  We study the macroscopic entanglement properties of a low dimensional quantum
spin system by investigating its magnetic properties at low temperatures and
high magnetic fields. The tempera- ture and magnetic field dependence of
entanglement from the susceptibility and magnetization data comparing the
experimental extraction with theoretical estimates are given. Extraction of
entan- glement has been made possible through the macroscopic witness operators
magnetic susceptibility. The protocol followed in doing so has been outlined in
some recent work. Various plots of entan- glement comparing the experimental
extraction with theoretical estimates are given. Quantitative comparison
between concurrence and entanglement witness is given for both the theoretical
and experimental results. Theory and experiments match over a wide range of
temperature and field. The spin system studied is a chain, which exhibits
dimerisation and yields fascinating entanglement properties when the
temperature and magnetic field is varied. These spin systems exhibit quantum
phase transition (QPT) at low temperatures, when the magnetic field is swept
through a critical value. We show explicitly for the first time, using tools
used in quantum information processing (QIP), that quantum phase transition
(QPT) can be captured experimentally using canonically conjugate observables.
Macroscopically, quantum complementarity relation clearly delineates entangled
states from separable states across the QPT. We have estimated the partial
information sharing in this system from our magnetization and susceptibility
data. The complementarity relation has been experimentally verified to hold in
this system
