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The evaluation of the minimal evolution time between two distinguishable states of a system is important for assessing the maximal speed of quantum computers and communication channels. Lower bounds for this minimal time have been proposed for unitary dynamics. Here we show that it is possible to extend this concept to nonunitary processes, using an attainable lower bound that is connected to the quantum Fisher information for time estimation. This result is used to delimit the minimal evolution time for typical noisy channels.

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We present a method for the deterministic generation of all the electronic Bell states of two trapped ions. It involves the combination of a purely dispersive with a resonant laser excitation of vibronic transitions of the ions. In contrast to other methods presented up to now, our proposal does not require differential laser addressing of the individual ions and may be easily implemented with present available techniques. It is further shown that this excitation scheme is highly adequate for the complete determination of the motional state of the ions.PACS number(s): 03.65. Bz, 42.50.Vk, 32.80.Pj