Imagine now feeding a short pulse to a process that is efficient only in a narrow region of the spectrum, such as conventional parametric down-conversion (a process that splits a photon into two): only the photons that are around a specific frequency will be split; the rest will just go through. This is unfortunate, because when timing is an important issue (think communications, quantum information processing, etc..), pulses must be short and this implies that they must have a large bandwidth. Does this mean we cannot make use of down-conversion for these technologies?
Kwang Jo Lee and colleagues found a workaround: a down-conversion crystal that is efficient for a wide range of frequencies! They grew it artificially while an external electromagnetic field was periodically switched, so the various layers had a spontaneous polarization axis pointing in different directions. This technique is called "periodic poling". By optimizing the directions and other parameters, they managed to produce a non-linear crystal that can be efficient at splitting photons in a 200 nm window of the mid-infrared frequencies (3-5 microns). This idea can find a variety of applications in quantum communications and information processing with unique advantages.
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