# Introduction to PDC

Parametric down-conversion (PDC) is a quantum optics process that involves the conversion of a high-energy photon into two lower-energy photons. This process is carried out by a nonlinear crystal, which splits the energy of an incoming photon into two daughter photons with half the energy. The daughter photons are usually emitted at a different angle than the incident photon, a phenomenon called non-collinear, and are often polarized perpendicular to each other.

# How PDC works

PDC is based on the principle of conservation of energy and momentum. During PDC, a high-energy photon interacts with a nonlinear crystal and generates two daughter photons. The process obeys the conservation of energy since the sum of the energies of the two daughter photons is always equal to the energy of the incident photon. The conservation of momentum is also obeyed since the direction of the daughter photons is determined by the direction of the incident photon and the crystal structure.

# Applications of PDC

PDC has several applications in quantum optics, including quantum cryptography, quantum teleportation, and quantum computing. PDC is used to generate entangled photon pairs, a process that is crucial in quantum cryptography and quantum teleportation. Entangled photon pairs are also used in quantum computing for quantum gate operations, a necessary step in quantum algorithms. PDC is also used in other areas such as precision measurement and imaging.

# Example of PDC in action

PDC has been demonstrated in several experiments, including the production of entangled photon pairs, quantum teleportation, and quantum computing. In a famous experiment, researchers used PDC to generate entangled photon pairs and demonstrated quantum teleportation over a distance of 10 miles. PDC has also been used in quantum computing experiments, where researchers used entangled photon pairs to perform quantum gate operations. PDC has a broad range of applications in quantum optics and is a critical tool for developing quantum technologies.