Spontaneous Parametric Downconversion

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Quantum Optics and Spontaneous Parametric Downconversion

The goal of this project is to use a series table-top laser-based optics experiments to investigate various quantum mechanical phenomena. These include, but are not limited to: quantization of the electric field (proof of the existence of photons), single-photon interference, violation of Bell inequalities, and quantum information measurement effects.

Physics Background

Spontaneous parametric downconversion (SPDC) is a non-linear optical process that takes place with the assistance of specially-engineered optical crystals. These optical crystals are designed with specific index of refraction properties along given crystalline axis. When light of a specific frequency is incident upon the lattice, it will experience preferential absorption and re-emission as a result of this design. This will result in an overall "splitting" of one incident light beam into two, at some well-defined angle. The net effect of this can be seen by looking at the bulk beam properties.

In a project-specific example, consider a 405nm wavelength laser is incident upon a downconversion crystal. The net effect of the crystal refraction and re-emission results in two output beams of 810nm. The incident beam has been "downconverted" to two output beams of half-the energy and twice the wavelength. See figure 1 for an illustration, and the WikiPedia link in references for more detail.


[fig 1 SPDC diagram]


If we reduce the incident beam to a series of single photons, whose existence is a central postulate of quantum theory, the above description need only be slightly altered. A single photon incident on the crystalline lattice has a certain probability of being downconverted via the interaction with the lattice (roughly 1 in 10^12). When this conversion takes place, the single photon, with its inherent polarization properties, is converted into a pair of polarization entangled photons at half the energy and wavelength.

Experimental Setup


External Resources


SPDC Web Page