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.
Pumped at approximately twice the natural frequency, a Josephson parametric oscillator called parametron or Kerr parametric oscillator shows self-oscillation. Quantum annealing anduniversal quantum computation using self-oscillating parametrons as qubits were proposed. However, controls of parametrons under the pump field are degraded by unwanted rapidly oscillating terms in the Hamiltonian, which is called counter rotating terms (CRTs) coming from the violation of the rotating wave approximation. Therefore, the pump field can be an intrinsic origin of the imperfection of controls of parameterons. Here, we theoretically study the effect of the CRTs on the accuracy of controls of a parametron: creation of a cat state and a single qubit gate along the x axis. It is shown that there is a trade-off relationship between the suppression of the nonadiabatic transitions and the validity of the rotating wave approximation in a conventional approach. We show that the tailored time dependence of the detuning of the pump field can suppress both of the nonadiabatic transitions and the disturbance of the state of the parametron due to the CRTs.
Quantum annealing (QA) provides us with a way to solve combinatorial optimization problems. In the previous demonstration of the QA, a superconducting flux qubit (FQ) was used. However,the flux qubits in these demonstrations have a short coherence time such as tens of nano seconds. For the purpose to utilize quantum properties, it is necessary to use another qubit with a better coherence time. Here, we propose to use a capacitive-shunted flux qubit (CSFQ) for the implementation of the QA. The CSFQ has a few order of magnitude better coherence time than the FQ used in the QA. We theoretically show that, although it is difficult to perform the conventional QA with the CSFQ due to the form and strength of the interaction between the CSFQs, a spin-lock based QA can be implemented with the CSFQ even with the current technology. Our results pave the way for the realization of the practical QA that exploits quantum advantage with long-lived qubits.
We propose a prime factorizer operated in a framework of quantum annealing (QA). The idea is inverse operation of a multiplier implemented with QA-based Boolean logic circuits. We designedthe QA machine on an application-specific-annealing-computing architecture which efficiently increases available hardware budgets at the cost of restricted functionality. The invertible operation of QA logic gates consisting of superconducting flux qubits was confirmed by circuit simulation with classical noise sources. The circuits were implemented and fabricated by using superconducting integrated circuit technologies with Nb/AlOx/Nb Josephson junctions. We also propose a 2.5Dimensional packaging scheme of a qubit-chip/interpose /package-substrate structure for realizing practically large-scale QA systems.