PART B. Towards Neutral-atom Space Optical Clocks:

Transcription

1 PART B Towards Neutral-atom Space Optical Clocks: Development of high-performance transportable and breadboard optical clocks and advanced subsystems Proposal acronym: SOC2 Type of funding scheme: Collaborative Project Work programme topics addressed: SPA Space technologies, and herein Optical Clock Time Referencing System Coordinator: Prof. Stephan Schiller, Ph.D. List of participants: Participant Participant organisation name Country 1 Heinrich-Heine-Universität Düsseldorf UDUS coordinator DE 2 Physikalisch-Technische Bundesanstalt PTB DE 3 Leibniz Universität Hannover LUH DE 4 Observatoire de Paris OBSPM FR 5 Università degli Studi di Firenze UNIFI IT 6 Istituto Nazionale di Ricerca Metrologica INRIM IT 7 University of Birmingham UBham UK 8 National Physical Laboratory Teddington NPL UK 9 TOPTICA Photonics AG TOPTICA DE 10 Kayser-Threde GmbH KT DE 11 EADS Astrium Friedrichshafen ASD DE 12 Menlo Systems GmbH Menlo DE 13 Kayser Italia Srl KI IT 14 Université de Neuchâtel UNINE CH 15 Centre Suisse d Electronique et de Microtechnique SA CSEM CH 16 Ecoles Polytechniques Fédérales Lausanne EPFL CH ANNEX I 1

2 Table of Contents B1: Scientific and Technical Description of project 1.1 Concept and project objectives Progress beyond the state-of-the-art State-of-the art Beyond the state-of-the-art S/T methodology and associated work plan Overall strategy of the work plan ii Detailed description of work 8 WP 1 Baseline laser system 12 WP 2 Advanced Laser Systems 15 WP 3 Advanced atomic package 18 WP 4 Integration and Characterization 20 WP 5 Qualification tests of critical components 21 WP 6 Preliminary design of 3 rd generation optical clock for space use 21 WP 7 Coordination Timing of the WPs and their components Risks and Contingency Plans Complementarity 25 B2. Implementation 2.1 Management structure and procedures Beneficiaries Consortium as a whole 46 i) Sub-contracting ii) Third party 2.4 Resources to be committed (including a table presenting a breakdown of the direct costs of each partner) 49 B3. Impact 3.1 Strategic impact Plan for the use and dissemination of foreground 57 Appendix 1: Complementarity, dissemination and risk management details 61 Appendix 2: Introduction to optical clocks and state-of-the-art 75 References 82 List of Acronyms 85 ANNEX I 2

3 B1: Scientific and Technical Description of project 1.1 Concept 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 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 times per second instead of 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 beginning to reach below (reference [Cho09], see end of proposal) 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 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 10 17, 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 [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 Thus, the development of a space optical clock of performance significantly higher than the current state-ofthe-art (microwave) space clock PHARAO [Cac09] is an important as well as challenging task for the first half of the upcoming decade. Topic addressed in the call Our proposal responds to Activity: 9.2. Strengthening the foundations of Space science and technology, sub-activity SPA 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. Objectives 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 (2013) [Cac09]. ANNEX I 3

4 Thus, the goal specifications we set for the demonstrator are an instability below /τ 1/2, and an inaccuracy below , 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 is 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 is 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. Approach The concept of the proposed demonstrator development is based on the following rationale and key points: (i) The atomic reference is selected to be a neutral atom ensemble stored in an optical lattice (see Appendix for an introduction). Compared to the approach of a single trapped ion, the large (~ ) number of atoms has the fundamental advantage of enabling an instability that is at least 100 times lower. This is a crucial advantage for many applications in space, in particular tests of fundamental physics. For a desired statistical error level, the frequency measurements can then be performed in much ( times) shorter time, so that both calibrations and actual measurements can be repeated more frequently, enhancing both the reliability and the statistics of the measurement. The actual atomic species selected, neutral Strontium (Sr) and Ytterbium (Yb) have already been demonstrated in the laboratory to exhibit an instability /τ 1/2, reaching at 2000 s [Oat09]. This level is already a factor of nearly 100 lower than that of PHARAO. The current inaccuracy of lattice clocks is at (1-3) [Lud08,Lem09], the same level as the specification of PHARAO. However, lattice clock characterization studies by the members of the consortium and world-wide [SOC-Wes09, Lem09, SOC-Lis09] indicate that an instability of /τ 1/2 and inaccuracy well below should be possible with accurate atom interrogation, control of atom number and density, improved interrogation laser, as well as improved thermal control of the atom chamber. Taking the specification of the PHARAO clock, and rescaling by the significantly lower black-body shift of Sr or Yb, an inaccuracy in the low level is considered feasible within the next several years. (ii) Neutral atom lattice optical clocks are the most widely studied clock types in terms of number of groups both worldwide as well as in Europe. This will ensure a broad and steadily increasing knowledge base and a large number of researchers able to contribute to their further development towards space clocks. In particular, researchers with suitable experience can be hired by space industry. Notably, in Europe 8 research groups, OBSPM (FR), UBham (UK), NPL (UK), PTB (DE), UDUS (DE), LUH (DE), UNIFI (IT), INRIM (IT), (see title page for acronym definitions) in four ESA countries actively pursue lattice clock development, and all participate in this cooperation project. (iii) The developments in the field of commercial optics have produced impressive improvements in size, mass, stability and reliability of lasers and other optoelectronic components. Nearly the complete laser system for lattice optical clocks can today be based on off-the-shelf commercial components of moderate size and high reliability. Thus, a further industrial development will be able to produce even more compact and performant systems adapted to the needs of the clocks of this proposal. Recently, very robust laser systems for cold atom sensors capable of withstanding 40 g deceleration in drop tower experiments have been developed. This experience will be transferred to optical clock lasers, leading to 2 nd generation laser systems. Both above developments can later lead to engineering models of space clocks.. (iv) Already, a significant effort has been undertaken by some of the proposers to study the physics of lattice clocks at the accuracy level of 10-16, and to develop the design and central components of breadboard and transportable lattice clocks, in the framework of both national projects and an ESA-DLR development project Space Optical Clocks (SOC) ( ) ( [SOC]. The main results are described below in Section ANNEX I 4

