  Here a special case of perturbation in quantum harmonic oscillator is
studied. Here we assume the perturbed potential to be a Harmonic Oscillator
that has been shifted in the position space.We construct the new creation and
annihilation operators for the new Hamiltonian to find out its energy
eigenstates. What is interesting about the solution of this system is that we
find out that the ground state of this system is a coherent state of the ground
state of the original harmonic oscillator. We know that the Hamiltonian of
electromagnetic field is similar to that of the harmonic oscillator where the
electric field and magnetic field are expressed in terms of time evolution of
annihilation and creation operators. According to the quantum electromagnetic
field theory, photon states are expressed as the coherent states of the ground
states of the unperturbed harmonic oscillator.In the previous research works,it
has been established how coherent states provide a better picture of Electric
field compared to the energy eigenstates. This paper mainly deals with why the
coherent states provide a more complete picture of electric and magnetic field
theory compared to just energy eigenstates. Here we find how our perturbation
act as a coherent state generator and adds a new term in the Hamiltonian. The
Perturbation is thought to arise from the only available interaction i.e.
observation. The perturbation also explains how observation leads to coherence
of photon states. We will find out how the new perturbation affects the time
evolution of electromagnetic field. This also explains how the classical
electromagnetic field theory takes into account the factor of observation, due
to which the classical laws are in perfect agreement with the quantum laws.
