  Quantum entanglement is notorious for being a very fragile resource.
Significant efforts have been put into the study of entanglement degradation in
the presence of a realistic noisy environment. Here, we present a theoretical
and an experimental study of the decoherence properties of entangled pairs of
qubits. The entanglement dynamics of maximally entangled qubit pairs is shown
to be related in a simple way to the noise representation in the Bloch sphere
picture. We derive the entanglement level in the case when both qubits are
transmitted through any arbitrary unital Pauli channel, and compared it to the
case when the channel is applied only to one of the qubits. The dynamics of
both cases was verified experimentally using an all-optical setup. We further
investigated the evolution of partially entangled initial states. Different
dynamics was observed for initial mixed and pure states of the same
entanglement level.
