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Towards Neutral-atom Space Optical Clocks: Development of high-performance transportable and breadboard optical clocks and advanced subsystems

Final Report Summary - SOC2 (Towards Neutral-atom Space Optical Clocks: Development of high-performance transportable and breadboard optical clocks and advanced subsystems)

Executive Summary:
The use of ultra-precise optical clocks in space (“master clocks”) will allow for a range of new applications covering the fields of fundamental physics (tests of Einstein's theory of General Relativity, time and frequency metrology by means of the comparison of distant terrestrial clocks), geophysics (mapping of the gravitational potential of Earth), and astronomy (providing local oscillators for radio ranging and interferometry in space).
Within the ELIPS program of ESA, the “Space Optical Clocks” (SOC) project aims to install and to operate an optical lattice clock on the ISS towards the end of this decade, as a natural follow-on to the ACES mission (which is based on a cesium microwave clock), improving its performance by at least one order of magnitude.
The payload is planned to include an optical lattice clock, as well as a frequency comb, a microwave link, and an optical link for comparisons of the ISS clock with ground clocks located in several countries and continents. Undertaking a necessary step towards optical clocks in space, the EU-FP7-SPACE-2010-1 project no. 263500 (SOC2) (2011-2015) had the goal to develop two “engineering confidence“, accurate transportable lattice optical clock demonstrators having relative frequency instability below 1E-15 at 1s integration time and relative inaccuracy below 5E-17. This goal performance is about 2 and 1 orders better in instability and inaccuracy, respectively, than today’s best transportable clocks. The devices are based on trapped neutral ytterbium and strontium atoms. One device would be a breadboard. The two systems would be validated in laboratory environments and their performance will be established by comparison with laboratory optical clocks and primary frequency standards.
In order to achieve the goals, SOC2 developed a large number of laser systems - adapted in terms of power, linewidth, frequency stability, long-term reliability, and accuracy. Novel solutions with reduced space, power
and mass requirements were implemented.
A completely new, compact strontium lattice clock was built, occupying approximately two racks (2 cubic meter) and having 1.1 kW power consumption. This apparatus was transported from Birmingham to Braunschweig, for optimization and characterization. It is fully functional ( The clock transition of bosonic strontium was observed with < 4 Hz linewidth and preliminary measurements
showed a frequency instability of approximately 1E-16 for integration times up to 2 000 s.
A transportable fermionic Yb clock apparatus was completed and underwent preliminary characterization (12 Hz clock transition linewidth). It was transported from Düsseldorf to Torino, where characterization is continuing.