5 1.2 Progress beyond the state-of-the-art State of the art for transportable clocks Transportable microwave clocks A transportable cold Cs microwave clock was developed by the team member OBSPM in Paris and is operated with instability /τ 1/2 and inaccuracy [Biz09]. This transportable clock has been an important aspect in the development of the space clock PHARAO. It has been transported to several European laboratories to serve as a frequency standard, in absence of or for the purpose of calibrating a local frequency standard. (Similar applications are foreseen for transportable optical clocks, but at higher accuracy level, see Sec. 3.2). In addition, the space-qualified Cs clock PHARAO is currently ready for flight model development, with specifications of /τ 1/2 instability and (1-2) inaccuracy when operated in space [Cac09]. Transportable optical clocks No such clocks exist at present at inaccuracy level better than However, the apparatuses developed by the applicants in the framework of a previous project ([SOC], see figures below) have laid the groundwork for high-performance transportable optical clocks capable of attaining the level and below. The current status of the work of the proposers on lattice optical clocks, in particular the transportable demonstrators, is as follows (see figures below for some of the developed hardware): full characterization of a Sr clock at the level, determination of collision shifts and decoherence in a bosonic Sr clock and determination of operation parameters for inaccuracy level [SOC-Lis09] non-destructive atom interrogation for improved clock stability [SOC-Lod09], development of an all-diode-laser based Yb clock apparatus with a compact 3D optical lattice and demonstration of atom trapping in the 2 nd stage magnetooptical trap (MOT) [SOC-Abo09] design, assembly and test of a compact cold Sr atom apparatus [SOC-Pol09], development and integration of a compact blue laser for Sr first-stage cooling [SOC-Pol09], implementation of two diode-laser-based optical lattices [SOC-Abo09], [Lem09b], development of ultrastable clock lasers with relative frequency instability [SOC-Mil09] development of two transportable clock laser systems with < 10 Hz and 2 Hz linewidth, respectively [SOC-Leg08,Gör09] Figure Left: the transportable Yb clock system (2 m 2 ), developed in the SOC project [Abo09,Nev08]. See also Figure A5. Middle: the transportable compact Yb clock laser system (90 60 cm). The silvery box contains the reference cavity of the clock laser. Right: Yb atoms trapped in the 1 st stage MOT (399 nm). ANNEX I 5

6 By the beginning of this proposed project (ca. Mid-2010), we expect that in the project SOC we will furthermore have achieved: Reduction of systematic effects in a stationary Sr clock to a level below Achievement of instability of a stationary Sr clock below (at averaging times of 1000 s) Reduction of systematic effects in an Yb transportable system to a level below Achievement of instability of a Yb clock below Transport of clock laser (Fig ) from PTB to UNIFI, integration with the compact Sr system (Fig ) and spectroscopy of the clock transition in the MOT. Demonstrate transportability of PTB Sr clock laser (Fig ) to UDUS for comparison with Yb clock laser From the project SOC, the experience, the designs, and the existing Sr and Yb cold atom apparatuses will be crucial input to the proposed project, ensuring strong progress and a realistic chance of achieving the project goals. Figure The 1 st generation cold Sr source breadboard (1.2 m 0.9 m) developed within project SOC [Pol09]. Top: overall view; the two boxes in the foreground contains the 461 nm cooling laser, the black box the frequency distribution system. Bottom left: schematic of compact Sr oven and transverse cooling cell (10 cm long), right: atomic beam in operation with atomic fluorescence excitation probability Hz clock laser detuning (Hz) Figure Compact, transportable Sr clock laser system with 10 Hz linewidth and 330 l volume developed in the project SOC (2007-9). Right: the clock transition of ultracold Sr atoms at 698 nm, obtained using the clock laser shown on the left. The line has an extremely large quality factor of ANNEX I 6

