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IEEE Transactions on Applied Superconductivity

Design of an SFQ Microwave Chopper for Controlling Quantum Bits

G. Matsuda, Yuki Yamanashi, Nobuyuki Yoshikawa

Published Jun 1, 2007• 14 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2007.898657

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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.
Superconducting qubits are generally controlled by irradiation with microwave pulses. We propose a 5-GHz-range microwave pulse generator based on pulse density modulation for controlling qubits using single-flux-quantum (SFQ) circuits. The proposed microwave pulse generator comprises a 100-GHz circular shift register and a band-pass filter (BPF). In this study, we investigated the relationship between the target envelope of the microwave amplitude and the SFQ pulse density based on the transfer function for the BPF and the frequency spectrum of an SFQ pulse. We implemented a 64-bit circular shift register and demonstrated its operation up to 68 GHz.
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.
We have been studying a superconducting quantum-computing system where superconducting qubits are controlled and read out by rapid single-flux- quantum (RSFQ) circuits. In this study, we designed and fabricated an RSFQ microwave chopper, which turns on and off an externally applied microwave to control qubit states with the time resolution of sub-nanosecond. The chopper is implemented in a microwave module and mounted in a dilution refrigerator. We tested the microwave chopper at 4.2K. The amplitude of the output microwave was approximately 100μV which is much larger than that of previously designed chopper. We also confirmed that the irradiation time can be controlled by RSFQ control circuits.
This paper discusses a class of filters utilized in a low-power superconductor rapid single-flux-quantum (rapid SFQ, RSFQ) microwave pulse generator for controlling quantum bits (qubits). Here, the filter plays an important role to convert SFQ pulse trains (generated by RSFQ digital gates) and output a microwave control pulse. Characteristic of the output pulse can be adjusted by the in-band and roll-off performances of the filter, which is analyzed through numerical examples. Moreover, a novel in-line topology lumped network is proposed for the filter implementation, which can satisfy complex terminal cases at source (connecting an RSFQ digital block) and load (connecting a qubit). For the validation, we carry out a fourth-order bandpass filter using multilayer niobium (Nb) then-film technology, then incorporate it into an RSFQ control-pulse generator, and finally confirm correct functionality by both simulation and measurement.