  Thermodynamics and information have intricate inter-relations. The
justification of the fact that information is physical, is done by
inter-linking information and thermodynamics - through Landauer's principle.
This modern approach towards information recently has improved our
understanding of thermodynamics, both in classical and quantum domains. Here we
show thermodynamics as a consequence of information conservation. Our approach
can be applied to most general situations, where systems and thermal-baths
could be quantum, of arbitrary sizes and even could posses inter-system
correlations. The approach does not rely on an a priori predetermined
temperature associated to a thermal bath, which is not meaningful for
finite-size cases. Hence, the thermal-baths and systems are not different,
rather both are treated on an equal footing. This results in a
"temperature"-independent formulation of thermodynamics. We exploit the fact
that, for a fix amount of coarse-grained information, measured by the von
Neumann entropy, any system can be transformed to a state that possesses
minimal energy, without changing its entropy. This state is known as a
completely passive state, which assumes Boltzmann-Gibb's canonical form with an
intrinsic temperature. This leads us to introduce the notions of bound and free
energy, which we further use to quantify heat and work respectively. With this
guiding principle of information conservation, we develop universal notions of
equilibrium, heat and work, Landauer's principle and also universal fundamental
laws of thermodynamics. We show that the maximum efficiency of a quantum
engine, equipped with a finite baths, is in general lower than that of an ideal
Carnot's engine. We also introduce a resource theoretic framework for
intrinsic-temperature based thermodynamics, within which we address the problem
of work extraction and state transformations.
