  We present a scheme for key distribution based on bi-partite correlation of
single photons. Alice keeps an ancilla photon and sends a signal photon to Bob,
where intrinsic bi-partite correlation of these photons is obtained through
first order intensity correlation in their detectors. The key bits are
distributed through sharing four bi-partite correlation functions and photon
counting. The scheme consists of two parts; first, Alice prepares deterministic
photon states and Bob measures the photon states based on his random choice on
correlation functions. Second, Alice guesses Bob's choice of correlation
functions and sets the key bits by sending out photon states. Bob verifies the
key bits through the photon states regardless Alice made a right or wrong
guess. We called this key distribution scheme as prepare-measure-guess-verify
(PMGV) protocol. We discuss the protocol by using a highly attenuated laser
light, and then point out the advantages of using a fiber based correlated
photon-pair to achieve better performance in security, communication distance
and success rate of key distribution.
