This thesis discusses four-wave mixing (4WM) in a warm ensemble of rubidium

using the diamond configuration level structure. Both classical 4WM and non-

classical photon-pair production are investigated.

Quantum information science has spawned a great amount of experimental

work on the interaction of light with collective modes of excitation in atomic ensem-

bles. Plans to build quantum networks and quantum repeaters with atom ensembles

take advantage of nonlinear interactions to produce and store non-classical states

of light. These technologies will require photon sources that not only generate non-

classical light, but also resonant, narrow band light. Here we investigate a system

which could be used as such a source.

We take advantage of the 4WM interaction in a warm ensemble of Rubidium

atoms. Our scheme utilizes the diamond energy level configuration which, in ru-

bidium, allows for correlated pairs at telecommunications wavelengths. We start by

examining the properties of classical 4WM in the system. We measure the reso-

nance structure and see that it can be understood in terms of velocity class selective

resonant enhancement and power splitting effects. The efficiency of the process is

low and limited by linear absorption of the pumps. Our observations agree with a

semi-classical Maxwell-Bloch theoretical treatment.

Next we observe pair generation by spontaneous 4WM from the warm ensem-

ble. The temporal profile of the cross-correlation function (CCF) for the photons

depends on pump-laser power and detuning. This allows us to produce biphotons

with controllable spectra. A simple quantum optical theoretical treatment based on

linear filtering gives qualitative agreement with the data.

We show that the photon pairs are polarization entangled, clearly violating

Bell's Inequality. A perturbative quantum optical treatment predicts the polariza-

tion state of the pairs and agrees with our measurements. We analyze the photon

statistics of the source and find the largest violation of the two beam Cauchy-Schwarz

inequality from a warm atomic source yet. We cast the system as a heralded sin-

gle photon source at telecommunications wavelengths and see that it is competitive

with other systems in terms of spectral brightness.