  A quantum-mechanical analog of the Carnot engine reversibly working at
vanishing temperature, shortly termed the quantum-mechanical Carnot engine, is
discussed. A general formula for the efficiency of such an engine with an
arbitrary confining potential is presented. Its expression is purely given in
terms of the structure of the energy spectrum. Dependency of the efficiency on
the form of a potential as an analog of the working material in thermodynamics
implies nonuniversality of the engine. This may be due to the absence of the
second-law-like principle in pure-state quantum mechanics.