7 Beyond the state-of-the-art Goals of this project 1.) Develop two transportable engineering confidence optical clock demonstrators with performance Instability < /τ 1/2 Inaccuracy < 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 today s stationary laboratory clocks, 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 2 nd 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). 1.3 S/T methodology and associated work plan 1.3 i) Overall strategy of the work plan. In order to respond to the challenges of the project, our strategy is based on involving a large number of European researchers, expertise and available transportable cold atom apparatuses, explore a wide variety of operational parameters, and minimize risks. This leads us to develop two demonstrator systems in parallel, whereby funding constraints and strategic aspects lead us to implement the aspect of compactness in only one device. The actual performance of a clock depends on a large number of parameters, many tests and optimizations have to be made to find a reliable set. Work on two rather than one system both increases the number of tests that can be performed, and reduces the risk of not achieving the desired specifications. On one hand, the two systems chosen share many common aspects, allowing for a cross-fertilization of the results obtained with one system to the other. On the other hand the different atomic species/isotopes, the different trapping schemes, and the different laser types used, allow us to explore in depth the physics and metrology possibilities that lattice clocks permit, and give us confidence that a demonstrator system will be delivered that will represent a major step toward the high performance and robustness of a space optical clock. We shall build on the extensive expertise provided by the ESA-DLR project Space Optical Clocks both on the atomics and the laser package side: 1.) The consortium provides as input a transportable lattice clock apparatus based on Yb, which is already essentially complete. In this system, 2 nd stage cooling was achieved in mid-2009, and first full operation as a clock will have occurred in mid-2010, before the start of this project. In the proposed project, this apparatus will be improved into a flexible and reliable system allowing for extensive systematic studies of lattice confinement of the Yb atoms and development into a clock with < inaccuracy. Its laser technologies (blue diode lasers, fiber lasers, DFB lasers) will be complementary to those of the Sr clock, allowing for an in-depth comparison of the two clock approaches. Development of on atomics package with simplified atom loading and high-level black-body control will also be undertaken. 2.) A second input is an assembled cold Sr source breadboard, currently under test. As the state-ofthe-art in Europe on laboratory Sr clocks is advanced (work at PTB, OBSPM) and the breadboard development is expected to be successfully completed into a complete 1 st generation breadboard clock, our strategy will be to develop a 2 nd generation Sr breadboard clock, in which the atomics package is very advanced, optimized w.r.t. volume, power consumption and black-body shift control. For this system on the laser side a modular laser system will be developed or upgraded, that includes robust and compact laser subsystems appropriately adapted to the clock requirements, This laser system will first be integrated in the existing Sr breadboard, thereby turning it into the first lattice clock breadboard worldwide. Characterization of this breadboard will occur for more than a year, giving crucial input data for the development of the 2 nd generation Sr breadboard. In the 2 nd half of the project, the new 2 nd generation Sr atomics package will be integrated with the laser system into a complete clock system and extensively characterized and tested. ANNEX I 7

8 In parallel to these activities, advanced laser systems will be developed for both Sr and Yb, building on recent laboratory results and field tests (project QUANTUS [Koe07]). These promise an important reduction in volume, mass and power, a must for future space clocks. Successfully developed advanced laser subsystems will be incorporated in the two clock systems, taking advantage of their modular concept. Clock qualification at the targeted frequency stability and accuracy levels, will be performed by two partner national metrology institutes (PTB, INRIM) in cooperation with several partners. A development toward compact, high-performance frequency comb subsystems with performance at the level will be undertaken. Because of the limited resources no comb is included in the demonstrators. Finally, some of the crucial laser technologies (laser diodes), after having been shown to be suitable for high-performance clock operation, will undergo preliminary robustness tests similar to those used for space qualification (thermal, thermal-vacuum, and radiation tests). These will indicate whether some of the present candidate laser technologies must be changed. Overall, the project will result in an advanced, compact, modular Sr lattice clock (< 1 m 2 footprint breadboard, < 1 m 3 volume) and a larger, but easily transportable Yb lattice clock (2 m 2 footprint, < 2 m 3 volume). The planned implementations are expected to lead to similar instability and inaccuracy for the two clocks (<5 x ); the difference being that by design the Sr clock will be of smaller volume. The project will also deliver a preliminary design and roadmap toward an engineering model of a space clock ii Detailed description of work ii.1. Overview The work is divided into two phases, separated by the mid-term review. First phase: - Upgrade of the transportable Yb clock developed in project SOC (WP 1, WP 4) - Operation and first characterization of the upgraded Yb clock (WP 4) - Completion of the first generation Sr breadboard into a complete clock with modular laser subsystems, detailed characterization - Development work on advanced laser subsystems, including frequency comb (WP 2) - Studies, design and development of the advanced atomics packages (WP 3) - Component qualification tests (laser diodes) (WP 5) Second phase: - Integration of 2 nd generation Sr clock breadboard (WP 4) - Continuation of advanced laser subsystems and comb development, followed by integration into the 2 nd generation Sr and Yb systems (WP 2) - Full characterization of 2 nd generation Sr and Yb clocks at the highest level at metrology laboratories (WP 4) - Synthesis of results and preliminary design of a 3 rd generation system, as basis for an engineering model (EM) (WP 6) Yb clock transportable demonstrator The almost complete 1 st generation system that is an input to this project will be upgraded in several respects both on the lasers side and on the atomic package side. This will allow reliable operation and high frequency stability, enabling extensive systematic investigations especially on different isotopes and lattice configurations, with the purpose of determining best operating conditions. A potentially much simpler approach for trapping and cooling, without Zeeman slower, will be tested and a 2 nd generation vacuum chamber developed, including black-body radiation control. The total volume of the optical, optomechanical and mechanical setup of the Yb clock is projected as < 2000 liter. Sr clock breadboard demonstrator Test on laboratory clocks, on the first generation breadboard, as well as hardware tests will explore various options concerning power and space requirements reduction. The results will be used to design an optimized, compact 2 nd generation Sr atomics package that will then be built. In parallel, building on the experience of the industrial partners, of projects SOC, QUANTUS, ESA studies and input from space industry partners, after a design phase (reviewed in month 5), the baseline laser system, consisting of compact Sr laser modules, will be developed, tested in the developers labs. In the second project ANNEX I 8