Project Context and Objectives:
Atomic clocks are essential tools in modern society. Clocks operate in computers, data networks, are ubiquitous in scientific applications and are even operated in satellites, especially in navigation systems such as GPS, GLONASS and the currently developed European Galileo system. Time, or more precisely, time interval, is the most precisely measurable quantity. Since the speed of electromagnetic waves in vacuum is invariable, distance measurements can be performed by propagation time measurements. Thus, ultimately the precision of distance measurements is limited by the available precision in time measurements.
In the International System of Units, the unit of time, the second, is based on an atomic hyperfine transition in neutral cesium (Cs) atoms. Laboratory clocks using cold Cs atoms have an inaccuracy of several parts in 1016 today. Although this is already by far the lowest inaccuracy of any physical unit, a major scientific development over the last decade, namely clocks based on optical rather than microwave transitions, has opened a new era in time/frequency metrology. In optical clocks the (laser) electromagnetic wave beats 1015 times per second instead of 1010 as in microwave clocks. Therefore, one can detect (and also correct) a minute change of the period much faster, allowing an enhanced stability. In addition, several perturbing effects on the energy levels of the employed atoms are smaller in relative terms for optical transitions compared to microwave transitions, and a large gain is therefore also possible in the accuracy. Optical clocks have now achieved a performance significantly beyond that of the best microwave clocks, at levels now below 1×10-17 relative inaccuracy. It is therefore expected that in the mid-future the unit of time will be redefined via an optical transition.
The essential techniques used in optical clocks are the confinement of the atoms to regions significantly smaller than the wavelength of light, provision of an environment as free of disturbing influences (magnetic and electric fields, residual gas, black-body fields) as possible, choice of adequate atomic species, and the narrowing of the spectral width of the clock laser to relative levels of 10-15 and less. Accurate comparisons of optical frequencies of these clocks or relative to the Cs atomic time unit are performed using femtosecond laser frequency combs. Several Physics Nobel prizes were awarded for methods that have enabled optical clocks.
With the rapidly improving performance of optical clocks, it will only be possible to take full advantage of it by operating them in space, since on Earth the clock frequency is influenced by the Earth’s gravitational potential at the location of the clock, contributing with an uncertainty on the order of one part in 1017, which may drift in time e.g. from tidal effects. Therefore, in the future, most applications requiring the highest accuracy will require operating optical clocks sufficiently far away from Earth, e.g. in a geostationary orbits. Such clocks will then become “master clocks in space”. A range of new applications will be enabled by
ultra-precise optical clocks in near or deep Space, in part in conjunction with terrestrial clocks. These applications have been widely discussed, proposed and evaluated by review panels of ESA. They cover the fields of fundamental physics (tests of General Relativity and its foundations), time and frequency metrology (comparison of distant terrestrial clocks, operation of a master clock in space), geophysics (mapping of the gravitational potential of the Earth), and potential applications in astronomy (local oscillators for radio ranging and interferometry in space). In particular, one project in the ELIPS program of ESA targets operation of a high-performance lattice optical clock on the ISS for fundamental physics and Earth science in approx. 2024. Thus, the development of a space optical clock of performance significantly higher than the current state-ofthe-art (microwave) space clock PHARAO is an important as well as challenging task for this decade.
Topic addressed in the call
Our proposal responded to “Activity: 9.2. Strengthening the foundations of Space science and technology”, sub-activity “SPA.2010.2.2-01 Space technologies”, and herein to the item “Optical Clock Time Referencing System” (Technology Domain: Opto-Electronics) as described in the EC-ESA-EDA LIST OF URGENT ACTIONS FOR 2009 [ESA09]. It calls for the “Development of an Optical Clock demonstrator for ultrahigh precision timing referencing applications”.
For the ultra-high precision required in the call we target a level beyond that of the best space clock, PHARAO, the compact microwave clock based on cold Cs atoms that has recently been developed as an engineering model for the ESA ACES mission (planned for 2017). Thus, the goal specifications for the demonstrator were set to be an instability below 1×10-15/¬1/2, and an inaccuracy below 5×10-17, in a package of less than 1000 liter, excluding electronics. With this enhanced performance, fundamentally new applications will become possible, as mentioned above. In addition it will allow the comparison of the best ground clocks, within much shorter averaging time even compared to ACES with higher accuracy.
The proposers are convinced that optical clocks based on neutral atoms trapped in a laser light lattice offer a good balance between the performance potential (instability and inaccuracy) and moderate complexity. Thanks to the developments already performed in the laboratories of the proposers, this project will permit to develop high-precision demonstrators that represent a first step towards a space lattice clock of significantly
improved performance compared to the best current space clock.
Thus, the main objective of this work was the development and characterization of two highperformance optical clock demonstrators with the above target performance and with dimensions and design that qualify them as breadboard and transportable, respectively.
The secondary objective was to develop and test components and sub-systems that will lead to enhanced compactness, robustness and longevity of the optical clocks.
Both objectives have a direct relevance to later implementation as a space instrument.
Objectives of this project
1.) Develop two transportable engineering confidence optical clock demonstrators with performance
Instability < 1×10-15/¬1/2
Inaccuracy < 5×10-17
This goal performance is about 2 – 3 orders better than today’s best transportable optical clock based on roomtemperature Doppler-free molecular spectroscopy, and better than the best microwave cold atom clock by a
factor 100 and approx. 10, in instability and inaccuracy, respectively.
Compared with the stationary laboratory clocks at the time of submission of the proposal (2009), the inaccuracy goal is about a factor 2 better than the level of today’s best neutral atom laboratory clocks and the
stability goal is equal.
The two systems are to be brought to TRL4 (validation in a laboratory environment).
2.) Develop the corresponding laser systems (adapted in terms of power, linewidth, frequency stability, longterm reliability), atomic package systems with control of systematic (magnetic fields, black-body radiation,
atom number), and an electronic and computer control system, where novel solutions with reduced space, power and mass requirements will be implemented. Some of the laser systems will be developed to 2nd
generation level with emphasis on even higher compactness and robustness. Also, some laser components will be tested at TRL 5 level (validation in relevant environment).

