Figure 1: Macroscopic and microscopic work.
From: Fundamental limitations for quantum and nanoscale thermodynamics
(a) A macroscopic heat engine that performs work by lifting a heavy weight a certain height. (b) In the quantum or micro-regime, we can think of work as the ability to excite a two-level system from one energy state to a higher one. Having many of these atoms would allow us to perform macroscopic work—for example, we could use the atoms in a laser. An amount of work W can be used to produce a transition from the state |00| to the state |11|, with Hamiltonian 
(we call such a two-level system the work qubit or wit). We can use such a system as a basic work storage unit, as our results will not depend on what physical system is used.
