Caramuel - Caramuel (GEO QKD Hosted Payload) Phase A

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The Caramuel project targets the full design, development, launch, and operation of the first Quantum Key Distribution (QKD) system implementation from geostationary orbit. The space segment will be embarked as a hosted payload on Hispasat’s next high-throughput satellite (HTS). The new ground segment to manage and control the QKD service enables its integration into the future European secure quantum communications infrastructure (EuroQCI).

During the Definition Phase of the project, the different technological and architectural alternatives were evaluated, in accordance with the state of the art and the requirements of the services to be delivered, leading to the definition and specification of the system and the associated development plan.


The Caramuel project goes beyond the state of the art since it is the first implementation of a QKD system from geostationary orbit.

Satellite quantum communications pose several challenges. First, the payload components must be built to withstand the demanding conditions of space operation, thus limiting in many cases the performance of optoelectronic components at the expense of robustness. Second, the quantum source optical beams shall be magnified with state-of-the-art telescopes to reduce the optical divergence and mitigate optical losses at the receiving optical terminal. Third, the atmospheric turbulence induces optical aberrations which are detrimental to the fiber optical coupling at the receiving terminal and need to be compensated with adaptative optics systems.

Therefore, aspects such as the low level of maturity of the technology, the large size and cost of the receivers and the low achievable key rate were tackled.


QKD systems provide the means to distribute symmetric random bit strings to be used as keys secure even against an eavesdropper with an unbounded computational ability. Due to the distance limitation of quantum communication via ground-based fibre networks (around 100 km), space-based QKD is a viable solution to extend such networks over global distances. Hence the integration of the satellite QKD system with the terrestrial networks.

Potential competitor systems such as satellite QKD from Low Earth Orbit (LEO) are not mature enough to deliver services; they provide lower availability (need of a satellite constellation); and the key management process can become very complex. Whereas one GEO satellite QKD covers nearly a third of the Earth with high availability.


QKD represents the highest level of certified security among all cryptographic solutions. The Caramuel QKD system, based on European quantum technologies know-how and integrated with EuroQCI, complements the terrestrial solutions providing longer-range key distribution with high availability from a GEO Satellite.

System Architecture

The Caramuel system architecture is illustrated in the next figure, which identifies the key state-of-the-art building blocks and major interfaces for the solution.

Caramuel system architecture
Caramuel system architecture

Due to the link budget limitations, the proposed payload operates on a single quantum link.

Caramuel being a hosted payload, strict constraints in terms of mass, centre of gravity, mechanical, electrical, and thermal compliance are imposed by the platform. On the other hand, the platform itself has an impact in the system performance, as it must provide telemetry information (satellite attitude, pointing, stability, and vibrations) for the proper the management of the link.

The main constituents of the payload are the: QKD processor, quantum random number generator (QRNG), photon source, optical assembly (telescope & optical bench), and synchronization channel.
The ground segment is made of two distinct building blocks, namely the: optical ground stations (OGS) and the operations centre.


The Definition Phase of the project (i.e., Phase A) lasted six months, with a Mid-Term Review (MTR) and a Final Review (FR).
In the MTR, the system architecture, building blocks and services were presented; along with the first system, operational and commercial mission feasibility assessments.

The FR was equivalent to a Preliminary Requirements Review (PRR) in terms of project maturity with respect to the outcomes: requirements, preliminary system architecture, development plan, risk register, and design and compliance matrix supported by analyses and simulations.

Current status

The Definition Phase of the project (i.e., Phase A) was successfully completed in December 2022.

The next Technology, Product, and Demonstration Phases (B, C/D, and IOV/Operation respectively) are planned to be carried out for the design, development, integration, launch, and operation of the QKD system.