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IEEE Transactions on Applied Superconductivity

Design of Single Flux Quantum Qubit Control Circuit With Adjustable Patterns

Bicong Weng, Yuxuan Feng, Liliang Ying, Xiaoping Gao, Minghui Niu, Jie Ren

Published Jan 16, 2024• 5 citations
Publisher

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. Previously, we designed a low-power SFQ qubit control circuit (SQCC) to produce an equidistant SFQ pulse train without high-frequency input. It was reported that the fidelity of qubit control could be further improved if the pattern of SFQ pulse train could be adjustable. In this paper, we present optimized designs for different pattern requirements of qubit control, building upon the SQCC without high-frequency input. These designs feature segmented frequency modulation circuits, as well as circuits capable of outputting arbitrary patterns. The circuit capable of generating arbitrary patterns currently achieves a maximum adjustable number of 400, with a corresponding frequency output of 20 GHz. It has been successfully fabricated using the SIMIT Nb03 process and has passed functionality measurement tests. The optimized circuit opens up possibilities for improved qubit control and advancements in quantum computing applications.

DOI
10.1109/tasc.2024.3354676

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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. Previously, we designed a low-power SFQ qubit control circuit (SQCC) to produce an equidistant SFQ pulse train without high-frequency input. It was reported that the fidelity of qubit control could be further improved if the pattern of SFQ pulse train could be adjustable. In this paper, we present optimized designs for different pattern requirements of qubit control, building upon the SQCC without high-frequency input. These designs feature segmented frequency modulation circuits, as well as circuits capable of outputting arbitrary patterns. The circuit capable of generating arbitrary patterns currently achieves a maximum adjustable number of 400, with a corresponding frequency output of 20 GHz. It has been successfully fabricated using the SIMIT Nb03 process and has passed functionality measurement tests. The optimized circuit opens up possibilities for improved qubit control and advancements in quantum computing applications.
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.
High-frequency input signals from room-temperature microwave sources present a major bottleneck for scaling qubit numbers in universal quantum computers. To address this challenge, we previously developed a superconducting single-flux quantum (SFQ) qubit control circuit (SQCC) capable of generating precise SFQ pulse sequences using only DC bias. This approach eliminates traditional microwave signal chains and enables scalable, low-loss DC-based interconnects. However, as the number of SQCCs increases, the total required bias current grows linearly, leading to significant cable-induced thermal dissipation that limits scalability. In this work, we propose and implement an architecture using recycled bias current to scale up multiple SQCCs (RB-MSQCC). Theoretical analysis indicates that the recycling method can reduce thermal load by a factor of 100 to 1000 compared to conventional biasing. A key enabler of this architecture is a custom-designed driver-receiver pair (DRP) cell, which supports SFQ signal transmission across isolated ground planes using the SIMIT Nb03 process. Based on this, we demonstrate a 4-bit RB-MSQCC prototype. Experimental results confirm stable SFQ pulse generation across all four channels, with output frequencies ranging from 3.42 to 5.27 GHz, consistent with prior results (2.40-8.11 GHz). These findings validate the feasibility and robustness of the proposed scheme under current fabrication constraints. Our study offers both theoretical and experimental support for scalable, energy-efficient SFQ-based qubit control, representing a critical step toward large-scale superconducting quantum processors.
A microwave chopper using single-flux-quantum (SFQ) circuits is proposed for the control of quantum bits (qubits). The proposed microwave chopper is composed of a DC/SFQ converter, an SFQ switch and a band-pass filter (BPF). In operation, an externally applied microwave is input to a DC/SFQ converter to generate an SFQ pulse train, which is chopped at high speed by the SFQ switch. The SFQ pulse train is then filtered by the BPF to remove higher harmonics. The transient response, the amplitude and the spectrum of the microwave output from the chopper were examined by circuit simulation, taking into account the thermal noise. The chopper was implemented using the SRL 2.5 kA/cm2Nb standard process and its characteristics were investigated. It was shown that the microwave output could be successfully chopped by the SFQ switch. The amplitude and line width of the microwave output were measured as 15 muV and 2 Hz at 7 GHz.
In order to control the state of qubits by a microwave pulse, the irradiation time and the amplitude have to be controlled precisely. We have developed a single-flux-quantum (SFQ) microwave chopper for high-speed switching of microwave pulses. The proposed chopper is composed of a DC/SFQ convertor, an SFQ switch, a PTL driver, and a superconducting low-pass filter (LPF). The chopper converts an input microwave, which is generated by an external microwave generator at the room temperature, into microwave pulses by using start/stop SFQ control signals. We designed and implemented a microwave chopper module, which can be attached to dilution refrigerators. SFQ chips were fabricated using the ISTEC 2.5 kA/cm2 Nb process. We tested the microwave chopper module at 4.2 K, and demonstrated that a 5-GHz microwave whose amplitude ranging from 0 μV to 150 μV can be chopped by the SFQ control signals.
Superconductor single-flux-quantum (SFQ) integrated circuits can provide classical, general-purpose digital processing at cryogenic temperature near quantum bits. We report high-frequency operation of low-power SFQ circuits using a lowered critical current density fabrication process and low-voltage design. We are developing a standard-cell library with integrated devices featuring four planarized Nb layers and a Pd resistor layer. SFQ shift registers successfully operated at up to 11 GHz at 0.3 K. The measured operating frequency is sufficient for SFQ-based digital controlling of quantum bits using impulse-shaped voltage pulses.
Superconducting single flux quantum (SFQ) circuit can operate at very low temperature. This is suitable for controlling a quantum computing system with Josephson junctions. However, it is difficult to integrate both SFQ circuits and qubits into a single-chip, because of the dissipative characteristics of SFQ circuits. Therefore, we have developed a multi-chip packaging technology for a qubit control module. The module consists of SFQ circuit chips, qubit chips, and a substrate all of which are fabricated with Nb and Al technology. The chips are flip-chip bonded with superconducting solder bumps. We also investigated SFQ control circuits for superconducting qubits and circuit parameter optimization for sub-Kelvin temperature operation. Using both multi-chip packaging and optimized SFQ control circuit makes the design of qubit control module more flexible.