F.H. Uhlmann

Flip-flopping fractional flux quanta.

The d-wave pairing symmetry in high–critical temperature superconductors makes it possible to realize superconducting rings with built-in π phase shifts. Such rings have a twofold degenerate ground state that is characterized by the spontaneous generation of fractional magnetic flux quanta with either up or down polarity. We have incorporated π phase–biased superconducting rings in a logic circuit, a flip-flop, in which the fractional flux polarity is controllably toggled by applying single flux quantum pulses at the input channel. The integration of p rings into conventional rapid single flux quantum logic as natural two-state devices should alleviate the need for bias current lines, improve device symmetry, and enhance the operation margins.

RSFQ Circuitry Using Intrinsic

The latching of temporary data is essential in the rapid single flux quantum (RSFQ) electronics family. Its pulse-driven nature requires two or more stable states in almost all cells. Storage loops must be designed to have exactly two stable states for binary data representation. In conventional RSFQ such loops are constructed to have two stable states, e.g. by using asymmetric bias currents. This bistability naturally occurs when phase-shifting elements are included in the circuitry, such as π-Josephson junctions or a π-phase shift associated with an unconventional (d-wave) order parameter symmetry. Both approaches can be treated completely analogously, giving the same results. We have demonstrated for the first time the correct operation of a logic circuit, a toggle-flip-flop, using rings with an intrinsic π-phase shift (π-rings) based on hybrid high-Tc to low-Tc Josephson junctions. Because of their natural bistability these π-rings improve the device symmetry, enhance operation margins and alleviate the need for bias current lines.

\pi

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.

-Phase Shifts

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.

Technology Related Timing Jitter in Superconducting Electronics

A technique is discussed in order to find an estimation of the basin of attraction for each stable equilibrium point in cellular neural networks. The technique is based on the determination of the direction of the trajectories for each of the regions in which the state space is divided via the piecewise linear output function of each cell.

Passive Phase Shifter for Superconducting Josephson Circuits

Highly sensitive measurement of magnetic fields can be made by analogue SQUIDs with analogue-to-digital conversion in a semiconductor environment. An alternative way is the use of digital SQUIDs consisting of superconducting digital Josephson electronics. Previously we presented a full digital SQUID device based on the bidirectional single flux quantum (SFQ) technique, which featured a very high dynamic range due to intrinsic flux compensation, a high slew rate and low complexity of the superconducting digital electronics. Analogue external feedback circuitry is replaced by the internal digital feedback. The intrinsic digital processing of measured flux makes external analogue-to-digital conversion dispensable. The remaining semiconducting electronics is reduced to a high-speed up–down counter. The whole system will be clocked by a bidirectional bias signal. The achieved tristate operation ensures a serial data output stream in the range of gigabits per second. The proper function of such a single-stage digital SQUID was recently proved by experiments. This concept of flux counting suffers intrinsically from its resolution of one flux quantum, which is not sufficient for the measurement of small magnetic fields. In this paper we present investigations on novel circuit topologies utilizing digital SQUID devices to overcome the restrictions in flux resolution without losing the dynamics advantages of a full digital SQUID device.

Estimation of the basin of attractions in CNNs

A vital precondition for the realization of rapid single-flux quantum (RSFQ) digital circuits with reduced critical currents of the Josephson junctions is the implementation of an efficient technique for superconductive phase dropping. In this paper, we present a novel phase shifting element consisting of a miniature superconductive ring located over a ground plane hole. Contrary to the solutions reported up to now, this topology can be simply integrated within complex digital RSFQ circuits realized with conventional fabrication technology.

New approach for a highly sensitive magnetometer utilizing a multi-stage digital SQUID

Magnetometers utilizing digital SQUIDs are good candidates for high sensitive measurements of rapidly varying magnetic fields in a wide range. The association of its very large dynamic range together with the very high field resolution of an analog SQUID leads to new generation of SQUID sensors. This paper describes a hybrid magnetometer system, which is designed for applications in unshielded environment. It uses the flux quanta counting digital SQUID to operate in large environmental fields. We present a detailed study for the dynamic properties of the sensor system with focus on the digital SQUID part. We use a numerical simulation technique to investigate the interference between analog and digital sub-systems and characterize the hybrid magnetometer system performance.

Superconductive passive phase shifter for integrated RSFQ digital circuits

Recently, we have proposed a novel technique for the design of Josephson transmission lines (JTLs). According to our theoretical estimations, this technique is able to increase the propagation speed of the JTLs and to reduce drastically their dc bias consumption. Here, a successful experimental verification of these estimations is presented.

Analysis of a Digital SQUID Magnetometer Utilizing a Direct Coupled DC SQUID for Improved Magnetic Field Resolution

Superconductive rapid single-flux-quantum (RSFQ) circuits are able to work in a digital mode with clock frequencies of tens to hundreds of GHz. They rely on shunted Josephson junctions assembled with thin films inductors. To date, there is no on-the-shelf instrumentation apparatus which allows to directly verify the digital operation of such circuits in the 100 GHz range and above. Nevertheless, it is of importance of being able to sample the output of RSFQ circuits, and RSFQ pulses themselves, in a time-resolved manner at ultrafast speed, in order to get direct information about the RSFQ circuit behavior and for some specific applications. At this stage, we want to prove that RSFQ circuits can be correctly triggered through optical means, i.e. with a femtosecond Ti-Sa pulsed laser synchronized with a readout setup. Superconducting bridges of different dimensions, based on niobium films, have been designed to act as photoswitches. They have been included in a microwave custom-made circuit connected to a simple RSFQ processing circuit based on the 1 kA/cm2 JeSEF RSFQ process of IPHT Jena [1]. The design of different geometries is presented, along with the expected electrical features of the bridges and RSFQ circuits. Preliminary experimental results are also given.