  We report on an experimental investigation of the dynamics of entanglement
between a single qubit and its environment, as well as for pairs of qubits
interacting independently with individual environments, using photons obtained
from parametric down-conversion. The qubits are encoded in the polarizations of
single photons, while the interaction with the environment is implemented by
coupling the polarization of each photon with its momentum. A convenient Sagnac
interferometer allows for the implementation of several decoherence channels
and for the continuous monitoring of the environment. For an
initially-entangled photon pair, one observes the vanishing of entanglement
before coherence disappears. For a single qubit interacting with an
environment, the dynamics of complementarity relations connecting single-qubit
properties and its entanglement with the environment is experimentally
determined. The evolution of a single qubit under continuous monitoring of the
environment is investigated, demonstrating that a qubit may decay even when the
environment is found in the unexcited state. This implies that entanglement can
be increased by local continuous monitoring, which is equivalent to
entanglement distillation. We also present a detailed analysis of the transfer
of entanglement from the two-qubit system to the two corresponding
environments, between which entanglement may suddenly appear, and show
instances for which no entanglement is created between dephasing environments,
nor between each of them and the corresponding qubit: the initial two-qubit
entanglement gets transformed into legitimate multiqubit entanglement of the
Greenberger-Horne-Zeilinger (GHZ) type.
