Theory

This will bring to light a few of the nuances of the lab. Under construction

Optical Pump

The blue diode laser, referred to as the pump laser, serves as the optical source in this experiment. The pump laser is sent through the BBO down-converting crystal in order to produce the twin 810nm beams.

Down-Conversion

The 405nm beam passes through the BBO, experiencing down-conversion. Upon exiting the crystal, the 405nm beam is split into two 810nm beams, each at half the incident energy. These beams each emanate 3 degrees from normal incidence.

• • Note that many of the incident photons travel straight through.

Entanglement

The two 810nm beams emanate from the BBO crystal at precisely the same time. Therefore, if a single photon is detected in one beam, you know that a single photon was in the other beam at the moment of detection. Consider: if the light was down-converted by a ratio other than 50-50, the energy of one beam directly determines the energy of the other beam provided the energy of the incident beam is known. Thus the down-converted photon beams are considered to be entangled.

Wave Plates

The following image, taken from "Wikipedia-Waveplate"(Image License: CC BY-SA 3.0) briefly describes how a wave plate works.

A wave plate is a birefringent material that changes the polarization of light by letting certain polarizations of light pass quickly while delaying other polarizations of light. A wave plate has two axes: a fast axis, which allows light polarized in that direction to pass quickly; and a slow axis, which causes light polarized in that direction to be delayed relative to the fast axis. The total delay depends on the type birefringent material used and the geometry of the material. In our experiment, the birefringent material is quartz. The length(along the axis of propagation) of the material directly affects change in polarization. Depicted in the picture above is a Half Wave Plate. The blue component of the light is along the fast axis and the green component is along the slow axis. If we trace the blue wave, we see that it completes 3 and 1/2 cycles. This material is deemed a Half Wave Plate because the fast axis was shifted one half wavelength. Specifically, this is a 3rd order Half Wave plate because the length of the material is three wavelengths plus one half wavelength. If we were to cut the crystal exactly in half, we would get a multi-order Quarter Wave Plate because the relative phase change would be 1/4 of a cycle.