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.
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.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.
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 number of bits in quantum computers needs to be increased since large problems must be solved to achieve quantum acceleration. Superconducting qubits are the most advanced qubit devices currently available. A method has been proposed to place a quantum-classical interface on a 4 K stage to ensure scalability. The practical implications of a simple counter circuit in the interface and its role in processing the qubit read results have been demonstrated. A method is needed for configuring a counter circuit with low power consumption to improve scalability. In this paper, we design a novel counter circuit using the Half-Flux Quantum (HFQ) technology. This technology operates in cryogenic environments and has lower power consumption than the Single-Flux Quantum (SFQ) tech-nology. In addition, we conducted a comparative evaluation of circuits dedicated to the NISQ algorithm using SFQ and equivalent circuits implemented using HFQ. Assuming the same power consumption technology applicable to SFQ can be applied to HFQ, we found that the proposed HFQ-based counter circuit can reduce the power consumption by up to 97%.
As superconducting quantum processors increase in size and complexity, the scalability of standard techniques for qubit control and readout becomes a limiting factor. Replacing room temperature analog components with cryogenic digital components could allow for the realization of systems well beyond the current state-of-the-art qubit arrays with tens of qubits. The standard technique for performing a qubit measurement with heterodyne readout uses a quantum-limited cryogenic amplifier chain and requires bulky microwave components inside the refrigerator with multiple control lines and pump signals. Additionally, the result is only accessible in software at room temperature. An alternative method for measuring qubits involves mapping the qubit state onto the photon occupation in a microwave cavity, followed by subsequent photon detection using a Josephson photomultiplier (JPM). The JPM measures the qubit and stores the result in a classical circulating current. To make use of this result, we can leverage existing single flux quantum (SFQ) circuitry. An underdamped Josephson transmission line (JTL) can be coupled to the JPM and fluxons traveling along the JTL are accelerated or delayed, depending on the circulating current state of the JPM. This fluxon delay can then be converted to an SFQ logic signal resulting in a digital qubit readout with a proximal microfabricated device, paving the way for cryogenic digital feedback necessary for error-correcting codes.
We report the measurement results of the low-power single-flux-quantum (SFQ) circuits below 20 mK using a dilution refrigerator. We have been developing the SFQ circuits using a lowered critical current density process (250 A/cm2) and low-voltage design (0.1–0.5 mV) toward digital signal processing near superconductor quantum bits. So far, we have designed several basic SFQ cells, including wiring elements, flip-flops, and logic gates, using the four Nb planarized layer process and obtained the correct operation at 300 mK. In this study, we tested several SFQ circuits aiming to evaluate the operation at the mixing chamber stage of a dilution refrigerator. Our experimental results show that there was almost no change of device parameters, such as critical current values of Josephson junctions, or operating margins of the basic cells between 300 mK and 16 mK.