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.
Single-flux-quantum (SFQ) digital logic is an attractive candidate for classical control and monitoring of large-scale quantum processors based on superconducting qubits. The authors describe a control approach based on repeated irradiation of the qubits with short registers of classical bits, which can be stored locally in compact SFQ registers and streamed to the quantum array at a high rate. Numerical simulations show achievable gate fidelity in excess of 99.99%; moreover, the approach allows control of many qubits resonating at different frequencies with a single, global SFQ clock. As a result, the scheme is well matched to the control of a scalable two-dimensional surface code.
Fluxoniums, as partially-protected superconducting qubits are promising to be employed to build high-performance large-scale quantum processor. The recently proposed ``integer fluxonium" operates at zero external flux bias, with a frequency of approximately 3 GHz. Single-qubit gate fidelity has been demonstrated to exceed
99.9%, while two-qubit gate schemes and scalable architectures remain underexplored. In this work, we investigate a fluxonium-transmon-fluxonium (FTF) coupling architecture using integer fluxoniums. We first confirm suppression of
ZZ interaction in the FTF system and then propose two high-fidelity controlled-
Z (CZ) gate schemes utilizing the coupler control: a flux-activated adiabatic gate scheme and a microwave-activated non-adiabatic gate scheme. Both schemes are capable of achieving low coherent error on the order of
1×10−6 within gate durations of several tens of nanoseconds. Additionally, we discuss a hybrid circuit system in which an integer fluxonium is coupled to a conventional fluxonium through a transmon coupler. Our proposal provides insights for future implementations of large-scale quantum circuits based on integer fluxonium devices.
Hybrid superconducting architectures combining different types of qubits offer a promising platform for exploiting their complementary advantages, yet high-fidelity entangling gates remain challenging because of strong nonlinearities and residual qubit-qubit interactions. Here, we propose a high-fidelity controlled-Z (CZ) gate for a hybrid circuit comprising a fluxonium qubit, a fixed-frequency transmon qubit, and a flux-tunable transmon coupler. By modulating only the external magnetic flux applied to the coupler, the qubit-qubit interaction is dynamically engineered for conditional-phase accumulation while the residual interaction is suppressed at idle, mitigating spectator-induced errors. We employ a low-dimensional Fourier-cosine pulse parameterization and a physically motivated cost function to independently suppress conditional-phase errors and leakage from the computational subspace. Numerical simulations demonstrate that a microwave-free CZ gate can be realized within
25ns, with an average gate fidelity exceeding
99.99% and leakage below
10−5. Using experimentally relevant superconducting-qubit parameters and accounting for decoherence, the proposed scheme maintains a CZ-gate fidelity of approximately
99.9%. We further extend the analysis to larger coupled architectures and find that the CZ-gate infidelity remains below
10−4 in the presence of spectator qubits. These results establish single-parameter flux control as a simple and robust approach for realizing high-fidelity entangling gates in heterogeneous superconducting quantum architectures.
The main source of error for superconducting quantum processors today is two-qubit gates, so understanding what limits their fidelity is central to improving these devices. This thesis studies the controlled-Z (CZ) gate between two transmon qubits, realized by tuning the
∣11⟩ and
∣20⟩ states through an avoided crossing with a flux pulse. There are two primary errors inherent to this approach. The first is leakage into non-computational states like
∣20⟩; the second is an exchange error from population transferring between the single-excitation states
∣01⟩ and
∣10⟩. Using a closed-system simulation, this work compares four flux-pulse shapes (unipolar, net-zero bipolar, Gaussian-filtered bipolar, and adiabatic) and constructs a CNOT from each CZ. The central question is which error channel sets the gate fidelity, and whether the widely used net-zero pulse, known to protect neighboring qubits, also helps inside the gate itself. An important finding of this research is that every type of flux pulse has an average gate fidelity limited by exchange error rather than leakage. While the exchange error is not apparent in the return of the
∣11⟩ population, because excitation-number conservation prevents
∣11⟩ from entering the single-excitation sector, it does appear as an off-diagonal term in the CZ unitary, and subsequently in the CNOT. A second finding concerns the net-zero pulse: although its zero net flux integral cancels the first-order phase imprinted on neighboring qubits, it does not suppress the exchange error. The strong suppression observed in the analytic bipolar gate happens because its idle time leaves the exchange oscillation near a cancellation point at the end of the gate, not because the pulse is net-zero. The flux operating point predicted by the simulation was tested on a physical transmon device accessed remotely, where a square pulse produced a
180∘ conditional phase close to the simulated flux and duration. Because the model is closed-system, the reported fidelities describe the coherent error of each pulse rather than the total infidelity of a physical device.
Today, most superconducting quantum processors are based on transmons, the modifications of charge qubits. Traditional transmon control schemes for implementing high-fidelity quantum gates are based on the analog microwave signals. Current problems with this type of control are bulkiness, high cost, and unwanted feedback from the microwave equipment, which increases with the number of qubits involved. A promising breakthrough is the use of superconducting digital circuits operating with picosecond (single flux quantum or SFQ) pulses. In our previous work, we have shown how to use SFQ control sequences to implement the rotation of a qubit at rest on the Bloch sphere around the Y-axis by an angle
π/2 with fidelity better than 99.99
% . The next important step is to implement two-qubit operations. Here we present the Genetic Algorithm for SFQ Control Sequence Search, which implements the original control protocol using the ZX operation as an example. The algorithm also takes into account the leakage to the higher energy states of the system. Its parallel implementation can find the appropriate sequence for arbitrary system parameters from a practical range in a reasonable time. As a result of numerical experiments, control sequences were found that implement the ZX operation with 0.999 target level population.