  Work belongs to the most basic notions in thermodynamics but it is not well
understood in quantum systems, especially in open quantum systems. By
introducing a novel concept of work functional along individual Feynman path,
we invent a new approach to study thermodynamics in the quantum regime. Using
the work functional, we derive a path-integral expression for the work
statistics. By performing the $\hbar$ expansion, we analytically prove the
quantum-classical correspondence of the work statistics. In addition, we obtain
the quantum correction to the classical fluctuating work. We can also apply
this approach to an open quantum system in the strong coupling regime described
by the quantum Brownian motion model. This approach provides an effective way
to calculate the work in open quantum systems by utilizing various path
integral techniques. As an example, we calculate the work statistics for a
dragged harmonic oscillator in both isolated and open quantum systems.
