Kerr-cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of errors. The ZZ coupling between the qubits can be utilized for a two-qubitentangling gate, but the residual coupling causes unnecessary always-on gates and crosstalk. 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 Rzz(Θ) gate (ZZ rotation with angle Θ), one of the two-qubit entangling gates. We numerically show that the fidelity of the Rzz(−π/2) gate is higher than 99.9% in a case of 16 ns gate time and without decoherence.
A Kerr-nonlinear parametric oscillator (KPO) is one of the promising devices to realize qubits for universal quantum computing. The KPO can stabilize two coherent states with oppositephases, yielding a quantum superposition called a Schrödinger cat state. Universal quantum computing with KPOs requires three kinds of quantum gates: Rz,Rx, and Rzz gates. We theoretically propose a two-qubit gate Rzz for highly detuned KPOs. In the proposed scheme, we add another two-photon drive for the first KPO. This leads to the Rzz gate based on the driving of the second KPO depending on the first-KPO state, which we call „conditional driving.“ First, we perform simulations using a conventional KPO Hamiltonian derived from a superconducting-circuit model under some approximations and evaluate the gate fidelity. Next, we also perform numerical simulations of the two-qubit gate using the superconducting-circuit model without the approximations. The simulation results indicate that two-qubit gates can be implemented with high fidelity (>99.9%) for rotation angles required for universality.