European Space Agency


Quantum Key Distribution (QKD) aims to solve the problem of key exchange by the use of quantum mechanics and thus offers the potential of ensuring “unconditional secure” key exchange for encrypted communication. For the identification of practical scenarios for QKD from space and in particular with regard to the limited computational/communication resources available on a satellite the following 4 objectives are covered by the project activities:

  1. Development of a model (analytical or numerical) to calculate and optimize the performance parameters of suitable QKD links, taking into account:
    1. Quantum channel characteristics
    2. QKD protocol parameters
    3. Post-processing requirements
  2. Integration of the model into a software suite, used to identify the performance limitations of the selected protocols and implementation scenarios. The obtained results shall be complemented with an assessment of:
    1. Technological state-of-the-art of required hardware/software components
    2. Potential side-channels
    3. Protocol security level under realistic conditions
  3. Assessment of the TRL level of the hardware components that need to be launched to space for implementing the QKD protocol between space and ground.
  4. Proposal of a technology development roadmap of the critical hardware for selected QKD implementations and analysis of the impact on the protocol performance.


QTL has a strong scientific background related to space-based quantum key distribution implicating an overview of scientific literature and the technology relevant for QKD implementations, in particular for space applications. Concerning the timely satisfaction of the requirements of the activity, the preliminary results already presented show that the basic software framework is already existing at QTL. Additionally, essential QKD protocols are already integrated, and large parts of the software can be reused for the integration of additional QKD protocols. Based on these facts, we don’t anticipate challenges that could have a severe impact on the work carried out during the activity.


While classical link budget calculations are widely known and sufficient for calculating the received photon rate, calculating the secure key rate strongly depends on many parameters of used QKD protocol, the post processing and the performance of the QKD hardware (transmitter and receiver). Knowledge and tools to calculate the secure key rate for a given optical link scenario are therefore important for assessing different QKD service architectures.


The basic software framework is already existing at QTL; some important QKD protocols are integrated and are ready to be used. In addition to calculations/optimizations directly related to the individual QKD protocols, the software features simulations/calculations of relevant parameters related to the optical quantum channel. It consists of 4 modules.

Module 1 allows us to calculate and optimize relevant parameters regarding the optical quantum channel (e.g., link distance, elevation angle, beam-size on ground, etc.) depending on a set of input parameters (e.g. orbital parameters, wavelength, up- or downlink, etc.).

The second module allows to simulate the expected experimental measurement results during the exchange phase of the quantum signal.

Module 3 takes into account “finite-size effects” and covers the analysis of the amount of classical communication that is required for the individual post-processing steps as well as the amount of pre-shared quantum key needed for the authentication of the classically exchanged messages. Consequently, the net secure key gain can be obtained.

The optimal mean photon numbers in a decoy-state DV-QKD protocol strongly depends on the noise in the quantum channel. Module 4 will set out a solution to this problem with the help of numerical optimization of the net-secure key gain.

System Architecture

The software framework consists of 4 modules:

Module 1: Optical Quantum Channel

  1. allows calculating and optimizing various quantum channel parameters

Module 2: Protocol specific performance and security analysis

  1. allows simulating the QKD protocol performance during the quantum communication phase for given parameter sets
  2. allows establishing the security level of specific QKD protocols

Module 3: Post-Processing

  1. allows simulating the parameter estimation phase of the QKD protocol
  2. allows to calculate post-processing requirements with respect to resources related to classical communication and computation

Module 4: Optimization

  1. allows optimizing the full parameters set for a QKD protocol with regard to different target functions

QTL’s software framework constitutes a relevant part for simulating the performance of selected QKD protocols but is not subject to any IP Rights.


Duration: 6 months

Milestone 1:

  • Negotiation Meeting, before contract signature
  • Kick-off Meeting, M0, video-/telephone conference
  • Progress Meeting 1, end of WP 200, M1, video-/telephone conference
  • Mid Term Review Meeting, end of WP 300, M4

completed with the acceptance of all deliverable items at the Mid Term Review

Milestone 2: 

  • Progress Meeting 2, end of WP 400, M5, video-/telephone conference
  • Final Review Meeting, end of WP 500, M6
  • Final Presentation, Contract Closure, Date is TBD, end of M6

completed with the final presentation and acceptance of the End-of-Contract documentation

Current status

Based on a broad literature research, the three most promising QKD protocols among the currently known and secure ones have been selected. It was shown that these protocols can reliably fulfil the theoretical assumptions in a practical scenario, even in harsh environmental conditions. Hence, they can safely be regarded as information-theoretically secure and thus protected by quantum physics laws. To further analyse the impact of the specific characteristics of the protocols, the employed satellite constellation (LEO, MEO, GEO), and other relevant technology-related parameters, QTLabs developed a simulation software for the different QKD protocols. As a result of a first broad performance study it was shown that suitable amounts of secure quantum key can be generated in general and that the technology is ready for further development towards space readiness. The current study, however, did not investigate the immense potential that lies in the general mission design, which constitutes an important next step for further investigations

Status date

Monday, January 11, 2021 - 08:55