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Switching dynamics of double barrier Josephson junction based qubit gate

S.E. Shafranjuk, I.P. Nevirkovets, J. Ketterson
Physics & Astronomy Dept., Northwestern University, US

Keywords: SINIS qubits dynamics

Abstract:
The double barrier SINIS junctions (here S, I, and N denote a superconductor, an insulator, and a normal metal, respectively) with a nanoscopic N spacer are potentially capable of performing quantum logic operations (so-called qubits) involving the superposition of two (macroscopic) quantum states. In our report we analyse the switching dynamics of three-terminal double barrier SINIS junction working as a qubit gate based on two quantum states. The quantum states are associated with conventional and unconventional Josephson current components observed in the SINIS junctions. In this work we study the switching time and decoherence (dephasing) time time of the mentioned device. Such characteristics are closely related to the longitudinal and transverse dynamics of the superconducting order parameter. Such a dynamics in particular is determined by the electron recombination time. The mentioned parameter strongly depends on the electron excitation spectrum inside N, which in turn is very sensitive to the presence of nonmagnetic impurities (i.e., to the magnitude of electron impurity scattering time ). In this work we computed the local electron density of states in the SINIS junction using the quasiclassical Eilenberger equation approach. We find that in a “clean” limit , the electron excitation spectrum inside N consists of quantized levels, while in the opposite “dirty” limit the spectrum of N is rather smooth versus the energy variable E. Such a difference affects drastically. In Fig. 2 we plot the energy dependence of inside the middle N spacer of SINIS gate for two different cases (curve A), and (curve B). One can see pronounced peaks in at (curve B for the “clean” case) which are absent for a “dirty” junction (curve A). For such reasons, the dynamics of SINIS qubit gates is quite distinct in the two mentioned limits.

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