9 phase, this laser system will be integrated with the 2 nd generation atomics package (with black-body radiation control) into a complete clock, with volume < 1000 liter. A first characterization phase will include extensive optimization the production of ultracold Sr atoms and the loading in the lattice. A subset of compact and robust lasers, including clock lasers, will be developed for both Yb and Sr. The performance of these advanced components will be carefully assessed before they are implemented in the clocks in order to minimize the interference with ongoing studies of the clock performance. One important subsystem of an optical clock is the frequency comb. Studies on existing and novel frequency comb architectures will address the challenge of achieving accuracy with a fiber-laser based comb. In year 4, the Sr and Yb systems will be operated at PTB and INRIM, respectively, for in-depth evaluation of the clock performance in comparison with stationary optical clocks and primary frequency standards (Cs). Main deliverables: 1. An advanced compact and modular 87 Sr optical clock breadboard, containing advanced atomics and laser subsystems. 2. A transportable Yb clock system, operating with alternative atomic species and laser technology, containing advanced laser subsystems These two transportable, engineering confidence demonstrators, are expected to achieve an uncertainty below and an instability better than in one second, dropping to after s averaging time. 3. Test results showing compact frequency comb capability at few level 4. Specifications for, preliminary design of, and a roadmap toward achieving inaccuracy in a space clock and a compact frequency comb. Note: A complete development of a compact comb to be integrated with the clock breadboard is beyond the funding limit of this project. However, the comb activities proposed are an important step in this direction. The consortium will make an effort to obtain the means to demonstrate, at the end of the project, operation of one of the optical clocks together with a fiber based comb of correspondingly high specifications ii.2. Specifications of the main deliverables of the project The 2 nd generation Sr breadboard clock The design of the advanced compact breadboard Sr lattice clock is based on a modular approach (Figure and Table 1), which means that the modules are connected by optical fibers. The modular approach allows for independent testing of the subcomponents and, during the course of the project, simple replacement of components by more advanced components. The different laser breadboards are described in Table 2. They contain lasers and the additional distribution optics, acousto-optical frequency shifters and switches and finally fibre couplers for interfacing the radiation to the atomics package, frequency control unit and the fs-comb. In the present approach a standard footprint for all laser breadboards of 60 cm 45 cm (or half size 30 cm 45 cm) is used to allow a compact overall package for the clock. Together, all these boxed breadboards will take up a volume of cm 3. Including the clock laser system and the atomics package, but without electronics and power supplies, the complete optical clock hardware will be < 1000 liter in volume. The electronics is estimated to fit into one standard 19 rack Due to the constraints of the project no hardware development toward very compact electronics will be undertaken. However, principal electronics issues for a future engineering model space clock will be identified by a space integrator partner (WP 6). The transportable Yb clock The transportable Yb lattice clock has the same basic layout as the Sr clock shown in Fig Since the clock will not be set up from scratch, but is already an almost complete 1 st generation system that is an input to this project, it is not modular, but an integrated system where most components including the optical lattice are mounted on one optical table (see Fig and Fig.A5 in the Appendix). In particular, the laser radiation required for laser cooling and repumping is provided through free-space beam lines rather than fibers. The optical table with a footprint of 1 2 m is movable (on wheels). The clock laser is placed on a separate optical breadboard with a current footprint of cm, and the light is delivered via fiber to the atomics package ANNEX I 9

10 on the optical table. All required components (with the exception of a repumping laser at 1388 nm) are already assembled and a postcooling MOT of up to Yb atoms has been successfully realized. In addition, a compact clock laser with a linewidth of 2 Hertz has been realized. Within the present proposal several components of the transportable Yb lattice clock will be replaced by newly developed, improved components (Table 4). This includes a simpler and more compact cooling laser system (WP 1, WP 2), a 4 times more stable and more compact clock laser (WP 2), as well as the atomic oven and possibly the main vacuum chamber (WP 3). The performance of all new components will be carefully assessed before they are implemented in the clock apparatus in order to minimize the interference with ongoing studies of the clock performance. As for the Sr clock, the Yb clock will not use specialized electronics. We estimate that all electronics including power supplies can be fit into two standard 19 racks. laser BB 1 1 st stage cooling atomics package User laser BB 2 2 nd stage cooling fibres FSS clock laser fibres laser BB 3 repumper frequency control fs laser frequency comb rf Link laser BB 4 Optical lattice internal M&C LAN Minicomputer Figure : General architecture of a transportable optical clock operator console 1st generation system (project SOC) This project Laser Breadboards 461 nm cooling cm 3 (laser) cm 3 (doubler) cm 3 (distribution) 40 kg total cm 3, 20 kg laser cm 3, 12 kg distribution 689 nm cooling not available cm 3, 20 kg cooling cm 3, 12 kg stirring nm repumpers not available cm 3, 12 kg 813 nm lattice not available cm 3, 12 kg 698 nm clock cm, 30 kg cm 3, 20 kg Reference Cavity cm, 50 kg cm 3, 30 kg Atomics Package cm, 50 kg cm 3, 30 kg Electronics, Control not available one 19 rack (not optimized *) Clock Performance Inaccuracy No studies possible goal: < Instability No studies possible goal: < , τ -1/2 Clock Laser Linewidth ~ 1 Hz (by mid-2010) 0.1 Hz Table 1: Main specifications of the Sr clock breadboard. (*) Due to time and funding restrictions, no particular development towards compact electronics is undertaken. ANNEX I 10

