Entangled photon pairs generated via spontaneous parametric down conversion have had many applications such as quantum key distribution(QKD), quantum teleportation, in quantum relay chip, interfacing in quantum memories and generally in quantum information systems. Are there any other possible potential applications of these photons?
1) Ghost imaging https://en.wikipedia.org/wiki/Ghost_imaging
"Specifically, quantum correlations between photon pairs were utilized to build up an image. One of the photons of the pair strikes the object and then the bucket detector while the other follows a different path to a (multi-pixel) camera. The camera is constructed to only record pixels from photons that hit both the bucket detector and the camera's image plane."
2) Measurements in coincidence. The so-called pair-coherence time of the two photons created in down-conversion is extremely short, τpair ~100 x 10-15 seconds. That means, if one of the two photons is detected, the other one is detected within this interval of time. In many experiments, this interval is completely negligible, s.t. we can consider that the two photons arrive at the same time.
2) Another application - however, rather for scientific research purpose than for engineering - is the creation of the Fock state |1>.
A Fock state |n> is a state containing a fix number, n, of particles. For instance, sometimes we are interested to examine the properties of a single photon. This is the Fock state |1>. It is not simple to create such a state. Sometimes we obtain it in an approximate way, from a very weak coherent state, the later being easily generated with lasers. But, an exact way is the use of the SPDC: if we catch one such photon, we know that within a window of time ~ 100 femtoseconds, we will get another, single particle, i.e. a particle in the Fock state |1>.
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