The implementation of high-fidelity swapping operations is of vital importance for executing quantum algorithms on a quantum processor with limited connectivity.We present an efficient pulse-control technique, the cross-cross resonance (CCR) gate, to implement iSWAP and SWAP gates with dispersively coupled fixed-frequency transmon qubits.The key to the operation of the CCR gate is the simultaneous driving of both of the coupled qubits at the frequency of another qubit, whereby a fast two-qubit interaction roughly equivalent to ZX + XZ entangling gates is realized without strongly driving the qubits.We develop a calibration technique for the CCR gate and evaluate the performance of the iSWAP and SWAP gates.The CCR gate shows remarkable improvements in the average gate error from the conventional decomposition based on the cross-resonance gate.
The implementation of high-fidelity swapping operations is of vital importance for executing quantum algorithms on a quantum processor with limited connectivity.We present an efficient pulse-control technique, the cross-cross resonance (CCR) gate, to implement iSWAP and SWAP gates with dispersively coupled fixed-frequency transmon qubits.The key to the operation of the CCR gate is the simultaneous driving of both of the coupled qubits at the frequency of another qubit, whereby a fast two-qubit interaction roughly equivalent to ZX + XZ entangling gates is realized without strongly driving the qubits.We develop a calibration technique for the CCR gate and evaluate the performance of the iSWAP and SWAP gates.The CCR gate shows remarkable improvements in the average gate error from the conventional decomposition based on the cross-resonance gate.
Abstract The cross-cross resonance (CCR) gate is an extension of the cross-resonance (CR) gate that has been theoretically implemented with transmon qubits. However, the weak anharmonicity of transmon qubits leads to decoherence and leakage errors, which reduce the fidelity of the gate operation. In this work, we propose employing fluxonium qubits to implement the CCR gate. With similar drive amplitudes, our scheme can achieve the required stronger ZX and XZ interactions compared to a transmon qubit system, resulting in shorter gate times. By combining the CCR gate with specific single-qubit gates, a iSWAP gate is realized, thereby enabling the construction of the iSWAP and SWAP gates. Moreover, through parameter optimization, these gates built on fluxonium qubits exhibit a significant reduction in operation time while simultaneously achieving higher fidelity. Our results provide valuable insights into achieving fast, low-error-rate swapping operations in superconducting circuits.
Fixed-frequency superconducting qubits demonstrate remarkable success as platforms for stable and scalable quantum computing. Cross-resonance gates have been the workhorse of fixed-coupling, fixed-frequency superconducting processors, leveraging the entanglement generated by driving one qubit resonantly with a neighbor’s frequency to achieve high-fidelity, universal controlled- () gates. Here, we use on-resonant and off-resonant microwave drives to go beyond cross-resonance, realizing natively interesting two-qubit gates that are not equivalent to gates. In particular, we implement and benchmark native i , , i SWAP , and b gates; in fact, any SU ( 4 ) unitary can be achieved using these techniques. Furthermore, we apply these techniques for an efficient construction of the B gate: a perfect entangler from which any two-qubit gate can be reached in only two applications. We show that these native two-qubit gates are better than their counterparts compiled from cross-resonance gates. We elucidate the resonance conditions required to drive each two-qubit gate and provide a novel frame tracking technique to implement them in Qiskit. Published by the American Physical Society 2024
The two-qubit entangler is one of the most important components of quantum computers. The cross-cross resonance (CCR) gate has recently been proposed and shown to operate faster than the cross-resonance (CR) gate as an iSWAP gate. However, unlike the CR gate, the CCR gate requires frequency modifications of the input microwave pulses induced by a Stark shift. We present a theoretical study on the CCR gate in the two-transmon system. Using the Schrieffer-Wolff transformation and the rotating-wave approximation, we obtain both the effective Hamiltonian and the input microwave pulse frequencies of the CCR gate. It is well known that the static ZZ term degrades the performance of the CR gate in the transmon system. For the CCR gate, an additional dynamic ZZ term arises from the combined coupling to the qubits by the input microwave pulses, whose strength increases as the product of the two pulse amplitudes ${\mathrm{\ensuremath{\Omega}}}_{0}{\mathrm{\ensuremath{\Omega}}}_{1}$. We perform numerical simulations using the set of parameters for the IBM Hanoi quantum processor. We show that the CCR gate has a phase tunability and that varying the relative phase of the input microwave pulses can significantly improve the gate performance. This demonstrates that the static ZZ term in the CCR gate can be effectively reduced by optimizing the relative phase of the input microwave pulses.
