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Applied Physics Letters

Compact RSFQ microwave pulse generator based on an integrated RF module for controlling superconducting qubits

Yuxing He, Hongxiang Shen, Shiori Michibayashi, Xihua Zou, Xiaojun Xie, Lianshan Yan, Wei Pan, Nobuyuki Yoshikawa

Published Feb 7, 2022• 14 citations
Publisher

Abstract

An on-chip superconducting microwave pulse generator (MPG), incorporating rapid single-flux-quantum (RSFQ) logic gates and passive radio frequency (RF) components, is an energy-efficient solution for controlling superconducting quantum bits (qubits). Traditional RSFQ MPG designs, however, suffer from a large footprint as multiple RF components (including impedance matching networks, filters, etc.) are separately implemented one after another. In this Letter, we propose a compact on-chip MPG by introducing an integrated RF module. Using a generalized filter synthesis technique, the integrated RF module combines the functionalities of impedance matching and bandpass filtering as a single inductor-capacitor (LC) lumped network. Plenty of circuit elements can, thus, be saved, and the resultant LC network is applicable to both real and complex terminal impedance cases. For validation, a superconducting MPG adopting a fourth-order, 5 GHz centered, 100 MHz bandwidth integrated RF module is designed and fabricated, whose correct operations are confirmed through both simulation and measurement. These results raise the availability of controlling large-scale quantum systems by low-power superconductor technology in the future.

DOI
10.1063/5.0083972

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An on-chip superconducting microwave pulse generator (MPG), incorporating rapid single-flux-quantum (RSFQ) logic gates and passive radio frequency (RF) components, is an energy-efficient solution for controlling superconducting quantum bits (qubits). Traditional RSFQ MPG designs, however, suffer from a large footprint as multiple RF components (including impedance matching networks, filters, etc.) are separately implemented one after another. In this Letter, we propose a compact on-chip MPG by introducing an integrated RF module. Using a generalized filter synthesis technique, the integrated RF module combines the functionalities of impedance matching and bandpass filtering as a single inductor-capacitor (LC) lumped network. Plenty of circuit elements can, thus, be saved, and the resultant LC network is applicable to both real and complex terminal impedance cases. For validation, a superconducting MPG adopting a fourth-order, 5 GHz centered, 100 MHz bandwidth integrated RF module is designed and fabricated, whose correct operations are confirmed through both simulation and measurement. These results raise the availability of controlling large-scale quantum systems by low-power superconductor technology in the future.
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.
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.
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.
Readout and control of qubits are limiting factors in scaling quantum computers. An ideal solution is to integrate energy-efficient cryogenic circuits close to the qubits to perform control and pre-processing tasks. With orders of magnitude lower power consumption and hence lower noise, Single Flux Quantum (SFQ) and Adiabatic Quantum Flux Parametron (AQFP) superconductor logic families can reach ultimate performance at cryogenic temperatures. We have created a superconductor-based on-chip function generator to control qubits. The generated signal is modulated up to tens of GHz based on the external input waveform applied to the superconductor mixer stage. This circuit works at 4.2K. A radiofrequency (RF) matching circuit transmits the signal to the ~mK stage after digital amplification and noise reduction.
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.