Magnetic Field Dependence of Retrapping Currents in DC-Squids

Abstract

The quantum computer is a novel architecture that is capable of extremely fast computation. The basic element of a quantum computer is the qubit, for which there are many candidates. Solid-state qubits have advantages in terms of integration, but the many quantum degrees of freedom affect their coherence. Of the solid-state qubits, superconducting qubits have a longer coherence time because of their superconducting energy gap. These superconducting qubits, which have already been confirmed experimentally, are based on the charge1 and the flux state 2,3. A charge qubit has already shown full one-qubit operation, but it has a large charge noise in its Josephson junction. By contrast, flux qubits are insensitive to charge noise. Moreover, flux qubits are expected to have a longer coherence time than charge qubits. We are studying 3-Josephson-junction flux qubits first proposed by Mooij et al. The readout for this flux qubit is performed by using a DC-SQUID coupled to the qubit via a mutual inductance. The dissipation of the qubit and the DC-SQUID affects the coherence of the qubit state. The retrapping current of the DC-SQUID is related to the dissipation of the SQUID and the qubit. Therefore, we are studying the retrapping current of the DC-SQUID. In this work, we studied the retrapping current of the DC-SQUID without the qubit to order to characterize the DC-SQUID itself.