We investigate the fast-forward and time-scaling properties of superconducting resonators under a coherent drive. We propose a scheme for perfect displacement of a superconducting resonator by modulating the drive amplitude based on fast-forward scaling theory. Furthermore, we propose a scheme exploiting both the fast-forward and time-scaling properties that enables perfect displacement through detuning modulation. The proposed schemes are also applicable to a subsystem that can be effectively represented by a driven resonator. In particular, we apply the latter scheme to fast and high-fidelity displacement of a coupler between Kerr parametric oscillators.
We investigate the fast-forward and time-scaling properties of superconducting resonators under a coherent drive. We propose a scheme for perfect displacement of a superconducting resonator by modulating the drive amplitude based on fast-forward scaling theory. Furthermore, we propose a scheme exploiting both the fast-forward and time-scaling properties that enables perfect displacement through detuning modulation. The proposed schemes are also applicable to a subsystem that can be effectively represented by a driven resonator. In particular, we apply the latter scheme to fast and high-fidelity displacement of a coupler between Kerr parametric oscillators.
Stabilized cat codes can provide a biased noise channel with a set of bias-preserving (BP) gates, which can significantly reduce the resource overhead for fault-tolerant quantum computing. All existing schemes of BP gates, however, require adiabatic quantum evolution, with performance limited by excitation loss and nonadiabatic errors during the adiabatic gates. In this paper, we apply a derivative-based leakage-suppression technique to overcome nonadiabatic errors, so that we can implement fast BP gates on Kerr-cat qubits with improved gate fidelity while maintaining high noise bias. When applied to concatenated quantum error correction, the fast BP gates not only can improve the logical error rate but also can reduce resource overhead, which enables more efficient implementation of fault-tolerant quantum computing.
Kerr cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of their errors. The ZZ coupling between the qubits can be utilized for a two-qubit entangling gate, but the residual coupling called ZZ crosstalk is detrimental to precise computing. In order to resolve this problem, we propose a tunable ZZ-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual ZZ couplings via the two couplers cancel each other out. We also apply our scheme to the ${R}_{ZZ}(\mathrm{\ensuremath{\Theta}})$ gate (ZZ rotation with angle $\mathrm{\ensuremath{\Theta}}$), one of the two-qubit entangling gates. We numerically show that the fidelity of the ${R}_{ZZ}(\ensuremath{-}\ensuremath{\pi}/2)$ gate is higher than 99.9% in a case of 16-ns gate time and without decoherence.
Kerr-cat qubits offer a hardware-efficient route to fault-tolerant quantum computing by suppressing bit-flip errors exponentially, while phase-flip errors increase only linearly with the mean photon number. To maintain this protection, entangling operations must be bias-preserving, ensuring that phase-flip errors occurring during the gate do not propagate into bit-flip errors. In this paper, we propose an asymmetrically threaded SQUID (ATS) parametric coupler to implement high-fidelity, bias-preserving entangling gates between Kerr-cat qubits. By engineering a photon-number asymmetry between the control and target modes, we enable fast gate operations that suppress non-adiabatic leakage without complex pulse shaping. Through numerical simulations of the full circuit Hamiltonian, we demonstrate a bias-preserving CX gate achieving 99.69% fidelity in the absence of photon loss for a gate duration of 50 ns. Under a single-photon loss rate of 100 kHz, our protocol yields a noise bias of 9922, exceeding the noise bias of a naive non-bias-preserving CX implementation by approximately a factor of 570. These results support the ATS coupler as a valuable building block for concatenated cat-qubit architectures.
Building large-scale quantum computers requires an interqubit-coupling scheme with a high on–off ratio to avoid unwanted crosstalk coming from residual coupling and to enable fast multi-qubit operations. We propose a ZZ-coupling scheme for two Kerr-cat qubits with a frequency-tunable coupler. By making four relevant states of the two Kerr-cat qubits quadruply degenerate, we can switch off the ZZ coupling. By partially lifting the level degeneracy, we can switch it on. We theoretically show that an experimentally feasible circuit model suppresses the residual ZZ coupling. Moreover, our circuit can realize RZZ(−π/2)-gate fidelity higher than 99.9% within 18 ns when decoherence is ignored. Our model includes the first-order terms in expansion beyond the rotating-wave approximation.
Stabilized cat codes can provide a biased noise channel with a set of bias-preserving (BP) gates, which can significantly reduce the resource overhead for fault-tolerant quantum computing. All existing schemes of BP gates, however, require adiabatic quantum evolution, with performance limited by excitation loss and non-adiabatic errors during the adiabatic gates. In this work, we apply a derivative-based leakage suppression technique to overcome non-adiabatic errors, so that we can implement fast BP gates on Kerr-cat qubits with improved gate fidelity while maintaining high noise bias. When applied to concatenated quantum error correction, the fast BP gates can not only improve the logical error rate but also reduce resource overhead, which enables more efficient implementation of fault-tolerant quantum computing.
Kerr parametric oscillators (KPOs), which can be implemented with superconducting parametrons possessing large Kerr nonlinearity, have been attracting much attention in terms of their applications to quantum annealing, universal quantum computation, and studies of quantum many-body systems. It is of practical importance for these studies to realize fast and accurate tunable coupling between KPOs in a simple manner. We develop a simple scheme of fast tunable coupling of KPOs with high tunability in speed and amplitude using the fast transitionless rotation of a KPO in the phase space based on the shortcuts to adiabaticity. Our scheme enables rapid switching of the effective coupling between KPOs, and can be implemented with always-on linear coupling between KPOs, by controlling the phase of the pump field and the resonance frequency of the KPO without controlling the amplitude of the pump field nor using additional drive fields and couplers. We apply the coupling scheme to a two-qubit gate, and show that our scheme realizes high gate fidelity compared with a purely adiabatic one, by mitigating undesired nonadiabatic transitions.