In a recent paper a quantum heat engine operating on radiation pressure from a single mode radiation field which drives a piston engine was presented. A thermally excited atomic beam serves as the high temperature energy source and a lower temperature heat bath acts as the entropy sink. We here extend the previous macroscopic (quantum thermodynamical) analysis by developing a microscopic (quantum statistical) analysis. This provides insight into engine operation and questions related to the second law. The same quantum phase effects that yield lasing without inversion and ultraslow light, also make possible extensions of Carnot cycle operation; e.g., extraction of energy from a single heat bath, and efficiency beyond the Carnot limit.