Boyko Dimov

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.

Phase engineering techniques in superconducting quantum electronics

Due to the pulse driven nature of the Rapid Single Flux Quantum electronics nearly every basic cell requires the capability of temporary data storing. Implementing phase shifting elements in this essential device leads to several advantages concerning the device characteristics. There are different concepts enabling phase shifting elements. We give a comparative overview about these approaches. The effect of this novel element on a basic cell is analyzed exampling a toggle-flip-flop. Based on the effective noise temperature determined from the experimental results of a standard flip-flop, the bit error rate for several toggle-flip-flop realizations containing different phase shifting elements was calculated. A significantly improved area of function could be shown by simulated error rates lower than 10-12 with a DC bias margin better than ±63.5%.

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.

Tuning of the RSFQ gate speed by different Stewart-McCumber parameters of the Josephson junctions

At the typical operating temperature of 4.2 K, the theoretical upper limit of the clock frequency of the Low-Temperature Superconductive (LTS) Nb based Rapid Single Flux Quantum (RSFQ) digital circuits is several hundred GHz. Nevertheless, the few middle-scale RSFQ circuits reported on up to now operate at only some tens of GHz. An important performance-limiting factor is in this case the clock signal distribution, so nowadays asynchronous RSFQ designs are often considered. Our previous studies have shown that it is an important advantage of a given asynchronous RSFQ cell library to contain gates with tunable delays. We have already shown by simulations that the RSFQ gate delays are best adjusted by changing the Stewart-McCumber parameter /spl beta//sub c/ of the Josephson junctions-this method tunes the delay within a large interval of values while scarcely affecting the margins and the fabrication yield of the RSFQ gates. In order to prove this statement experimentally, we have designed, fabricated, and tested several ring-shaped oscillators with identical topologies but different Stewart-McCumber parameters of the Josephson junctions. Their operating speeds and margins are measured and compared.

Design issues for interconnects in densely packaged RSFQ structures

The manufacturing process of LTS RSFQ circuits is quite similar to that of the semiconductor chips, thus providing the possibility of an ultra high-density packaging similar to the modern semiconductor logic circuits. However, the miniaturization of the interconnects does not enhance their performance. The present work highlights the impact of the parasitic interactions between the superconductive interconnects on the correct logical functionality and the upper bias current margins of the LTS RSFQ circuits.

Analysis of electromagnetic coupling effects in integrated Josephson junction logic devices by the FDTD technique

A very important step of the design of circuits and devices in the Josephson junction technology is the complete and correct calculation of their electrical characteristics. Due to the very high clock speed of up to 100 GHz, dynamic effects like the electromagnetic coupling start to play a significant role over the operation of the system. The presented work reports on the implementation of the FDTD technique for the description of the electromagnetic coupling effects in the Josephson devices. Some typical microstrip layouts are considered strictly taking in account the technological specifications. The obtained results are analyzed in respect to the constraints, which the coupling effects impose on the lateral dimensions of the microstrip lines.

Superconductive passive phase shifter for integrated RSFQ digital circuits

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.

Universal asynchronous RSFQ gate for realization of Boolean functions of dual-rail binary variables

We report on the design, optimization, fabrication, and successful testing of an universal asynchronous RSFQ logic gate based on the dual-rail data coding. Properly connecting its inputs and outputs, one can perform most of the basic Boolean functions over a pair of dual-rail input variables. Therefore, this gate is fundamental component for the development of high-speed complex asynchronous RSFQ digital devices.

Optimal signal propagation speed of a Josephson transmission line

We study the dependence between the JTL inductance and its SFQ pulse propagation speed. An optimal value, being far away from the generally established design rules, is found. The dependence of this optimal value on the JTL parasitics is also considered. The bias margins behaviour around the new optimum is presented.

New optimized elements for the rapid single-flux quantum shift register family

We present a scheme of a new buffered RSFQ shift register, having reversible input and output. This scheme is extended to a new dual-rail reversible buffered RSFQ shift register, suitable for asynchronous applications. Finally, a new proposal for a buffered RSFQ shift register performing both shift-left and shift-right operations is given.