11 Laser Breadboard Wavelength Stability Size in cm 3 Power at fiber output Cooling #1 461 nm ECDL+SHG 1 MHz kg 140 mw to distribution breadboard cooling #1 distribution 461 nm 1 MHz kg 50 mw MOT,30 mw slower, 1 mw setection,1 mw (IR) FSS cooling #2 689 nm ECDL < 1 khz in 1 h, linewidth < 1 khz with additional kg 10 mw MOT, 1 mw FSS 1 mw to stirring breadboard FM Stirring and spin 689 nm Offset phase locked to mw MOT, polarisation. repumper ECDL 707 nm, 679 nm ECDL nm cooling FM +/- 3 GHz with a few khz modulation frequency; center: 100 MHz 12 kg kg 2 1 mw spin polarization 2 mw each wavelength to MOT, 1 mw to FSS Lattice 813 nm ECDL < 10 MHz in 10 h kg >200 mw to atomics 1 mw to FSS Clock 698 nm ECDL <1 Hz kg 2 mw to atomics, 2 mw to comb, 0.5 mw to cavity, 1 mw to FSS Clock cavity 698 nm thermal noise , 30 kg Frequency stabilization All, exclud., To levels indicated above , fiber input for the above fiber system (FSS) 698 nm 10 kg outputs Table 2: Detailed specifications of the laser subsystems for the Sr clock to be achieved in this project. Current status (project SOC) This project Subsystem Source of ultracold Yb optical table 1 m 2m Optical table 1 m 2m including optic lattice 1388 nm repumper not available Included on optical table Clock laser including cm cm 2 reference cavity Electronics, Control not available two 19 racks (not optimized ) Clock Performance Inaccuracy No studies performed yet; goal: < goal: in 6/2010 Stability goal: in 6/2010 goal: < τ -1/2 Clock Laser Linewidth 2 Hz Goal: 0.4 Hz Laser Wave-length Stability Power at fiber output Cooling #1 399 nm (ECDL blue diode) 1 MHz 30 mw to MOT, 5 m W to Slower Cooling #2 556 nm (fiber laser + SHG) < 20 khz 20 mw to MOT Repumper 1388 nm (DFB diode laser) 10 MHz 5 mw Lattice 759 nm (ECDL diode laser) < 10 MHz >200 mw Clock 578 nm (ECDL diode laser + SHG) <0.5 Hz 2 mw to atomics, 2 mw to comb 0.5 mw to clock cavity Clock cavity 578 nm thermal noise Table 3 (top) : Main specifications of the transportable Yb clock. Table 4 (bottom): Detailed specifications of the laser subsystems for the Yb clock to be achieved. ANNEX I 11

12 1.3.1.ii.3. Detailed description of workpackages WP 1 Baseline laser system WP 1.1: Cooling and manipulation laser breadboard development In this WP the various cooling, stirring and spin-polarization lasers sources are set up on compact breadboards. The designs are based in part on commercially available components with proven reliability, appropriately enhanced and specially adapted to the needs of the breadboard. The specific designs will reviewed by a space integrator to ensure the future compatibility with space environment. These lasers will be integrated into the different distribution breadboards. The breadboards are expected to be operational within the first project year. They can be tested on stationary clocks at PTB, and integrated into the Sr clock breadboard as soon as it is ready. The cooling and manipulation lasers will be specially designed diode laser systems, optimized for lowest vibration sensitivity, highest long term stability, as well as low sensitivity to temperature and acoustics. All lasers will be equipped with appropriate optical isolators (> 30 db for 679 nm and 707 nm; > 60 db for 689 nm) and high bandwidth laser diode current modulation access for fast locking. To ensure reliable locking, reproducible parameter setting and external computer control, special locking modules based on FPGA technology will be integrated into the laser control racks. All electronics modules (current/temperature/scan/monitor/locking) will be integrated in standard 19 rack systems. Based on the opto-mechanical DL pro external cavity resonator design (see Figure 1.3.2), modifications of the resonator will be investigated and different laser diode types will be tested in order to achieve the best compromise between short term linewidth of the free running laser and size of the single mode plateaus (as a measure of single mode stability) for each individual laser application. The 679 nm and 707 nm repumping lasers will be optimized for largest single mode plateaus by trading laser linewidth (up to 1-2 MHz). On the other hand, the 689 nm second stage cooling and stirring lasers will be optimized for free running short term linewidths below 100 khz. Fig : DL pro type ECDL resonator (left: closed; right: inside view). Size of device on left is 5 5 cm First Stage cooling 461 nm laser: The 461 nm cooling laser will be a special design tapered amplified and resonantly frequency-doubled diode laser system (See figure 1.3.3). A fiber-coupled probe beam output at 922 nm will be available for the FSS (frequency stabilization unit, WP 2.1). The master laser will again be an optimized version incorporating the DL pro technology for best single mode and thermal stability. Optional output of the 922 nm high power radiation will be provided to be used for the waveguide doubler. The second harmonic generation (SHG) resonant enhancement cavity will be manufactured from a full metal block in order to provide transport and long term stability. Entrance and exit windows for the laser beams will prevent dust from entering the resonator. As special development, we will integrate a special AR coated LBO crystal instead of a KNbO 3 crystal in order to achieve high fiber coupling efficiencies of the 461 nm SHG output on the order of 70% even at power levels of 170 mw after the single mode fiber. Such high coupling efficiencies are necessary to increase the lifetime of used SM PM fibers. A consequence of using LBO at this wavelength and requiring special development, the SHG resonator has to be air-sealed and maintained humidity-free. A flip mirror will provide access to the fundamental wave for operation with the waveguide doubler of WP 2.6. Second Stage cooling 689 nm: The compact second stage cooling laser and distribution breadboard ( BB-2 in Figure 1.3.1) is based on the stationary design developed at PTB and used successfully in the stationary Sr clock, using the miniature optics technology of QUANTUS heritage. For the second stage cooling a laser linewidth of about 1 khz is required, ANNEX I 12

