  Recently highly-efficient quantum engines were devised by exploiting the
stochastic energy changes induced by quantum measurement. Here we show that
such an engine can be based on an interaction-free measurement, in which the
meter seemingly does not interact with the measured object. We use a modified
version of the Elitzur-Vaidman bomb tester, an interferometric setup able to
detect the presence of a bomb triggered by a single photon without exploding
it. In our case, a quantum bomb subject to a gravitational force is initially
in a superposition of being inside and outside one of the interferometer arms.
We show that the bomb can be lifted without blowing up. This occurs when a
photon traversing the interferometer is detected at a port that is always dark
when the bomb is located outside the arm. The required potential energy is
provided by the photon (which plays the role of the meter) even though it was
not absorbed by the bomb. A natural interpretation is that the photon traveled
through the arm which does not contain the bomb -- otherwise the bomb would
have exploded -- but it implies the surprising conclusion that the energy
exchange occurred at a distance despite a local interaction Hamiltonian. We use
the weak value formalism to support this interpretation and find evidence of
contextuality. Regardless of interpretation, this interaction-free quantum
measurement engine is able to lift the most sensitive bomb without setting it
off.
