  The Szilard engine (SZE) is the quintessence of Maxwell's demon, which can
extract the work from a heat bath by utilizing information. We present the
first complete quantum analysis of the SZE, and derive an analytic expression
of the quantum-mechanical work performed by a quantum SZE containing an
arbitrary number of molecules, where it is crucial to regard the process of
insertion or removal of a wall as a legitimate thermodynamic process. We find
that more (less) work can be extracted from the bosonic (fermionic) SZE due to
the indistinguishability of identical particles.
