  One of the peculiar features in quantum mechanics is that a superposition of
macroscopically distinct states can exits. In optical system, this is
highlighted by a superposition of coherent states (SCS), i.e. a superposition
of classical states. Recently this highly nontrivial quantum state and its
variant have been demonstrated experimentally. Here we demonstrate the
superposition of coherent states in quantum measurement which is also a key
concept in quantum mechanics. More precisely, we propose and implement a
projection measurement onto the arbitrary superposition of the SCS bases in
optical system. The measurement operators are reconstructed experimentally by a
novel quantum detector tomography protocol. Our device is realized by combining
the displacement operation and photon counting, well established technologies,
and thus has implications in various optical quantum information processing
applications.
