Lord Kelvin’s formulation of the second law of thermodynamics forbids any cyclic process with
the sole result that heat is absorbed by a system from a “bath” and transformed into work; i.e., there
can be no single-bath engine. An exception to this rule is that an observer acting as “Maxwell’s
demon” can commute information acquired by measuring the system into work in a single-bath
engine. Here we show that frequent measurements of the energy of a quantum system embedded
in a single bath enable the system to do work in a cycle even if the measurement results are unread
(or unknown).This finding cannot be attributed to “Maxwell’s demon” operation, since unread
quantum measurements provide no information. Nor can it be ascribed to quantum coherence in
the system, which is the source of work in recently explored quantum heat engines (QHEs),
since unread measurements destroy such coherence. Instead, unread measurements are shown
to enable the extraction of work from the system-bath quantum-correlation energy , which
constitutes a hitherto unexploited work resource. It becomes available only when the cycle is shorter
than the bath memory time. Strikingly, this resource allows more work to be obtained by Maxwell’s
demon when the measurements are read than anticipated by the standard relation of information
and work. The second law is reformulated to account for this resource. The predicted work
resource may be the basis of novel nanomechanical or spin-based QHEs  embedded in a single
solid-state bath.