Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, Teck Seng Koh, John King Gamble, X. Wu, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson
We report coherent oscillations in a charge qubit formed in a top-gated Si/SiGe double quantum dot. We observe Rabi oscillations (x-rotations on the Bloch sphere) between the (2,1) and (1,2) charge states, with a figure of merit (number of oscillations per T2* coherence time) of 10 at the charge degeneracy point, where T2* is 2.1 ns. Unlike previous measurements of charge qubits in quantum dots, the longest T2* of 3.7 ns is found away from the charge degeneracy point; this feature arises from an anticrossing between the (2,1) ground state and a (1,2) excited state, indicating that internal structure of the quantum dots can be used to enhance coherence. In this regime we find a figure of merit of 37. We also observe Ramsey fringes (z-rotations on the Bloch sphere) and use them to extract a T2* of 179 ps at detunings away from any protective energy level structure.
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http://arxiv.org/abs/1208.0519
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