The ability to generate entangled photons is a key prerequisite for several quantum applications like quantum information processing, quantum repeaters, or quantum cryptography. In particular, for the implementation of long distance quantum communication, as a matter of example for fiber-based quantum key distribution, it becomes compulsory to employ low loss transmission channels.

In this context, semiconductor quantum dots have shown enormous potential in fulfilling essential requirements of quantum technologies. These quantum dots are tiny man-made crystals of the size of few nanometers. They are very powerful devices capable of converting light into nearly any color in the visible spectrum with very high efficiency. The novelty is the use of these quantum dots inside the already developed global glass fiber network. But for that a quantum emitter must match the telecom bands, especially the C-band for which an absolute minimum of the absorption is found.

The electronic characteristics of quantum dots are determined by their size and shape. It is possible to control the color of light given off by a quantum dot just by changing its size. Bigger dots emit longer wavelengths like red, while smaller dots emit shorter wavelengths like green.

A team from Germany developed a new approach for the emission of polarization-entangled photons from a single semiconductor quantum dot using the telecom C-band (1530 nm–1565 nm). In this work, they reported the possibility of emitting in the telecom C-band using photonic state tomography. With this device, they demonstrated the first emission of polarization-entangled photon pairs from a QD near 1.55 μm. To reach this telecommunication window, they used an InAs quantum dots embedded in InGaAs barriers.

illustration of energy band diagram of the active region

The team was impressed by the results obtained. The quality of the signal was great and it let hope for great possibilities.

“The chance to find a quantum dot that is able to emit polarization-entangled photons with high fidelity is quite high for our specific study. […] The hard part now is to combine all the advantages of the system and fulfill prerequisites such as high photon indistinguishability, high temperature operation, increased photon flux and out coupling efficiency that would make them work.”

Fabian Olbrich, Center for Integrated Quantum Science and Technology, Germany

Researchers hope that one day, entangled photons will impact cryptography and secure satellite communications.

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