Coupling and coherent electrical control of two dopants in a silicon   nanowire
  Electric control of individual atoms or molecules in a solid-state system offers a promising way to bring quantum mechanical functionalities into electronics. This idea has recently come into the reach of the established domain of silicon technology, leading to the realization of single-atom transistors and to the first measurements of electron spin dynamics in single donors. Here we show that we can electrically couple two donors embedded in a multi-gate silicon transistor, and induce coherent oscillations in their charge states by means of microwave signals. We measure single-electron tunneling across the two donors, which reveals their energy spectrum. The lowest energy states, corresponding to a single electron located on either of the two donors, form a two-level system (TLS) well separated from all other electronic levels. Gigahertz driving of this TLS results in a quantum interference pattern associated with the absorption or the stimulated emission of up to ten microwave photons. We estimate a charge dephasing time of 0.3 nanoseconds, consistent with other types of charge quantum bits. Here, however, the relatively short coherence time can be counterbalanced by fast operation signals (in principle up to 1 terahertz) as allowed by the large empty energy window separating ground and excited states in donor atoms. The demonstrated coherent coupling of two donors constitutes an essential step towards donor-based quantum computing devices in silicon. 
E. Dupont-Ferrier, B. Roche, B. Voisin, X. Jehl, R. Wacquez, M. Vinet,   M. Sanquer, S. De Franceschi
cond-mat.mes-hall quant-ph