13 Fig : Left: reference system for the cooling laser at 461 nm. Middle: Zoom onto the SHG resonator which has to be modified, see text. Right: 461 nm frequency distribution breadboard developed in project SOC [Pol09], serving as baseline design for this project. which will be ensured by locking the laser to a ULE cavity, which is contained in the FSS. To cover a broad range of atomic velocities, during the first phase of the second stage cooling, the laser spectrum is broadened by modulation of the frequency with an amplitude of 1.5 MHz and a modulation frequency of 15 khz. Thus various AOMs for switching and well defined modulation are included on the breadboard. Use of the fermionic 87 Sr atomic species requires an additional laser, tuned to the hyperfine transition F=9/2 - F =9/2, which is detuned by 1.4 GHz from the cooling laser. It needs similar linewidth as the cooling laser, and needs to follow its frequency modulation. This is achieved by phase locking the laser using the beat to the cooling laser. For geometrical reasons this laser will be set up on an additional breadboard, connected by fibers to the atomics package (MOT, 2 spin polarization). Repumping lasers 679 nm and 707 nm: A similar breadboard ( BB-3 in Figure 1.3.1) contains the repumping lasers at 679 nm and 707 nm. The lasers will be specially designed ECDLs for highest single mode stability with free running linewidth ~1 MHz, to be developed by TOPTICA. To cover the hyperfine structure of 87 Sr the frequency will be modulated by a few GHz. Also here fiber outputs are provided to the atomics package and to the FSS. WP 1.2: Clock laser development In this WP the existing clock laser breadboard from SOC (Figure 1.2.2) will be modified into a new one with the same footprint as the other compact laser distribution breadboards. The reference cavity will be modified to reduce the thermal noise instability level ~ Using smaller, active vibration isolation stages and an aluminium vacuum system will reduce the size of the cavity package including acoustic isolation to cm 3 (see also Fig ). Highly reliable extended cavity diode lasers will be developed, that will deliver 698 nm radiation with high spectral purity and narrow intrinsic linewidth. Here the use of narrow-band interference filters as wavelength selective element promises many advantages compared to the conventional grating design. As the clock wavelength lies at the edge of currently available laser diodes, special care is required to achieve reliable operation. The design will be based on simulation of the mechanical setup (e.g. thermal, vibrational stability etc.), see Figure left. Prototypes will be set up to test different filters and diodes, parameters of temperature control and parameters of frequency stabilization. Finally, optimization will be performed and well characterized laser modules will be built. One module will go on the clock-breadboard while a second module with integrated isolator and fiber coupling will be made available for cavity tests. Also for these tests and for use with the reference cavity, a compact, stable fibre coupling module for mode-matching light to the reference cavity will be developed. The clock laser distribution breadboard will contain the AOMs for tuning the laser, for probing the Sr absorption line and AOMs to provide the fiber-noise cancellation of the fibers that connect the breadboard to the femtosecond comb, to the atomics package and to the reference cavity. Modifications of the available cavity (Figure middle) will reduce the thermal noise level by replacing the current ULE mirrors by Fused-Silica mirrors with compensation of the thermal expansion mismatch using additional ULE-rings at the rear side of the mirrors (Figure right). ANNEX I 13

