  We propose quantum engines powered entirely by the quantum measurement
process. Our theoretical construction of the engine requires no work from the
system Hamiltonian, and takes energy only from the process of observation to
move a particle against a force. We present results for the work done and the
efficiency for different values of the engine parameters. Feedback is required
for optimal performance. We find that unit efficiency can be approached when
one measurement outcome prepares the initial state of the next engine cycle,
while the other outcomes leave the original state nearly unchanged.
