  It had been a long standing problem that there is no consistent definition of
photon position operator nor photon number density in the context of quantum
theory. In this paper we derive the photon detection operator, which defines
location of photon absorption, by applying the theory of indirect measurement
to quantum electrodynamics. It is shown that the photon detection probability
depends on electric properties of a photon-absorbing atom, in particular, on
both electric and magnetic dipole moments of the atom. An experiment is
proposed, in which the complementarity of wave-particle nature of light will be
tested. It is also discussed that the complementarity is related to the
non-commutativity of the electric and the magnetic fields.
