View into the telescope system for quantum communication

Experten des Fraunhofer-Instituts für Angewandte Optik und Feinmechanik in Jena haben eine neue satellitengestützte Quelle für verschränkte Photonen entwickelt. Solche verbundenen oder »verschränkten« Photonen kommen bei sicheren Verschlüsselungstechnologien zum Einsatz. Von der Photonenquelle aus werden über Faserleitungen oder den freien Raum eine Reihe von Photonen an einen oder zwei Empfänger versendet. Nachdem diese umgewandelt wurden, dienen die Photonen als Schlüssel zur Kodierung der ursprünglichen Nachricht. Diesen Vorgang nennt man Quanten-Schlüssel-Verteilung (QKD). Die QKD-Technologie wird Teil einer neuen Generation weltraumgestützter Lasersysteme sein, die eine sicherere und schnellere Kommunikation zwischen Satelliten sowie zwischen Satelliten und Bodenstationen ermöglicht. Experts from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena, developed a new satellite-based source for entangled photons. Twinned or »entangled« photons can be used for secure encryption. A quantum photon source may send a series of photons to one or two receivers through fibers or free space, and after some processing, the series may serve as a key for the encryption of the actual message. This is called quantum key distribution (QKD). QKD technology will be part of a new generation of space-based laser systems that will allow faster and more secure communication among satellites and between satellites and ground stations. Stable sources for polarization or wavelength entangled photons - Applicable to quantum imaging | quantum key distribution (QKD) - Suitable for harsh environments (vacuum, space) - Providing key parameters (brightness, efficiency, visibility) Polarization-entangled photon source - Based on a Sagnac-interferometer - Spontaneous parametric down-conversion - Bulk periodically poled potassium titanyl phosphate crystal - Visibility: > 98 % - Pumping power: ca. 5 mW - Entangled photon pairs per second: > 300.000

Entangled photon pair source for satellite based quantum communication

Quantum Communication Technologies

Researching complex quantum states of light is at the core of our work in the Quantum Communication Technologies Group at Fraunhofer IOF with emphasis on developing innovative technologies in long-range quantum communications, remote sensing, distributed quantum networks, and quantum information processing. Our research areas comprise:

  • Employing adaptive optics and quantum hardware for satellite-based quantum communication
  • Exploiting high-dimensional photonic entanglement and hyperentanglement for distributed quantum information processing
  • Engineering the spatial and spectral structure of biphotons
  • High-dimensional quantum information processing in the spatial and temporal domain
  • Developing technologies for ranging and remote sensing with non-classical states of light

Quantum research methods

Equipped with cutting-edge technology, our research group employs the following novel experimental methods:

  • Single photon detectors and time-tagging electronics
  • Ultra-fast electro-optic modulators and pulse shaping technology
  • Ultra-bright entangled photon pair sources
  • Wavelength-division multiplexing technology
  • Spatial light modulators and adaptive optics
  • Pulsed and cw-lasers at different wavelengths  


Recent quantum research results

Our research is focused on applied quantum photonics – from quantum optical concepts to system-level application in quantum communication networks. Our latest results include the demonstration of polarization-entangled photon sources with exceptional pair generation rates, innovative approaches to quantum-enhanced sensing using multi-photon interference, as well as harnessing spatially-encoded and frequency-encoded states for high-dimensional quantum information processing. Propelling quantum technology to market-ready applications is also one of our goals which we pursue in tandem with other partners, focusing on the development of quantum payloads for satellites and quantum communications systems for free-space and fiber networks in metropolitan areas.


Scientific publications in the field of quantum communication

The research strength of our research group is also reflected in the scientific publications.

A list of scientific papers on the subject of quantum communication research can be found below:

Scientific publications of the Quantum Communication Group

Our research highlights in the field of quantum communication systems

Hybrid quantum communication via free-space and fiber links

Complete telescope system.
© Fraunhofer IOF
Telescope system for quantum communication.
View into the golden telescope system.
© Fraunhofer IOF
View into the telescope system without obscuring secondary mirror.

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.

Authors: Daniel Rieländer, Fabian Steinlechner, Matthias Goy, Christopher Spiess, Andrej Krzic, Gregor Sauer, Sakshi Sharma, Sebastian Töpfer

Entangled photon pair source for quantum communication

EPS – Entangled photon pair source.
© Fraunhofer IOF
EPS – Entangled photon pair source.
View inside the periodically poled  KTP crystal, which is elementary for the creation of stable and bright polarization entangled photon pairs.
© Fraunhofer IOF
View inside the periodically poled KTP crystal, which is elementary for the creation of stable and bright polarization entangled photon pairs.

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.

© Fraunhofer IOF
Pair count rate at 35 °C homogeneous crystal temperature.
© Fraunhofer IOF
Pair count rate at 35 °C homogeneous crystal temperature.

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