  Quantum Szilard engine constitutes an adequate interplay of thermodynamics,
information theory and quantum mechanics. Szilard engines are in general
operated by a Maxwell's Demon where Landauer's principle resolves the apparent
paradoxes. Here we propose a Szilard engine setup without featuring an explicit
Maxwell's demon. In a demonless Szilard engine, the acquisition of which-side
information is not required, but erasure and the related heat dissipation still
take place implicitly by the very nature of the work extraction process. We see
that the insertion of the partition in a quantum Szilard engine does not
localize the particle to one side, instead it creates a superposition state of
the particle being in both sides. To be able to extract work from the system,
particle has to be localized at one side. The localization occurs as a result
of quantum measurement on the particle, which shows the importance of the
measurement process regardless of whether one uses the acquired information or
not. In accordance with the Landauer's principle, localization by quantum
measurement corresponds to a logically irreversible operation and for this
reason it has to be accompanied by the corresponding heat dissipation. This
shows the validity of the Landauer's principle even in quantum Szilard engines
without Maxwell's demon. Furthermore, we take quantum confinement effects fully
into account to analyze the Szilard cycle in the quantum regime thoroughly and
obtain highly accurate analytical expressions for work and heat exchanges. Our
results show that Landauer's principle holds the key role to understand the
thermodynamics of the localization of the particle by quantum measurement,
which explicitly saves the second law in demonless engines and shows that
quantum-mechanical considerations are essential to reconcile thermodynamics and
information theory.
