced critical current density. We demonstrate an improved operation range and advanced noise immunity of basic cells resulting from an implemented phase shifting element. One of those elements is the π-phase shifter. It consists of a single flux quantum trapped in a superconducting loop. The π-phase shifter can be easily produced in standard niobium technology without any process modifications. Utilizing a mature process brings advantages concerning the reliable fabrication of complex circuits.

Experimental analysis of a digital SQUID device at 4.2 K

The application of superconducting rapid single-flux quantum (RSFQ) digital electronics for highly sensitive measurement of magnetic fields can provide significant advantages in the use of conventional analogue SQUIDs, especially in terms of operation speed and dynamic range. Furthermore, utilizing an unconventional generalized single-flux-quantum (SFQ) logic with a bidirectional operation principle allows an additional decrease in effort in superconducting electronics. Our novel fully digital SQUID based on the SFQ technique can be assumed to be operating at frequencies in the gigahertz range corresponding to slew rates of several 109 Φ0 s−1. We present first experimental results for the proper digital function as a preliminary stage for a digital SQUID magnetometer device. The measurements presented are performed for a reliable low temperature superconductor technology at liquid helium temperature; nevertheless the very low complexity of the superconducting digital circuitry holds promise as regards prospects for a working digital SQUID based on high temperature superconductor technology.

High-Speed Experimental Demonstration of Adiabatic Quantum-Flux-Parametron Gates Using Quantum-Flux-Latches

We experimentally demonstrated high-speed logic operations of adiabatic quantum-flux-parametron (AQFP) gates through the use of quantum-flux-latches (QFLs). In QFL-based high-speed test circuits (QHTCs), the output data of the circuits under test (CUTs), which are driven by high-speed excitation currents, are stored in QFLs and are slowly read out using low-speed excitation currents. We designed and fabricated three types of QHTCs using QFLs with different circuit parameters, where the CUTs were buffer gates and and gates. We confirmed the correct operation of buffer gates and and gates at 1 GHz. The obtained bias margins of the 1 GHz excitation currents were more than ±30% for each QHTC, which is wide enough for high-speed logic operations of AQFP gates.

HTS basic RSFQ cells for an optimal bit-error rate

Thermal noise strongly influences the operation of RSFQ (rapid single flux quantum) logic circuits made of high-temperature superconductors (HTS). In the past, the circuit design was based on fabrication yield optimization. A new theoretical study using a method of general determination of the digital bit-error rate (BER) gives hope to develop such devices with a large immunity against noise. With regard to this study the design parameters of a circuit optimized with respect to fabrication yield are far from its minimum bit-error rate. Only for temperatures close to 4 K the parameters determined for fabrication yield match the parameters obtained with BER optimization. For this reason, a new reliable technology for fabrication of HTS circuits is required. We have developed a new fabrication process to serve as a basis for a proof of this new design approach. We have calculated the bit-error rate of a newly designed RSFQ chip with realistic values derived from a test chip which has been fabricated with this new multilayer technology. The new technology contains three superconductor thin films. An inductance smaller than 0.5 pH per square has been reached by using a ground plane.

Design and Demonstration of Interface Circuits between Rapid Single-Flux-Quantum and Adiabatic Quantum-Flux-Parametron Circuits

We investigated interface circuits between rapid single-flux-quantum (RSFQ) circuits and adiabatic quantum-flux-parametron (AQFP) circuits for a high-speed and low-power hybrid computing system. In addition, the interface circuits allow us to use long-distance interconnections based on single-flux-quantum (SFQ) passive transmission lines between AQFP gates. The target frequency of interface circuits is several gigahertz. There are two types of interface circuits. The RSFQ/AQFP interface circuit, which transmits a signal from RSFQ circuits to AQFP gates, was realized by coupling an RSFQ toggle flip-flop (TFF) to an AQFP buffer gate. The simulated bias margins of the RSFQ TFF and the excitation current margin of the RSFQ/AQFP interface were 2.22 mA ± 24.7% and 1.71 mA ± 25.0%, respectively. The AQFP/RSFQ interface circuit, which transmits a signal from AQFP gates to RSFQ circuits, was designed by using a high-sensitive magnetically coupled dc/SFQ converter (MC-dc/SFQ) to detect an output current of an AQFP buffer gate. The simulated bias margins of the MC-dc/SFQ and the excitation current margin of the AQFP/RSFQ interface were 0.558 mA ± 20.5% and 1.95 mA ± 36.1%, respectively. We also fabricated and measured the interface circuits.

Implementation of superconductive passive phase shifters in high-speed integrated RSFQ digital circuits

The reduction of the critical current density in rapid single-flux quantum (RSFQ) circuits enables new application fields, like quantum computing and photonic detector readout. The low current density fabrication process creates new design challenges, such as lower stability against thermal fluctuations, violation of the lumped elements condition for microstrip inductances and increased sensitivity to the technological spread. To overcome these issues, we suggest a passive phase shifter as a promising alternative technique for superconductive phase dropping in the RSFQ electronics. Here, we study experimentally their applicability in high-speed RSFQ digital circuits. Conclusions are drawn about the impact of the passive phase shifters on the complexity, the speed and the bit error rate of the investigated RSFQ circuits. We demonstrate the successful operation of different circuits with implemented passive phase shifters at low and high speeds.