TY - JOUR
T1 - Recycling of quantum information: Multiple observations of quantum systems
JF - arXiv
Y1 - 2007
A1 - Rapcan, Peter
A1 - Calsamiglia, John
A1 - Muñoz-Tapia, Ramon
A1 - Bagan, Emili
A1 - Bużek, Vladimir
AB - Given a finite number of copies of an unknown qubit state that have already been measured optimally, can one still extract any information about the original unknown state? We give a positive answer to this question and quantify the information obtainable by a given observer as a function of the number of copies in the ensemble, and of the number of independent observers that, one after the other, have independently measured the same ensemble of qubits before him. The optimality of the protocol is proven and extensions to other states and encodings are also studied. According to the general lore, the state after a measurement has no information about the state before the measurement. Our results manifestly show that this statement has to be taken with a grain of salt, specially in situations where the quantum states encode confidential information.
UR - http://arxiv.org/abs/0708.1086
ER -