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Superconductor Science and Technology

Demonstration of an efficient single flux quantum logic circuit by introducing a local magnetic flux biasing

Shunta Asada, Yuki Yamanashi, Nobuyuki Yoshikawa

Published Mar 25, 2021• 2 citations
Publisher

Abstract

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.

DOI
10.1088/1361-6668/abf23a

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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.
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).
Novel reconfigurable superconductive single flux quantum (SFQ) logic devices, the functions of which can be dynamically reconfigured by inputting control signals, have been investigated. The characteristics of single flux quantum circuits can be easily modulated by applying currents or magnetic fields because of the high sensitivity of superconductive circuits. We investigated several design approaches for the realization of dynamically reconfigurable SFQ logic devices. We employed direct current injection to the logic gate using a non-destructive read-out flip-flop. By changing the internal state of the non-destructive read-out flip-flop, the logic function can be dynamically reconfigured. We have designed and tested a dynamically reconfigurable Josephson transmission line (JTL)/delay flip-flop (DFF) circuit. The measured dc bias margin was 72.5-107.2%. Furthermore, we have demonstrated the operation of a variable bit-length shift register, composed of the dynamically reconfigurable JTL/DFF circuits. The investigated design approach can be applied to more sophisticated dynamically reconfigurable single flux quantum logic gates.
We designed several single flux quantum (SFQ) flip-flops and logic gates composed of Josephson junctions (JJs) and π-shifted JJs (π-JJs) to quantitatively evaluate effectiveness of introduction of π-JJs into the SFQ logic circuit. One-output flip-flops and logic gates were designed on the basis of the circuit design methodology we built for the SFQ circuit containing π-JJs. The designed flip-flops and logic gates have wide operating margins, the dc-bias margins of larger than ±30%, and device parameter margins of ±18%, though the static power consumption are reduced compared to conventional ones composed of JJs. We found that the difference in the critical current density between JJs and π-JJs does not affect the operating margins of the SFQ flip-flop composed of JJs and π-JJs. We devised a circuit structure of the delay flip-flop with complementary outputs composed of JJs and π-JJs (π-DFFC). The analog circuit simulation shows the dc-bias margin of the π-DFFC is larger than ±33%. These results indicate that the large-scale SFQ logic circuit system can be implemented using the flip-flops and logic gates containing π-JJs.
A promising application of a single-flux quantum (SFQ) circuit is read-out circuitry for a multi-channel superconductive sensor array. In such applications, the SFQ read-out circuit is expected to operate outside a magnetic shield. We investigated an SFQ circuit structure, which is tolerant to an external magnetic field, using the AIST 2.5kA/cm2 Nb standard 2 process, which has four Nb wiring layers including the ground plane. By covering the entire circuit using an upper Nb wiring layer called the control (CTL) layer, the influences of the external magnetic field on the SFQ circuit operation can be avoided. We experimentally evaluated the sheet inductance of the wiring layer underneath the CTL shielding layer to design a magnetic-field-tolerant SFQ circuit. We implemented and measured test circuits comprising toggle flip-flops (TFFs) to evaluate their magnetic field tolerances. The operating margin and maximum operating frequency of the designed TFF did not deteriorate with increases in the magnetic field applied to the test circuit, whereas the operating margin of the conventional TFF was reduced by applying the magnetic field. We have also demonstrated the high-speed operation of the designed TFF operated in an unshielded environment at a frequency of up to 120GHz with a wide operating margin.
Single flux quantum (SFQ) technology has garnered significant attention due to its low switching power and high operational speed. Researchers have been actively pursuing more advanced devices and technologies to further reduce the reliance on inductors, bias, and dynamic power. Recently, innovative magnetic Josephson junction devices have emerged, enhancing the field of superconductor electronics logic. This article introduces a novel cell library design that relies entirely on Josephson junctions, showing promising potential for eliminating the need for inductors in conventional SFQ cells. This results in a 55% reduction in cell size and an 80% decrease in both static and dynamic power consumption. The proposed library implements a half flux quantum logic, where each pulse duration is half that of a SFQ pulse. This article presents the schematics of the basic cells, emphasizing critical circuit parameters and their margins. In addition, it examines layout blueprints, showcasing the advantageous area-saving characteristics of the proposed design.
We report on low-power operation of the half-flux-quantum (HFQ) shift-register circuit. We used a superconducting quantum interference device composed of two conventional switching junctions (0-junctions) and one π-shifted non-switching magnetic Josephson junction (π-junction) as a switching element, called 0-0-π SQUID. Because the π-shift assists switching by inducing a spontaneous current, the 0-0-π SQUID shows a nominal small critical current value and is easily switched by a weak driving force. This feature allows us to significantly reduce static and dynamic power consumption at junctions and bias-feeding resistors. We carefully designed the HFQ circuit elements using a circuit parameter optimization tool and introducing additional non-switching 0-0-π SQUIDs to compensate for superconductor phase shifts. We fabricated the test circuit of the 4-bit shift-register by forming Nb/PdNi/Nb π-junctions on the Nb four-layer, 10-kA/cm2 device. We successfully obtained correct operation with measured power consumption of 0.12-0.18 μW/bit, which was about 1/10 of the conventional single-flux-quantum shift-register designed with the 0-junctions of the same critical currents and the standard bias voltage.