Project Results:
please see attached document

Potential Impact:
The space mission “SOC” on the ISS
The main goal of the present project was to make a significant step towards a first prototype of a space clock, to be flown eventually on the ISS. In February 2014, the SOC consortium was asked to submit a report on its results and a roadmap for further development. The report submitted in March 2014 was accepted by ESA and “SOC” remained a mission candidate in the ELIPS program. This positive result would not have been possible without the intermediate results achieved in this project.
Through our interactions with US colleagues, we supported NASA’s ISS fundamental physics program to fund some US groups (e.g. the group of C. Oates at NIST) to conduct collaborative researches with European
colleagues for the SOC project. Both the US scientists and the NASA sponsor have strong interests in international collaboration in these fundamental physics areas.
In 2016, ESA prepares the next phase of Life and Physical Science research in the ESA Human Spaceflight and Exploration programme (this is the program that uses the ISS) covering the period up to 2024. ESA called the consortia active in the ELIPS program to develop or update their roadmaps.
A roadmap update concerning the use of optical clocks in space, in particular on the ISS, was produced by the coordinator of SOC with support from the SOC consortium. It is enclosed as an appendix in the report of year 4.
On the demand of the national delegations to the programme, a Research Community Consultation Workshop open to the broad scientific community, took place at ESTEC on the 18th-20th January 2016. Here the
coordinator presented the SOC project, in particular to the Human Spaceflight and Exploration Science Advisory Committee (HESAC). This committee is called to formulate recommendations to ESA as the
roadmaps should become key elements of the programme starting in 2017 that will be proposed for funding at the ESA Council at Ministerial level of end of 2016.
The roadmap document was endorsed by the following scientists, mostly members of the SOC consortium:
In alphabetical order of institution acronym:
Dr. Steve Lecomte, CSEM Centre Suisse de Microtechniques, Neuchâtel
Prof. Claus Braxmaier, DLR Deutsches Zentrum für Luft- und Raumfahrt, Bremen
Dr. Davide Calonico, INRIM Istituto Nazionale di Ricerca Metrologica Torino, Italy
Dr. Nan Yu, JPL Jet Propulsion Laboratory, Pasadena, USA
Prof. Ernst M. Rasel, LUH Leibniz-Universität Hannover
Dr. Chris Oates, NIST National Institute of Standards, Boulder, USA
Dr. Nathan R. Newbury, NIST National Institute of Standards, Boulder, USA
Prof. Patrick Gill, NPL National Physical Laboratory, Teddington, UK
Dr. Rodolphe Le Targat, SYRTE Paris
Dr. Uwe Sterr, PTB Physikalisch-Technische Bundesanstalt Braunschweig
Dr. Christian Lisdat, PTB Physikalisch-Technische Bundesanstalt Braunschweig
Prof. Stephan Schiller, (Coordinator), UDUS Heinrich-Heine-Universität Düsseldorf
Prof. Guglielmo M. Tino, UniFi Università di Firenze
Dr. Stephane Schilt, UniNe Université de Neuchâtel
Prof. Kai Bongs, UoB University of Birmingham
Prof. Claus Lämmerzahl, ZARM Zentrum für Angewandte Raumfahrttechnologie und Mikrogravitation, Bremen
Based on the results obtained in the SOC project (summarized in the publications Kai Bongs, et al. C. R. Physique 16, 553–564 (2015); and S. Origlia, et al., Proc. SPIE 9900, Quantum Optics, 990003; doi: 10.1117/12.2229473; ), ESA has decided to perform three technology development projects for a Sr lattice clock in space:
- Development of a laser at 461 nm and a laser at 689 nm
- Development of a clock control unit (actually: a device like the FSS)
- Development of a lattice laser at 813 nm
Some members of the SOC consortium most closely associated with the use of these devices consulted ESA on the specifications of the devices to be developed.
These projects are organized by the TEC department of ESA. The projects have started in 2016.
These developments are aimed to raise the TRL of the lasers currently used in the SOC project.
Some members will be involved in supporting these technology developments by providing test facilities of the contractors of the developments.
ESA has given the SOC team the task to develop the detailed experiment scientific document, that will be the foundation of a following phase-A study, including supporting funding.
ESA is about to launch a Phase-A project on the mission SOC in 2016. It will include a predesign of the space clock instrument.

List of Websites:
This webpage (red text on the left menu of ) is intended to give an overview of the topics on which the teams can offer collaborations, small and large, ranging from consulting to joint measurements and technology transfer. A number of topics on which the consortium partners can offer collaborations is displayed.
The webpage is a wiki, which each SOC partner can modify or extend, after entering a password. The wiki is rather straightforward to use. There is an online help.