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Superconductor Science and Technology

Low power single flux quantum qubit control circuit without high-frequency input

Bicong Weng, Wei Peng, Jie Ren

Published Jul 11, 2023• 4 citations
Publisher

Abstract

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.

DOI
10.1088/1361-6668/ace660

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•Jan 1, 2007•1 citation
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 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.
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.
Low-Power Single-Flux-Quantum Standard Cell Library Using 250 A/cm2^{2}2 Process for Qubit Control Applications
In this paper, we report the development of a low-power single-flux-quantum (SFQ) standard cell library and experimental results, including the cell design, high-speed test of shift-registers, delay measurement using ring oscillators, and demonstration of simple demultiplexers. We fabricated the SFQ circuits using a lowered critical current density (250 A/cm2^{2}2) process and low-voltage design (0.5 or 0.1 mV). We reduced the power consumption to 1/50 or 1/250 from the conventional SFQ circuits operating at 4.2 K. We designed several essential SFQ cells, including wiring elements, flip-flops, and logic gates with the fabrication process using four planarized Nb layers and obtained the correct operation at 0.3 K. We tested 8-bit shift-registers aiming to evaluate the operating frequency of the developed SFQ cells. The 0.1-mV shift-registers operated up to 8.5 GHz. The experimental results of ring oscillators showed good agreement with the analog circuit simulation results. We believe that our experimentally-verified standard cell library is ready to use for SFQ-based digital control of qubits, such as demultiplexing of control signals, and the timing information and power consumption will be valuable information as the baseline to discuss future scalable quantum computer systems.
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.
Abstract To achieve large-scale quantum processors, cryogenic quantum-bit (qubit) interface circuits that can control qubits inside a dilution refrigerator are indispensable. Superconducting single-flux quantum (SFQ) circuits are a promising building block for qubit interface circuits because SFQ circuits can operate with high clock frequencies and low power dissipation. In the present study, we developed an SFQ-based qubit interface circuit that we refer to as a pulse-pair microwave pulse generator (PP-MPG). The PP-MPG can generate microwave pulses for qubit control, with the microwave amplitude controlled based on the delay between paired SFQ pulses. By finely adjusting the delay with delay controlling circuits, the microwave amplitude can be controlled over a wide range. We fabricated a PP-MPG chip and tested it at 4.2 K, showing that the microwave amplitude can be controlled over a range of approximately 30 dB. Our results show that the PP-MPG has the potential for use in future large-scale quantum processors.
We present a Rapid Single Flux Quantum (RSFQ) microwave controller for superconducting qubits. Applying microwaves from room temperature sources is the conventional approach to excite quantum state transitions. Finding a method to apply short microwave bursts with a narrow linewidth is a challenge for on-chip Josephson based oscillators. We present a scheme containing an RSFQ pulse generator, ultra narrowband filter, SQUID based resonator and coupler. The operating frequency of the designed controller is 3-6 GHz. The unique feature of this design is a SQUID based resonator. By varying magnetic field in the SQUID, the\nresonance length can be varied between λ/4 and λ/2. The\nresonator is added as a shunt between the filter and coupler. Transmission of microwave signals from the filter to the output is determined by the shunt resonance. RSFQ T flip-flop circuit is designed to control the magnetic field applied to the SQUID in the resonator. Miniaturization of the coupler of 2-10 GHz has been done using a quasi-lumped element designed. We present the simulations results for the frequency response of\ndifferent resonance lengths of the modulator.