  We study the phenomenon of absorption refrigeration, where refrigeration is
achieved by heating instead of work, in two different setups: a minimal set up
based on coupled qubits, and two non-linearly coupled resonators. Considering
ZZ interaction between the two qubits, we outline the basic ingredients
required to achieve cooling. Using local as well as global master equations, we
observe that inclusion of XX type term in the qubit-qubit coupling is
detrimental to cooling. We compare the cooling effect obtained in the qubit
case with that of non-linearly coupled resonators (multi-level system) where
the ZZ interaction translates to a Kerr-type non-linearity. For small to
intermediate strengths of non-linearity, we observe that multi-level quantum
systems, for example qutrits, give better cooling effect compared to the
qubits. Using Keldysh non-equilibrium Green's function formalism, we go beyond
first order sequential tunneling processes and study the effect of higher order
processes on refrigeration. We find reduced cooling effect compared to the
master equation calculations.
