For certain applications with an enduring information-assurance requirement, concepts like confidentiality, integrity, availability and eavesdropper’s detectability are becoming of paramount importance in our increasingly networked world.
For some space communications, it is essential to provide new secure-communications methodologies that have superior long-term security assurances. Based on the rules of Quantum Physics, Quantum Key Distribution provides means for two (or more) parties to exchange with complete security an enciphering key over a private quantum channel, meaning that the presence of an eavesdropper will always be detected. Over the past decade, novel quantum cryptographic protocols have been proposed, important security proofs established, and experiments that implement the principles of QKD have been demonstrated in laboratories and universities around the world.
On Earth, quantum communication channels are limited because optical fibre link losses and current photon-detector technology limit the maximum span length without using amplification to several hundreds of kilometres; while for free space transmission the limit is the visible horizon. In principle, limitations in transmission length are nearly absent in space, and are less severe in ground to space links. In fact, quantum links in free space combined with fibre counterparts could extend secure communication between points on earth to a global level.
It is for what the development of a Quantum Photonic Transceiver (QPT) that is capable of generating and detecting secure encrypted key and its future accommodation in an optical communication terminal on board of a satellite has become one of the most promising activities.
Therefore, the objective of the project was to develop a QPT source that is capable of generating and detecting entangled photons (EPS) as well as faint laser pulses (FPS). The QPT source design and development was mainly focused on the transmitter part of the photonic transceiver (i.e. implementation of an EPS and FPS source) because the receiver part was not subjected to general requirements, hence it can be built upon commercially available components.
In addition to the design and implementation of these two QKD sources, their performance and compatibility with space environment was also demonstrated by a suitable combination of analysis and testing, in order to determine the operational limits and to identify any design or technological weaknesses. Recommendations based on the achieved performances were also included to establish the way towards a future space qualification.
The development of this type of sources (i.e. EPS and FPS) proved to be a challenging task because of the highly demanding conditions in alignment precision, mechanical stability and also in terms of its optical/electronic specifications. In relation to space compatibility, the aim was not to space qualify all electro-optical parts but to verify the correct functional performance of those parts under environment constraints. For individual optical parts, the preliminary assessment plan carried out in the frame of this project revealed the potential space compatibility and feasibility in future FM system.
In conclusion, encouraging results were accomplished proving that the proposed photonic transceiver approach could be a worthwhile option to fulfil the technical specifications required in the contract. Based on the achieved performances, the project finished with recommendations to be taken into consideration on both QKD sources in order to establish the way towards a future space qualification unit.
Trade off of several entangled photon and faint pulse sources optical design technologies were evaluated to select the most suitable options in terms of optical requirements fulfilment, robustness and maturity.
These sources require highly demanding conditions of alignment precision and mechanical stability to meet optical and electronic requirements.
The development of proximity electronics capable of accomplishing technical requirements has been also achieved. There is no indication that the developed electronics could not go through a qualification process as there is no need of any special processes or materials to implement it.
A robust and compact opto-mechanical design and manufacturing process was implemented for both sources. Parameters limiting their performance were identified so proposal of improvements in the opto-mechanical assembly to overcome these limitations was also included. The opto-mechanical assembly improvements should be taken into consideration in case that future QPT implementation.
The study has concluded that a suitable photonic transceiver capable of distributing quantum keys in an inherently secure way may be implemented using space qualified materials, optical and electronic components.
The study has been divided into the following work packages:
- Preliminary design of the photonic transceiver- The preliminary design of the photonic transceiver took into consideration the EPS and FPS as two separated sources. It was defined considering: technology assessment, its performance model and predictions as well as the identification of critical optical components/parts whose suitability for space use had not been assessed or substantiated beforehand.
- Critical component selection and integration assessment - the selection and evaluation of critical optical components for EPS and FPS source bearing in mind integration aspects into the overall photonic transceiver, was carried out in this work package. The critical aspects of each component were established taken into consideration their opto-mechancial tolerances and their performances verification under selected environmental test. These environmental tests included: thermal stability, vibration hardness, radiation hardness to gamma and proton and vacuum stability.
- Manufacturing design of the photonic transceiver- Detailed optical, electronic, mechanical and thermal design of each QKD source was established in this work packaged. It was done based on the findings encountered during the integration assessment. The complete performance verification programme was also defined at subsystem/component and at breadboard level.
- Procurement, manufacturing, subsystem assembly and complete system integration - The work package dealt with the manufacturing and procurement of the established EPS and FPS design. It also included all the tasks related to the QKD sources mounting, alignment at subsystem and system level.
- Performance verification - the performance of both sources was experimentally carried out under the specified operational conditions. Test results were compared with the predictions (compliance matrix) identifying potential discrepancies.
- Critical Appraisal and recommendations- Recommendations based on the achieved performances were established in order to face future space qualification.
The study has been completed. Test results, critical appraisal and recommendations based on achieved performance have been presented for one Entangled Photon and Faint Pulse source respectively.