We derive a semi-classical nonequilibrium work identity by applying the Wigner-Weyl quantization scheme to the Jarzynski identity for a classical Hamiltonian. This allows us, to the leading order in , to overcome the problem of defining the concept of work in quantum mechanics. We propose a geometric interpretation of this semi-classical relation in terms of trajectories in a complex phase space and illustrate it with the exactly solvable case of the quantum harmonic oscillator. PACS numbers: 03.65.Sq,03.65.YzAccording to the second principle of thermodynamics, macroscopic phenomena tend to evolve towards states corresponding to a maximum number of underlying microstates, i.e. states that maximize entropy. Combined with the first principle, this leads to a more operational statement for isothermal processes: the minimal amount of work to modify a system from a state A to a state B is given bywhere F = U − T S is the free energy. Statistical mechanics has shown that the interpretation of macroscopic thermodynamics is statistical, by endowing microscopic states with a probability measure. Hence, the second principle should be understood as an average of a random process; one should write, in fact, ∂ 2 S ∂q f ∂q i 1/2 is of higher order in than the exponential of S.