  The essence of both classical and quantum engines is to extract useful energy
(work) from stochastic energy sources, e.g. thermal baths. In Maxwell's demon
engines, work extraction is assisted by a feedback control based on
measurements performed by a demon, whose memory is erased at some nonzero
energy cost. Here we propose a new type of quantum Maxwell's demon engine where
work is directly extracted from the measurement channel, such that no heat bath
is required. We show that in the Zeno regime of frequent measurements, memory
erasure costs eventually vanish. Our findings provide a new paradigm to analyze
quantum heat engines and work extraction in the quantum world.