14 ULE ring ULE spacer fused silica mirror Figure 1.3.4: Monolithic Sr clock laser housing, middle: ULE reference cavity, right: thermally compensated mirrors WP 1.3: Lattice laser development The optical lattice laser at 813 nm will be a specially designed tapered amplified diode laser system (MOPA, Master Oscillator Power Amplifier), optimized for highest long term stability, resistance to shock during transportation, as well as low temperature and acoustic sensitivity. It will consist of a pro technology ECDL laser optimized with respect to laser linewidth vs. single mode stability according to the specifications from PTB, amplified in a tapered amplifier chip. Since high power output is needed for the optical lattice, the laser system will be optimized for high seed laser power, increased heat dissipation, high power TA chip and especially for best beam profile (target: fiber coupling efficiencies of ~ 60%). The aim of this development is high power output at best long term stability. Another special development will be a high bandwidth output power control option which can be used to either modulate or stabilize the output power to compensate for output noise and long term power drifts. Active power control requires an intensity monitor signal generated on the laser breadboard. Two (one AC coupled, one DC coupled) high bandwidth laser diode current modulation accesses with laser diode protection will be integrated (electrical bandwidth up to 60 MHz). These channels can be used for fast frequency modulation as well as for fast frequency locking. Frequency locking modules and electronics similar to the 422 nm laser will be included. The MOPA will be integrated in a distribution breadboard. This breadboard will include the power control and stabilization by AOM, an optical shutter and 2 fiber outputs to the atomics package (>200 mw) and FSS (~1 mw). The AOM will serve to keep the power level at the optical lattice stable, in order to reduce parametric heating of the atoms and to allow the automatic evaluation of residual light shifts from deviations from the magic wavelength. WP 1.4: 461nm cooling laser frequency distribution breadboard An advanced 461nm frequency distribution breadboard will be developed based on the QUANTUS and SOC heritage in terms of ultrastable optics mounts, full fiber coupling and stable aluminium enclosures (see Figure 1.3.3). This breadboard will integrate the 461 nm laser from WP 1. In the second half of the project, the waveguide-based frequency doubling unit from WP 2.6 will be integrated and tested toward a simpler replacement of the baseline SHG resonator solution. The breadboard will feature a 922nm fiber output for frequency stabilization as well as 461nm fiber outputs for laser cooling and trapping as well as for detection. The frequencies and intensities of these outputs will be individually controllable by AOMs and mechanical shutters will enable complete removal of cooling light during the clock interrogation phase. The fiber outputs will be realized using commercial fiber couplers directly attached to the system and polarization maintaining fibers. Polarization control is enabled by miniaturized rotatable waveplates and ensured by high-extinction polarization filters. The final frequency distribution breadboard will be delivered to WP 4.2 for integration with the Sr atomics package. 461 nm Laser and distribution breadboard together form the BB-1 laser 1 st stage cooling breadboard shown in Fig WP 1.5 Robust repumper laser for Yb The Yb clock requires a single 1389 nm repumper laser compared to the two required for Sr. In addition, industrial-type DFB-lasers in the O-band can be employed, e.g. manufactured by NTT-Electronics (Fig ). These are in a butterfly-type 14 pin package with thermo-electric cooler., where the pigtail fiber is connectorized. Available power is 20 mw and the free-running linewidth is specified at 2 MHz. ANNEX I 14 Fig Industrialtype repumper laser for Yb.

15 We will develop a compact frequency stabilization unit, based on a molecular absorption cell, to which the laser is fiber coupled. The on/off switching as well as frequency shifting relative to the molecular reference line will be done with integrated-optic modulator. Overall, the complete stabilized laser will be packaged in a 2 liter housing (excluding electronics). WP 2 Advanced Laser Systems WP 2.1 Development of FSS, a compact system for frequency stabilization of the Sr lasers A novel, compact (<10 liter), all-fiber coupled device will be developed that will stabilize 5 lasers 922 nm, 813 nm, 698 nm, 689 nm, and 707 nm according to the specifications in Table 2. The setup (see Figure 1.3.6) includes the reference cavity sub-unit (V) with two temperature-stabilized ULE cavities in a single evacuated housing. Alternatively, a single ULE-block with two embedded cavities can be used. The five waves are delivered from the respective lasers via fibers, pass through fiber microwave phase modulators (FOL 1, 5) and are delivered to the cavities. These modulators are driven individually with a frequency-modulated microwave signal. The lasers are locked to the cavities on their microwave modulation sidebands, so that changing the modulation frequency allows for a precise tuning of the lasers with respect to the atomic transitions. The required frequency stability of the 689 nm wave is the highest, hence the ULE2 cavity with relatively high finesse is solely dedicated to this laser. Requirements for the other wavelengths are looser and DLs 1, 3-5 can be addressed using the same reference cavity with relatively low finesse (and hence broadband mirrors). The light reflected from this cavity is detected on a single detector that produces four error signals in the signal processor sub-unit using corresponding rf local oscillators. Five feedback servos (S) lock the lasers. WP 2.2, WP 2.4 Advanced ultrastable Sr and Yb clock laser reference cavities The clock lasers for the two clocks will be significantly improved compared to our previous transportable systems by equipping them with 2 nd generation optical cavities and vacuum and thermal housings. The designs will built on the results of partner OBSPM in the project SOC [SOC-Mil09], using a dedicated cavity shape and holding system for achieving low sensitivity to vibrations along all directions, an optimized thermal environment (see Figure 1.3.3), and adapted mirror substrates for minimizing the thermal noise limit to the cavity stability. The objective is to obtain state-of-the-art frequency stability of for averaging time between 0.1 s and 10 s (limited by thermal noise), and sub-hz linewidth clock lasers as in [SOC-Mil09] but on a transportable system. One of the cavity systems (for Yb) will be built by the partners, using the vacuum and thermal housing design already developed (Figure 1.3.3), extended by zero-cte shift rings (see Figure 1.3.4, right). For the second one we plan to subcontract to a company with expertise in optomechanical design and lasers. The design guidelines and specifications will be provided to them, and the company will execute a complete finite element thermo-mechanical modelling of the cavity including the holding system and construct an optimized system. The company will also perform an analysis of the issues related to the spatialization of the system: identification of critical components, criticality under loads and vibrations conditions, analysis of weight and dimensions issues, analysis of the materials used in the prototype with respect to space environment. A suitable candidate subcontractor has already been identified (see Sec.2.3i). Finally, this new cavity will be fully characterized by the partners and integrated into the 2 nd generation Sr clock system, allowing to reach its optimal level of performance. WP 2.3 Integration and tests of 2 nd generation ref. cavity First, temperature stabilization of the cavity will be optimized to a long-term stability at the sub-mk level. The reference cavity subsystem will be integrated with the clock laser developed in WP 1 and a compact vibration isolation system at PTB, where subsequently the integration with the 2 nd generation Sr breadboard will occur. The optimization of performance will occur with respect to the stationary PTB Sr clock laser by using the heterodyne beat as monitor. Goal is achieving a thermal-noise limited instability of 5 x ANNEX I 15

