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Superconductor Science and Technology

Controlled initialization of superconducting π-phaseshifters and possible applications

Olaf Mielke, Thomas Ortlepp, J. Kunert, Hans‐Georg Meyer, Hannes Toepfer

Published Apr 1, 2010• 6 citations
Publisher

Abstract

The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digital circuit family which is currently close to being commercially applied. RSFQ is outstanding because of its very low switching energy resulting in very low power consumption. This advantage causes, however, a significant influence of thermal noise. For industrial applications, a certain noise immunity is required which is still a challenge, especially for circuits of higher complexity. Integrating phase-shifting elements is a new concept for further improvements concerning stability against the influence of thermal noise. We have already shown that the implementation of phase-shifting elements significantly reduces the influence of thermal noise on circuit behavior by experimentally analyzing the bit-error rate (Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which are easily implementable in standard niobium technology are especially promising. The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. The loop current related to the stored flux creates a well-defined phase shift. To achieve the correct functionality of complex circuits it is essential to store exactly one flux quantum in each π-phaseshifter during the cooling down of the chip. Thus, for studying the feasibility of this new approach, the initialization reliability of the π-phaseshifter needs to be verified. We present an experimental investigation of this reliability to obtain a general assessment for the application of the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shielded by a solid ground plane with a configuration with a ground-plane hole below the π-phaseshifter. Justified by the experimental results we suggest programmable RSFQ circuits based on π-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization of the π-phaseshifter. The fabrication was performed by FLUXONICS Foundry.

DOI
10.1088/0953-2048/23/5/055003

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The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digital circuit family which is currently close to being commercially applied. RSFQ is outstanding because of its very low switching energy resulting in very low power consumption. This advantage causes, however, a significant influence of thermal noise. For industrial applications, a certain noise immunity is required which is still a challenge, especially for circuits of higher complexity. Integrating phase-shifting elements is a new concept for further improvements concerning stability against the influence of thermal noise. We have already shown that the implementation of phase-shifting elements significantly reduces the influence of thermal noise on circuit behavior by experimentally analyzing the bit-error rate (Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which are easily implementable in standard niobium technology are especially promising. The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. The loop current related to the stored flux creates a well-defined phase shift. To achieve the correct functionality of complex circuits it is essential to store exactly one flux quantum in each π-phaseshifter during the cooling down of the chip. Thus, for studying the feasibility of this new approach, the initialization reliability of the π-phaseshifter needs to be verified. We present an experimental investigation of this reliability to obtain a general assessment for the application of the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shielded by a solid ground plane with a configuration with a ground-plane hole below the π-phaseshifter. Justified by the experimental results we suggest programmable RSFQ circuits based on π-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization of the π-phaseshifter. The fabrication was performed by FLUXONICS Foundry.
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.
The Rapid Single Flux Quantum electronics is a superconducting, naturally digital circuit family which is currently close to getting commercially applied.RSFQ is outstanding by its very low switching energy resulting in a very low power consumption which is the reason for a significant influence on thermal noise.For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity.In superconducting electronics resistors are the only on-chip noise sources.We analyzed the influence of each individual resistor in a toggle Flip-Flop circuit to the overall bit-error rate.The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.We analyzed in detail the types of thermal introduced switching errors of an toggle Flip-Flop which was optimized with respect to the fabrication yield.Simulation studies were accomplished to investigate the influence of circuit parameters on quality and on quantity of switching errors.By identifying and manipulating the parameters with the most significant influence on the noise susceptibility an enhanced noise immunity of the device can be achieved.The goal is to improve the robustness of RSFQ circuits against the influence of thermal noise for an specific topology by changing the circuit parameters of the device.
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.
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.
Rapid single flux quantum (RSFQ) electronics is based on the Josephson junction as an active switching element. In standard RSFQ circuits its switching energy is much lower than the static power consumption caused by the resistive current distribution network. Due to this thermal heating of the chip, the maximum number of junctions on a single chip is limited to about 1 million. The frequency-dependent contribution to power dissipation from junction switchings is only about 2 percent of the static one. This fact limits the direct construction of VLSI systems for high-performance computing as well as small-scale circuit applications in the vicinity of ultra-sensitive detectors or even quantum circuits. We present an assessment of different approaches for reducing the static power consumption by investigating the potential of inductive bias distribution networks as well as reduced critical currents. We analyse the operation stability of simple digital circuits with 5 times smaller critical currents at 4.2 K. The combination of the reduced critical currents and inductive biasing can provide digital superconductive circuits with significantly reduced static power consumption.
Within the classical Rapid Single-Flux Quantum (RSFQ) digital designs, the superconductive phase dropping necessary for the optimal SFQ data processing is realized with short sections of transmission lines serving as lumped inductances. This approach meets several hard restrictions about some promising future RSFQ applications requiring a drastic reduction of the junctions' critical currents. In this paper, we analyse these restrictions, and present an alternative and experimentally verified technique for superconductive phase dropping, which is compatible with any RSFQ fabrication technology. This technique provides compact and robust RSFQ digital circuits with highly improved fabrication yield.