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Physica C Superconductivity

Signal integrity in large-scale single-flux-quantum circuit

Hirotaka Terai, Shinichi Yorozu, Akira Fujimaki, Nobuyuki Yoshikawa, Zhen Wang

Published Jul 13, 2006• 12 citations
Publisher
DOI
10.1016/j.physc.2006.05.090

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Abstract The use of high-frequency input signals from room-temperature microwave sources makes it difficult to scale up the number of quantum bits in universal quantum computers. To address this issue, superconducting single flux quantum (SFQ) integrated circuits are being explored as suitable candidates for qubit manipulation in universal quantum computers. This paper deals with a scalable SFQ qubit control circuit (SQCC) structure that requires only low-frequency input. The circuit mainly consists of a pulse generator and a counter, that output the SFQ pulse train with adjustable frequency and a controllable number of pulses, which is applicable to control single-qubit Clifford operations. The design of low-voltage rapid single flux quantum (LV-RSFQ) and energy-efficient rapid single flux quantum (ERSFQ) for the SQCC achieves low power consumption and provides a basis for scaling up SQCC to control more qubits. The proposed circuits are fabricated under the SIMIT-Nb03 process and successfully pass test verification. The achieved test results reveal that the adjustable output frequency ranges of the SQCC based on the LV-RSFQ and ERSFQ designs in order are [2.40, 8.11] GHz and [4.81, 5.14] GHz. In the operating frequency range, the circuit is able to generate the correct number of SFQ pulses under control. The controllable number range is from 1 to 128. When the circuits operate at 5 GHz, the total power consumptions of the above circuits in order are 23.88 μW and 6.2 μW. All input signals are low-frequency signals, which frees the control of large-scale qubits from limitations caused by high-frequency inputs produced by room-temperature microwave sources.
The increasing complexity of modern superconductive circuits, and single flux quantum (SFQ) circuits in particular has made the issue of flux trapping of growing importance. The use of wide superconductive striplines for signal routing has exacerbated this issue. Trapping residual magnetic fields in these striplines degrades performance while reducing margins, damaging the operability of superconductive circuits. In this article, an area efficient topology for striplines is introduced to manage flux trapping in large scale SFQ circuits. This topology is composed of narrow parallel lines in series with small resistors. The proposed topology decreases the length of the striplines by exploiting the mutual inductance between the narrow parallel lines. The topology requires significantly less area while preventing flux trapping within wide superconductive striplines. The narrow parallel line topology also reduces coupling capacitance between striplines. The proposed approach is compatible with automated routing of large scale SFQ integrated circuits.
To improve the accuracy of astronomical data analysis, an increase in the number of output bits from the A/D converter is effective. We investigated a single-flux-quantum (SFQ) autocorrelator and an integrator that supports multiple-bit signal output from an SFQ A/D converter for astronomical data processing. The SFQ binary counter was used as the integrator that decimates the output from the SFQ autocorrelator for communication with equipment at room temperature. We designed the test circuits composed of 2-bit inputs, an autocorrelator, and an integrator that contains 5631 Josephson junctions using the 10 kA/cm2Nb process. We confirmed the correct operation of the test circuit with a normalized bias voltage margin of 102%-114% by the low-speed test. We obtained its correct high-speed operation up to a 55.1 GHz clock frequency. In terms of circuit design, scaling up the test circuit is easy. These results indicate the effectiveness of a high-precision and large-bandwidth radio astronomical observation using SFQ circuit technology.