Fig. 4.

Tomography of the demon state. Reconstructed density matrix ${\rho }_D$ by maximum likelihood at the end of the work extraction step 4, for ${\alpha }_{in}=0.25$ and when the system is initially (A) at temperature 0.10 K, (B) close to the excited state, (C) a superposition of ground and excited states, and (D) a maximally mixed state (see Bloch vector in Insets). Each pixel represents the amplitude of a density matrix element in the Fock basis, and the von Neumann entropies $S_D$ are given. (Wigner function is shown in SI Appendix.)