Published in

Elsevier, Physica C: Superconductivity and its Applications, (426-431), p. 1552-1560

DOI: 10.1016/j.physc.2005.01.076

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Coherent manipulation of coupled Josephson charge qubits

This paper is available in a repository.
This paper is available in a repository.

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Abstract

We have analyzed and measured the quantum coherent dynamics of a circuit containing two-coupled superconduc-ting 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 of the 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 pair 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 of the inter-qubit coupling which is also responsible for the avoided level crossing that we observed in the qubitsÕ spectra. Our simulations show that in the absence of decoherence and with a rectangular pulse shape, the system remains entangled most of the time reaching maximally entangled states at certain instances.