Quantum Communication Technologies

Quantum communication enables the tap-proof exchange of keys for encoding security-relevant information. In contrast to algorithmic cryptography methods, whose security is guaranteed by the computing effort involved in decryption, the security of quantum cryptography is based on physical principles such as quantum entanglement or the superposition principle. Fraunhofer IOF is developing actively, together with partners in research and industry, quantum communication systems, which can be applied to existing fiber-networks as well as in terrestrial-free space links.

Long-term data security

The goal of the QuNET initiative is to develop the physical-technical fundamentals of quantum communication systems and optical link technologies for use in real infrastructure - targeted for application in high-security networks. R&D contributions of Fraunhofer IOF are supported by expertise and competence along the entire quantum photonic process chain. In addition to components and key technologies, overall systems for quantum communication in real free-beam and fiber network infrastructure are investigated and demonstrated.

Quantum communication experiment

In the first phase of the project QuNet a key experiment is conducted at the interface between quantum channels in different wavelength bands and transmission media: a necessary development for future integration into a heterogeneous network architecture. In a technology demonstrator, a secure connection between two buildings is made possible via an optical free-space link. Elementary building blocks of the demonstrator are reflective telescopes with precision optics for the optimized transmission of polarization-encoded quantum states, a polarization-entangled photon-pair source, and post-processing units, which include single-photon state analyzers and key management system to generate quantum keys.

The telescopes

Each of the telescopes is equipped with an active beam tracking system and they provide an aperture of 20 cm, which allow a free beam link. The entangled photon-pair source emits photon-pairs with a rate of up to 1 Million photons per second at a wavelength of 810 nm. A second source, which is non-degenerate and thus emits at 810 and 1550 nm, can be implemented to connect the optical free-space link to an optical fiber network. The detection system guarantees tap-proof communication between the endpoints using high time resolution and precise synchronization in the pico-second range.

The security of quantum communication in this case is based on the so-called BBM92 protocol for polarization-entangled photons.

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.

Setup

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.

Space suitability

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