Implementing high-fidelity multiqubit gates is critical for reducing circuit depth in near-term quantum processors and fault-tolerant architectures. However, realizing multiqubit interactions and entanglement remains a critical challenge that limits the implementation of multiqubit gates. Here we propose an all-microwave scheme to realize single-step multiqubit gates incorporating n control and m target qubits, tailored for frequency-tunable superconducting transmon networks. By leveraging cross-resonance (CR) drives, this approach induces effective two-body ZX interactions that are significantly stronger than those achieved in conventional resonant regimes, circumventing the need for tunable couplers. The system's effective Hamiltonian is analytically derived using both quad frame rotation and rigorous block diagonalization. These combined theoretical methods facilitate the identification of optimal parameter regimes that simultaneously enhance desired target couplings and suppress parasitic higher-order terms, such as ZXX interactions. Through numerical simulations and gradient-based pulse-shape optimization, we demonstrate three-qubit gates achieving fidelities exceeding 99.9% within a 60 ns gate duration. Furthermore, we evaluate the fundamental scalability of this framework by extending the optimization to a five-qubit CXXXX architecture, confirming the physical viability of the multi-target driving scheme. Because these multiqubit operations naturally emulate stabilizer measurements, this architecture provides a hardware-efficient strategy for reducing the circuit depth of stabilizer-type operations by replacing sequential two-qubit gate decompositions with single-step multiqubit gates.
Implementing high-fidelity entangling operations between qubits is a key challenge in quantum computing. Multiple sources of error and a large parameter space make optimization of control schemes for high-fidelity operations a complex technical challenge. This study presents a complete recipe for calibrating a high-fidelity cross-resonance gate implemented between two dispersively coupled transmon qubits, showing that even large amounts of crosstalk can be effectively canceled. This approach should be useful as a general tool for those working on experiments with coupled quantum systems, in particular for computing.
Maintaining or even improving gate performance with growing numbers of parallel controlled qubits is a vital requirement for fault-tolerant quantum computing.For superconducting quantum processors, though isolated one-or two-qubit gates have been demonstrated with high fidelity, implementing these gates in parallel commonly shows worse performance.Generally, this degradation is attributed to various crosstalks between qubits, such as quantum crosstalk due to residual inter-qubit coupling.An understanding of the exact nature of these crosstalks is critical to figuring out respective mitigation schemes and improved qubit architecture designs with low crosstalk.Here we give a theoretical analysis of quantum crosstalk impact on simultaneous gate operations in a qubit architecture, where fixed-frequency transmon qubits are coupled via a tunable bus, and sub-100-ns controlled-Z (CZ) gates can be realized by applying a baseband flux pulse on the bus.Our analysis shows that for microwave-driven single-qubit gates, the dressing from the qubit-qubit coupling can cause non-negligible cross-driving errors when qubits operate near frequency collision regions.During CZ gate operations, although unwanted nearest-neighbor interactions are nominally turned off, sub-MHz parasitic next-nearest-neighbor interactions involving spectator qubits can still exist, causing considerable leakage or control error when one operates qubit systems around these parasitic resonance points.To ensure high-fidelity simultaneous operations, a request could be raised to figure out a better way to balance the gate error from target qubit systems themselves and the error from nonparticipating spectator qubits.Overall, our analysis suggests that towards useful quantum processors, the qubit architecture should be examined carefully in the context of high-fidelity simultaneous gate operations in a scalable qubit lattice.