Recent experiments by Hayashi et al. [Phys. Rev. Lett. 91, 226804 (2003)]demonstrate coherent oscillations of a charge quantum bit (qubit) in laterallydefined quantum dots. We study the intrinsic electron-phonon decoherence andgate performance for the next step: a system of two coupled charge qubits. Theeffective decoherence model contains properties of local as well as collectivedecoherence. Decoherence channels can be classified by their multipole moments,which leads to different low-energy spectra. It is shown that due to thesuper-Ohmic spectrum, the gate quality is limited by the single-qubit Hadamardgates. It can be significantly improved, by using double-dots with weak tunnelcoupling.
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