Collective thermalization, work reliability, and resource bounds in a population-inverted Dicke Otto engine

Population inversion and collective relaxation can both enhance the performance of a quantum heat engine, but through distinct mechanisms. We study a quantum Otto engine whose working medium is a symmetric collective spin of N two-level constituents. For commuting work strokes and collective reservoir coupling, the dynamics reduces exactly to a finite birth--death process on the Dicke ladder, allowing a unified treatment of stationary operation, work fluctuations, and finite-time relaxation. In the complete-reset limit, passive work per cycle saturates with system size, whereas population inversion yields work that grows linearly with N. The work reliability shows the same linear scaling, exceeding the square-root behavior of N independent engines. This enhancement originates from a macroscopic polarization displacement between hot and cold states, while collective coupling instead accelerates the finite-time dynamics through Dicke transition rates. For matched passive and inverted hot states, the extra gross work equals the Otto efficiency times the ergotropy of the inverted state. Charging the corresponding reversible excess state-formation cost removes this advantage, showing that the enhanced output converts a pre-existing active-state resource rather than generating a free thermodynamic gain. Exact finite-contact trajectory statistics further quantify how collective kinetics, fluctuations, correlations, and resource accounting remain linked away from complete reset.