Abstract Controlling a large number of superconducting qubits is a key challenge for realizing a fault-tolerant quantum computer. Implementing a qubit control system operating at the proximal millikelvin stage of the dilution refrigerator is a promising approach to reduce the number of coaxial cables required for qubit control. Single-flux-quantum (SFQ) logic circuits are one candidate for realizing such cryogenic control systems; however, their power dissipation must be minimized to avoid any negative impact on qubit performance. In this study, we demonstrate the operation of a flux-bias-free (FBF) flux qubit controlled by an SFQ driver on a monolithic chip. Here, the critical current ( I C ) of the Josephson junctions in the SFQ driver was set to a few µA, which is two orders of magnitude smaller than that in a typical SFQ circuit operating at 4 K, significantly reducing power consumption. Using this low-power SFQ driver, we successfully demonstrate single-qubit operation without any reduction of the energy relaxation time of the FBF qubit.
Abstract Controlling a large number of superconducting qubits is a key challenge for realizing a fault-tolerant quantum computer. Implementing a qubit control system operating at the proximal millikelvin stage of the dilution refrigerator is a promising approach to reduce the number of coaxial cables required for qubit control. Single-flux-quantum (SFQ) logic circuits are one candidate for realizing such cryogenic control systems; however, their power dissipation must be minimized to avoid any negative impact on qubit performance. In this study, we demonstrate the operation of a flux-bias-free (FBF) flux qubit controlled by an SFQ driver on a monolithic chip. Here, the critical current ( I C ) of the Josephson junctions in the SFQ driver was set to a few µA, which is two orders of magnitude smaller than that in a typical SFQ circuit operating at 4 K, significantly reducing power consumption. Using this low-power SFQ driver, we successfully demonstrate single-qubit operation without any reduction of the energy relaxation time of the FBF qubit.
As quantum computing processors increase in size, there is growing interest in developing cryogenic electronics to overcome significant challenges to system scaling. Single flux-quantum (SFQ) circuits offer a promising alternative to remote, bulky, and power-hungry room temperature electronics. To meet the need for digital qubit control, readout, and co-processing, SFQ circuits must be adapted to operate at millikelvin temperatures near quantum processors. SEEQC's SFQuClass digital quantum management approach proximally places energy-efficient SFQ (ERSFQ) circuits and qubits in a multi-chip module. This enables extremely low power dissipation, compatible with a typical dilution cryostat's limited cooling power, while maintaining high processing speed and low error rates. We report on systematic testing from 4 K to 10 mK of a comprehensive set of ERSFQ cells, as well as more complex circuits such as programmable counters and demultiplexers used in digital qubit control. We compare the operating margins and error rates of these circuits and find that, at millikelvin, bias margins decrease and the center of the margins (i.e., the optimal bias current value) increases by ~15%, compared to 4.2 K. The margins can be restored by thermal annealing by reducing Josephson junction (JJ) critical current Ic. To provide guidance for how circuit parameters vary from 4.2 K to millikelvin, relevant analog process control monitors (PCMs) were tested in the temperature range of interest. The measured JJ critical current (of the PCM JJ arrays) increases by ~15% when decreasing temperature from 4.2 K to millikelvin, in good agreement with both theory and the empirically measured change in the center of bias margins for the tested digital circuits.
The digital circuits in a superconducting quantum computer must be cooled to cryogenic temperatures in a dilution refrigerator for control and information readout of the qubit information. Because the dilution refrigerator's heat capacity is limited, extremely low-power digital circuits are required. Single-flux-quantum (SFQ) circuits and Josephson photomultipliers (JPMs) are expected to serve as control and readout circuits for superconducting quantum computers. These circuits generate minimal heat, but further reductions in power consumption are required to make it possible to build larger, more scalable faulttolerant quantum computers (FTQC). In this paper, we propose a low-power and reliable peripheral circuit for JPM-based qubit readout utilizing an SFQ counter circuit. The counter performs majority voting across repeated readouts, significantly improving the reliability of the JPM readout process. Additionally, we quantitatively evaluate switching errors in SFQ circuits using Josephson junctions with low-critical current values. By integrating these circuit techniques and simulation results, we find that the proposed approach achieves a 90% reduction in power consumption while effectively suppressing the increase in readout error.
Current superconducting quantum computers (QC) rely on a brute-force scaling approach where quantum chips operating at the millikelvin stage inside dilution refrigerators are connected to room-temperature electronics via an ever-increasing number of signal wires. In order to drastically scale up such systems without facing refrigeration challenges, these quantum interface electronics need to be integrated at cryogenic temperatures, ideally at the same stage as qubits. In this paper, we show that, by employing energy-efficient digital superconducting microchips based on single flux quantum (SFQ) technology, all essential functions to operate a full stack quantum computer can be integrated at 10 mK. They include digital qubit charge control and magnetic flux control for single- and two-qubit gates, as well as digital qubit readout.
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.
Quantum processors have the potential to accelerate specific computing tasks but are difficult to scale up due to engineering limitations, such as the number of available cables and cooling power for dilution refrigerators for quantum bits (qubits). Hence, it is very important to develop scalable, energy-efficient interface circuits that can control many qubits via a few control lines inside a dilution refrigerator. One of the most important interface circuits is the flux controller (FC), which generates arbitrary dc flux bias to adjust the characteristics of component devices such as qubits. In this paper, we propose and demonstrate FCs using an energy-efficient superconductor logic family, adiabatic quantum-flux-parametron (AQFP) logic. We develop two types of FCs: the AQFP FC and the AQFP/single-flux-quantum (AQFP/SFQ) FC. Both FCs require only a few control lines and have extremely small power dissipation, thus exhibiting high scalability. Furthermore, the AQFP/SFQ FC can control flux bias using ballistic SFQ transmission, which is crucial for integration with qubits. As a proof of concept, we demonstrate AQFP and AQFP/SFQ FCs at 4.2 K, fabricated by the AIST high-speed standard process. Our results indicate that AQFP logic is highly suitable for use as qubit interface circuits for very large-scale quantum processors, especially from the viewpoint of the control line count, power dissipation, and amount of supply currents.
Quantum computation requires high-fidelity qubit readout, preserving the quantum state. In the case of superconducting (SC) qubits, readout is typically performed using a complex analog experimental setup operated at room temperature, which poses significant technological and economic barriers to large system scalability. An alternative approach is to perform a cryogenic on-chip qubit readout based on a Josephson Digital Phase Detector (JDPD): a flux switchable device capable of digitizing the phase sign of a coherent input. The readout operation includes the flux excitation of the JDPD to evolve from a single to a double-minima potential. In this work, the effect of the flux bias characteristics on the JDPD performances is studied numerically. To meet the identified requirements that maximize detection fidelity and tackle the engineering challenges, a cryogenic on-chip Single Flux Quantum based flux bias driver is proposed and discussed.