Quantum Communication Technologies at Fraunhofer IOF

Quantum Key Distribution (QKD) is the distribution of keys to sender and receiver for encrypting and decrypting data. The key information is carried by photons that are quantum mechanically entangled. This means that a pair of entangled photons, where one photon is going to the sender and the other to the receiver, cannot be eavesdropped by a third party without destroying the entanglement state, which can be observed by sender and receiver. QKD is thus believed to be the future of encrypted communication; it provides keys that are safe from being hacked by means of the laws of physics.

Due to the required long coherence length of entangled photons that can only currently be realized by free-space optics, satellite optical links are preferred to distribute keys over large distances to different customers. At Fraunhofer IOF, the prototype of a space-suitable Entangled Photon Source (EPS) has been developed for such satellite links within the framework of the European Space Agency ARTES telecom program.

The EPS is based on a hybrid setup, where a non-linear, periodically poled KTP crystal is double-side pumped using up to 8 mW pumping power at 405 nm in a sagnac-interferometer scheme. The pumping power triggers spontaneous down conversion (SPDC) in the crystal that generates polarization entangled photons within sender and receiver channel at rates up to 300.000 pairs per second and at visibilities between 96 - 99 %. At higher pumping power, it is expected that up to 1 mio. pairs/s can be achieved, which will be the basis for efficient key transmission at highly attenuated optical links.

For the space-suitable design of the EPS, a compact, precision mechanics, thermo-mechanically stable platform has been selected, on which the optical setup of the source was integrated via effective and deterministic assembly algorithms, achieving high accuracies. Temperature leveling of the ppKTP crystal within this platform was realized at temperature homogeneity of 0.1 K along the 30 mm long optical axis of the crystal. To fixate the sensitive alignment state of the EPS with respect to long-term stability under space conditions, specific laser based soldering and optics glueing technologies have been used. The source was positively evaluated for its quantum optical parameters within and after typical test cycles for space assembly for thermal and mechanical loads as well as thermal-vacuum.

As a result of the development activities and results, it is planned to further optimize the EPS and implement it in an upcoming satellite mission for the demonstration of QKD links.

Authors: Erik Beckert, Oliver deVries, Christoph Damm