Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime $T_1$, coherence time $T_2^*$ and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for $T_1$ and $T_2^*$, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of $0.46$% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.
Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime T1, coherence time T2∗ and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for T1 and T2∗, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.
Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energy-efficient, stable, and reproducible qubit control and readout electronics. In comparison to traditional semiconductor control electronics (TSCE) located at room temperature, the signals generated by Josephson junction (JJ) based rf sources benefit from small device sizes, low power dissipation, intrinsic calibration, superior reproducibility, and insensitivity to ambient fluctuations. Previous experiments to co-locate qubits and JJ-based control electronics resulted in quasiparticle poisoning of the qubit; degrading the qubit's coherence and lifetime. In this paper, we digitally control a 0.01~K transmon qubit with pulses from a Josephson pulse generator (JPG) located at the 3~K stage of a dilution refrigerator. We directly compare the qubit lifetime T1, coherence time T2∗, and thermal occupation Pth when the qubit is controlled by the JPG circuit versus the TSCE setup. We find agreement to within the daily fluctuations on ±0.5μs and ±2μs for T1 and T2∗, respectively, and agreement to within the 1\% error for Pth. Additionally, we perform randomized benchmarking to measure an average JPG gate error of 2.1×10−2. In combination with a small device size (<25~mm2) and low on-chip power dissipation (≪100μW), these results are an important step towards demonstrating the viability of using JJ-based control electronics located at temperature stages higher than the mixing chamber stage in highly-scaled superconducting quantum information systems.
Scaling of quantum computers to fault-tolerant levels relies critically on the integration of energyefficient, stable, and reproducible qubit control and readout electronics.In comparison to traditional semiconductor-control electronics (TSCE) located at room temperature, the signals generated by rf sources based on Josephson-junctions (JJs) benefit from small device sizes, low power dissipation, intrinsic calibration, superior reproducibility, and insensitivity to ambient fluctuations.Previous experiments to colocate qubits and JJ-based control electronics have resulted in quasiparticle poisoning of the qubit, degrading the coherence and lifetime of the qubit.In this paper, we digitally control a 0.01-K transmon qubit with pulses from a Josephson pulse generator (JPG) located at the 3-K stage of a dilution refrigerator.We directly compare the qubit lifetime T 1 , the coherence time T * 2 , and the thermal occupation P th when the qubit is controlled by the JPG circuit versus the TSCE setup.We find agreement to within the daily fluctuations of ±0.5 μs and ±2 μs for T 1 and T * 2 , respectively, and agreement to within the 1% error for P th .Additionally, we perform randomized benchmarking to measure an average JPG gate error of 2.1 × 10 -2 .In combination with a small device size (< 25 mm 2 ) and low on-chip power dissipation ( 100 μW), these results are an important step toward demonstrating the viability of using JJ-based control electronics located at temperature stages higher than the mixing-chamber stage in highly scaled superconducting quantum information systems.
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.
Comparative study of high-Jc and low-Jc SFQ microwave generators for transmon control
Abstract Scalability of superconducting quantum processors is increasingly constrained by the thermal load and wiring overhead associated with room-temperature control hardware. Cryogenic control interfaces based on single-flux-quantum (SFQ) logic offer a potential route to alleviating these bottlenecks, although their dissipation depends sensitively on the Josephson-junction critical current and the underlying fabrication process. In this work, we comparatively investigate SFQ-synthesized microwave generators implemented in high- and low-critical-current processes, and evaluate their suitability for transmon qubit control through a combination of experimental characterization and hybrid QuTiP–JoSIM analysis. By modulating SFQ pulse intervals, we synthesize coherent microwave drives and obtain single-qubit gate fidelities above 99.9% at 10 mK in QuTiP-based simulations for both implementations. The low- Jc J c implementation exhibits reduced phase-noise-induced error, achieves higher gate fidelity within its valid bias window, and shows improved robustness against frequency detuning, while the high- Jc J c implementation maintains a lower broadband noise floor. Using an explicit model of thermal dissipation and circuit footprint, we further estimate the integration limits of cryogenic control channels. The results show that reducing the switching current substantially relaxes the cooling-budget constraint and increases the number of channels that can be deployed simultaneously. These results clarify the trade-offs associated with implementing SFQ microwave generators in different fabrication processes and provide practical guidance for the design of scalable cryogenic qubit-control interfaces.
Scalability of today’s superconducting quantum computers is limited due to the huge costs of generating/routing microwave control pulses per qubit from room temperature. One active research area in both industry and academia is to push the classical controllers to the dilution refrigerator in order to increase the scalability of quantum computers. Superconducting Single Flux Quantum (SFQ) is a classical logic technology with low power consumption and ultra-high speed, and thus is a promising candidate for in-fridge classical controllers with maximized scalability. Prior work has demonstrated high-fidelity SFQ-based single-qubit gates. However, little research has been done on SFQ-based multi-qubit gates, which are necessary to realize SFQ-based universal quantum computing.In this paper, we present the first thorough analysis of SFQ-based two-qubit gates. Our observations show that SFQ-based two-qubit gates tend to have high leakage to qubit non-computational subspace, which presents severe design challenges. We show that despite these challenges, we can realize gates with high fidelity by carefully designing optimal control methods and qubit architectures. We develop optimal control methods that suppress leakage, and also investigate various qubit architectures that reduce the leakage. After carefully engineering our SFQ-friendly quantum system, we show that it can achieve similar gate fidelity and gate time to microwave-based quantum systems. The promising results of this paper show that (1) SFQ-based universal quantum computation is both feasible and effective; and (2) SFQ is a promising approach in designing classical controller for quantum machines because it can increase the scalability while preserving gate fidelity and performance.
An approach for scalable quantum computing infrastructure based on the use of low-power digital superconducting single flux quantum (SFQ) circuits is described. Rather than replicating the room-temperature microwave control and measurement infrastructure solutions dominating the current systems, we use the inherent to superconducting technology methods - the use of SFQ pulses directly at the base temperature. For qubit control, we irradiate qubits with the coherent SFQ pulse sequences computed using optical control theory. For qubit measurement, Josephson photon counter performs projective quantum measurement, the result of which is converted to digital SFQ output. These operations are aided by a high-speed digital SFQ coprocessor located at higher temperature stage (e.g., 3 K) to process the measurement results and load new control sequences to 20 mK SFQ quantum-classical interface circuits.