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Two-qubit interactions are at the heart of quantum information processing. For single-spin qubits in semiconductor quantum dots, the exchange gate has always been considered the natural two-qubit gate. The recent integration of a magnetic field or g-factor gradients in coupled quantum dot systems allows for a one-step, robust realization of the controlled-phase (C-phase) gate instead. We analyze the C-phase gate durations and fidelities that can be obtained under realistic conditions, including the effects of charge and nuclear field fluctuations, and find gate error probabilities of below 10-4, possibly allowing fault-tolerant quantum computation.
We investigate the behavior of N atoms resonantly coupled to a single electromagnetic field mode sustained by a high quality cavity, containing a mesoscopic coherent field. We show with a simple effective Hamiltonian model that the strong coupling between the cavity and the atoms produces an atom-field entangled state, involving N+1 nearly coherent components slowly rotating at different paces in the phase plane. The periodic overlap of these components results in a complex collapse and revival pattern for the Rabi oscillation. We study the influence of decoherence due to the finite cavity quality factor. We propose a simple analytical model, based on the Monte Carlo approach to relaxation. We compare its predictions with exact calculations and show that these interesting effects could realistically be observed on a two or three atoms ...
We propose and implement a nondestructive measurement that distinguishes between two-electron spin states in a quantum dot. In contrast to earlier experiments with quantum dots, the spins are left behind in the state corresponding to the measurement outcome. By measuring the spin states twice within a time shorter than the relaxation time T1, correlations between the outcomes of consecutive measurements are observed. They disappear as the wait time between measurements becomes comparable to T1. The correlation between the postmeasurement state and the measurement outcome is measured to be ~90% on average.
Comment: 7 pages, 5 figures. To be published in Journal of Applied Physics, proceedings ICPS 2006
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