## ABSTRACT

In Sec. 15.4 we introduced polychromatic single-photon states in Hilbert space [Eq. (15.80)],

|Φ〉 = L− 32 ∑ i

φi|Pi〉, i = (q, s), (16.1)

by linear superposition of monochromatic plane (P) one-photon states

|Pi〉 ≡ |1i〉 = aˆ†i |0〉. (16.2)

In the one-photon subspace the |Pi〉 states satisfy the orthonormality,

〈Pj |Pi〉 = δij , (16.3)

and completeness ∑ i

|Pi〉〈Pi| = 1 (16.4)

relations. Instead of taking the |Pi〉’s as a basis set for the expansion of |Φ〉, one may use different sets of one-photon wave-packet states, as we now shall see. The generalization to wave-packet expansion is of importance from a fundamental point of view, and in this chapter we shall realize the usefulness of such an expansion for single-photon correlation studies.