Multiple observations of quantum clocks - art. no. 062309

How much information about the original state preparation can be extracted from a quantum system which has already been measured? That is, how many in dependent (noncommunicating) observers can measure the quantum system seque ntially and give a nontrivial estimation of the original unknown state? We investigate these questions, and show from a simple example that informatio n about the original preparation is not entirely lost as a result of the me asurement-induced collapse of the quantum state, and that an infinite numbe r of independent observers who have no prior knowledge about the initial st ate can gain partial information about the original preparation of the quan tum system.