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Quantum coherent dynamics of two coupled superconducting charge qubits

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  • Yu. A. Pashkin
  • O. Astafiev
  • T. Yamamoto
  • Y. Nakamura
  • D. V. Averin
  • T. Tilma
  • F. Nori
  • J. S. Tsai
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Abstract

We have analyzed and measured the quantum coherent dynamics of a circuit containing two coupled superconducting charge qubits. Each qubit is based on a Cooper pair box connected to a reservoir electrode through a Josephson junction. Two qubits are coupled electrostatically by a small island overlapping both Cooper pair boxes. Quantum state manipulation ofthe qubit circuit is done by applying non -adiabatic voltage pulses to the common gate. We read out each qubit by means of probe electrodes connected to Cooper pair boxes through high -Ohmic tunnel junctions. With such a setup the measured pulse -induced probe currents are proportional to the probability for each qubit to have an extra Cooper pai1r after the manipulation. As expected from theory and observed experimentally the measured pulse -induced current in each probe has two frequency components whose position on the frequency axis can be externally controlled. This is a result ofthe inter -qubit coupling which is also responsible for the avoided level crossing that we observed in the qubits' spectra. Our simulations show that in the absence of decoherence and with a rectangular pulse shape the system remains entangled most ofthe time reaching maximally entangled states at certain instances.