Erasing memory ? Protecting quantum coherence and entanglement

Suhail Zubairy (Texas A&M University)

In a quantum measurement, an operator projects the system into one of its eigenstates with a 
specific eigenvalue as the readout. Once the measurement is done, the unknown initial state of 
the system is destroyed, and in general there is no way to recover it from the result. 
However, such quantum measurement, the so-called strong or von Neumann measurement, 
is only part of the story. 
There is another type of quantum measurement called weak measurement, in which
 the outcome is not precise or sharp but nevertheless reveals some information about the 
system. Since this weak measurement does not totally collapse the system, the information of 
the initial state is passed over to the final state. If such retained information is complete, it 
would be possible to recover the initial state with some operations. In this talk, I shall discuss 
the processes and the issues related to protecting quantum coherence and entanglement.