  We study the indirect detection of Cosmological Constant from an open quantum
system of interacting spins, weakly interacting with a thermal bath, a massless
scalar field minimally coupled with the static de Sitter background, by
computing the spectroscopic shifts. By assuming pairwise interaction between
spins, we construct states using a generalisation of the superposition
principle. The corresponding spectroscopic shifts, caused by the effective
Hamiltonian of the system due to Casimir Polder interaction, are seen to play a
crucial role in predicting a very tiny value of the Cosmological Constant, in
the static patch of de Sitter space, which is consistent with the observed
value from the Planck measurements of the cosmic microwave background (CMB)
anisotropies.
