  We show that frequent nondemolition measurements of a quantum system immersed
in a thermal bath allow the extraction of work in a closed cycle from the
system-bath interaction (correlation) energy, a hitherto unexploited work
resource. It allows for work even if no information is gathered or the bath is
at zero temperature, provided the cycle is within the bath memory time. The
predicted work resource may be the basis of quantum engines embedded in a bath
with long memory time, such as the electromagnetic bath of a high-Q cavity
coupled to two-level systems.
