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.
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.
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.
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.
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.
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.
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.