The main objective of this study is the development of core technological elements for an optical clock in space. The statement of work asked hereby to reach TRL 4/5 on subsystem level for the components of a medium term optical clock, which aims at an application in space in the next 10-15 years, and to reach TRL 3/4 for those of the "advanced clock type". The long term development goals for the advanced clock are expected to have clear synergies with the medium term solutions without the need for major modifications, and should improve on the medium term the clock performance by at least an order of magnitude.
A clock performance at the 10E-16 level both in fractional frequency instability and inaccuracy shall be furthermore demonstrated by testing after having implemented the developed core technological elements into an operational optical atomic clock. The possibility of reaching the 1-3 x10E-17 level shall then be made obvious.
In the successfully performed Phase 1 of the study, different clock concepts have been reviewed with respect to:
• Performance
• Engineering feasibility
• Space qualification aspects
• Calibration and operation
• TRL level
• Development effort associated with the complete clock
The focus of the clock review was laid on lattice clocks, single ion clocks, and quantum logic clocks, respectively. The survey concluded that an 88Sr+ ion clock is the best suited candidate for a medium term clock (within the next 10-15 years). The evolution to a quantum logic clock is regarded as the next logical step in the development. A high commonality for components was identified between the two clock types. For these reasons the survey resulted in a selection of an 88Sr+ ion clock as the medium term clock and an extension to a quantum logic clock based on 27Al+ as the advanced clock.
The critical subsystems of these clocks, for which a strong need for technology development has been identified to increase the respective TRL, are the following items:
• The Optical local Oscillator (OLO)
• The Physics Package (PP)
• The Clock Control Unit (CCU)
• A blue Auxiliary Laser (AL)
Based on this selection, the consortium has proposed a preliminary design of these subsystems on a conceptual basis laying a high emphasis on a commonality between the two clock types. A preliminary design report for the identified subsystems and necessary subsequent developments and tests was delivered to ESA at the end of Phase 1. A selection was finally made and justified for those subsystems to be breadboarded during the course of the next Phase 2 of this study.
To reach TRL 4/5 for the respective subsystems, the following issues have to be considered to bring the corresponding technologies to the feasibility of being operated in a space environment:
• Optimisation in consumption of valuable resources like power, volume, and weight
• Mechanical stability with respect to launch loads
• Robustness in design and workmanship to cope with potential harsh space environmental conditions
• Fostering of long-term operation with reliable performance figures
• Compatibility with at least semi-automated operations