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Superconductor Science and Technology

A single flux quantum circuit with a ferromagnet-based Josephson π-junction

M. Khabipov, D Balashov, F. Maibaum, Alexander B Zorin, V. A. Oboznov, V. V. Bol’ginov, Anna N. Rossolenko, V. V. Ryazanov

Published Mar 19, 2010• 66 citations
Publisher

Abstract

We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor–ferromagnet–superconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ± 20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).

DOI
10.1088/0953-2048/23/4/045032

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We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor–ferromagnet–superconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ± 20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).
A methodology for designing single-flux-quantum (SFQ) flip-flops composed of both conventional (0-) and π -shifted Josephson junctions is investigated. We investigated the implementation of a storage loop, which can store flux quantum and is indispensable to express binary logic states in superconductor logic circuits. As all SFQ flip-flops have storage loops, the investigated design methodology can be applied to their design. We designed several SFQ flip-flops composed of 0- and π -shifted Josephson junctions using the investigated design methodology. The performances of the designed SFQ flip-flops were quantitatively evaluated by using an analog circuit simulator which we developed. We confirmed the correct operation of various SFQ flip-flops composed of 0- and π -shifted Josephson junctions with wide operating margins. Moreover, we observed that the investigated design methodology is suitable for SFQ flip-flops with complementary outputs because a storage loop composed of both 0- and π -shifted Josephson junctions has a symmetric structure and the complementary output function can be realized by using the storage loop. Our investigation indicates that the number of Josephson junctions and static power consumption of a non-destructive read-out flip-flop with complementary outputs (NDROC) can be reduced to less than half of those of the conventional NDROC, which has two storage loops composed of 0-Josephson junctions, to realize the complementary output function. The investigated design methodology is expected to be applied to not only SFQ circuits but also other superconducting logic circuits and novel reconfigurable logic devices using programmable 0- π Josephson junctions.
Abstract Half-flux-quantum (HFQ) circuits are based on 0–π superconducting quantum interference devices (SQUIDs) and is one of the energy-efficient superconductor digital circuits. The bit energy is determined by the critical current I cn of 0–π SQUID, which can be easily tuned with the loop inductance and junction critical current. In this work, an alternative π–π–π SQUID is adopted to demonstrate HFQ circuits to simplify the fabrication process and enhance circuit energy efficiency. The properties of superconductor/ferromagnet/insulator/superconductor Josephson junctions (π-JJs) are measured with temperature dependence from 4.2 K down to 10 mK. HFQ toggle flip-flops (TFFs) are successfully demonstrated at frequencies of up to 6.7 GHz and 44.5 GHz at temperatures of 4.2 K and 10 mK, respectively. Comparing the HFQ TFF with its rapid single-flux quantum counterpart under the same fabrication process, it is anticipated that the HFQ TFF will exhibit approximately 70% reduction in both static and dynamic energy dissipation. This research establishes the foundation for developing cryogenic interface control and readout circuits for large-scale quantum computing in the future.
Abstract Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.
Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.
Abstract We investigated local magnetic flux biasing (LFB) that induces a phase shift in superconductor circuits by locally applying a magnetic field through the superconductor loop with Josephson junctions. The arbitrary phase shift can be achieved using LFB without modifying the circuit fabrication process. To quantitatively evaluate the effects of introducing LFB for practical superconductor circuit applications, we designed a single flux quantum (SFQ) based non-destructive read-out flip-flop with complementary outputs (NDROC) and a delay flip-flop with complementary outputs (DFFC). The circuit area and static power consumption of the NDROC based on LFB architecture (LFB-NDROC) are approximately 67% and 36% of a conventional NDROC, respectively. The measured bias margin of the LFB-NDROC was in the range of 69%–129%. Using LFB, we were able to reduce the circuit area and power consumption for the DFFC by 67% and 83%, respectively. The measured bias margin of the DFFC with LFB was between 115% and 128%. LFB enabled us to implement a 5-to-32 SFQ decoder which comprises NDROC trees with a reduced circuit area of approximately 60% of a conventional decoder. The results obtained in this study can be applied to not just SFQ circuits but other superconductor circuits also, as they improve the area and power efficiency of such circuits.