  The unavoidable and irreversible interaction between an entangled quantum
system and its environment causes decoherence of the individual qubits as well
as degradation of the entanglement between them. Entanglement sudden death
(ESD) is the phenomenon wherein disentanglement happens in finite time even
when individual qubits decohere only asymptotically in time due to noise.
Prolonging the entanglement is essential for the practical realization of
entanglement-based quantum information and computation protocols. For this
purpose, the local NOT operation in the computational basis on one or both
qubits has been proposed. Here, we formulate an all-optical experimental set-up
involving such NOT operations that can hasten, delay, or completely avert ESD,
all depending on when it is applied during the process of decoherence.
Analytical expressions for these are derived in terms of parameters of the
initial state's density matrix, whether for pure or mixed entangled states.
After a discussion of the schematics of the experiment, the problem is
theoretically analyzed, and simulation results of such manipulations of ESD are
presented.