16 s DL1 461 nm FOL1 s s s DL3 707 nm DL4 679 nm DL5 813 nm FOL3 FOL4 FOL5 mux V DM ULE1 ULE2 signal processor s DL2 689 nm FOL2 Figure Left: Concept of the compact frequency stabilization system (FSS) for the Sr lasers. Right: the vacuum and thermal control chamber for a clock laser cavity (in green, 10 cm long). The 2 nd generation Sr clock laser chamber will be an improved version of this prototype. WP 2.4 Development of an improved and compact Yb clock laser system For the improved clock laser for the Yb clock we aim not only at better frequency stability using the above 2 nd generation cavity but also at a higher output power. The current output power at 578 nm is limited to 200 µw because of the need of placing an electrooptic modulator into the ECDL, reducing the output power. In cooperation with the company Innolume (Dortmund, Germany) we will test new quantum dot diode lasers at 1156 nm which they manufacture, which aims at obviating this need. If this is not successful, we will choose the approach of an injection-locked quantum dot amplifier. An improvement in efficiency, robustness and compact size for the laser system is furthermore to be implemented by using a frequency-doubling waveguide crystal with intergrated fiber coupling. 5 mw output power at 578 nm should then be reachable. WP 2.5 High-power stable laser system The complexity of an optical clock requires simplification at all levels. For the demonstrator of the optical clock, we aim for replacing conventional MOPA systems by a laser self-seeded high-power diode laser which uses an interference filter for wavelength selection (Fig ). Such lasers were developed in our laboratory for compact inertial sensors as used for QUANTUS. These lasers will be based on a design already realized at LUH, running at the Rubidium wavelength of 780 nm [Gil07] which is an extension of the work of [Bai06]. At low power, such a design is also implemented in the space clock PHARAO. These are interference filter stabilized extended cavity diode lasers. Due to the linear setup and the interference filter as the wavelength selective element, these lasers provide a high mechanical stability. The design aims to realize a single stage high-power semiconductor laser with at least 300 mw output power and Lorentz linewidth of 300 khz. In order to adapt this laser design to the wavelength of 759 nm for Yb, the design will include a new interference filter as well as a new tapered amplifier chips. At the output of a fiber, one should be able to obtain 150 mw (laser output of 300 mw in stable conditions passing through an optical isolator and being coupled into a single-mode fibre). To ensure the outstanding robustness and stability needed for a future development towards space-qualified lasers, an extensive characterization and careful initial alignment process has to be performed. Figure 1.3.7: Two self-seeded interference filter stabilized extended cavity diode lasers based on tapered amplifiers already realized by LUH [Gil07]. The 1 coin represents the scale in this figure. Figure Ridge waveguide periodically poled lithium niobate waveguide frequency doubler; example for SHG of 580 nm (NTT Electronics) ANNEX I 16

17 WP 2.6 Simplified SHG of 461nm The process of second harmonic generation will be made very robust by implementing periodically poled nonlinear crystal waveguides, which allow high efficiency single pass generation of 461nm light from the high power 922 nm semiconductor laser system of WP 1. The crystals will be equipped with a directly attached optical fiber input to ensure highest stability of coupling parameters and will deliver a free space beam into the frequency distribution breadboard in WP 1.4. The main task will be to ensure long-term crystal stability even under operating conditions delivering over 100 mw at 461 nm in routine operation. Different materials will be evaluated and the most promising chosen for the realisation of the device. In particular, mechanically cut waveguide doublers from NTT (Fig ) which have shown very good performance for 580 nm generation, will be produced for 461 nm generation. After long-term (1000 h) testing the SHG system will be delivered in month 24 to WP 1.4 for integration in the 461nm distribution breadboard and testing on an optical clock. WP 2.7 Development of an interference-filter based ECDL at 399 nm While the currently used grating-stabilized ECDLs do in general exhibit the performance required for laser cooling of Yb and are routinely used in the Yb apparatus, they have typically low quality of the spatial mode profile and have exhibited a significant degradation of the holographic gratings due to the short-wavelength radiation. Therefore, we will develop interference-filter based ECDLs at 399 nm with the goal to use such a laser as cooling laser in the transportable Yb clock. One of the goals of this development is to investigate the possibility to obtain enough power (> 40 mw) from a single interference-filter based ECDL to operate the 1 st stage MOT and the Zeeman slower. This would permit the replacement of the current master-slave setup. Another issue that will be addressed is the possibility to reduce the linewidth of interference-filter based ECDLs at 399 nm to a level of 100 khz. The currently achieved level of several MHz is sufficient for laser cooling in the 1 st stage MOT and the Zeeman slower but is a limiting factor for the achievable signal-to-noise level of the atomic clock, as it determines the noise level of the detected atom number. WP 2.8 Development of a robust 2 nd stage cooling laser for Yb (556 nm) Fiber laser technology, combined with efficient second-harmonic generation, offers a novel approach for near- IR and visible lasers. The advantages are very long lifetime, no open optical setup due to an all-waveguide design, very small linewidth and very high passive frequency stability. A Yb 3+ :glass fiber laser with efficient SHG in a waveguide periodically poled crystal with fiber output (similar to Figure 1.3.7) will be developed. This source will be turn-key and further simplify the Yb clock apparatus. It will lower the requirements on the required bandwidth for laser stabilization and reduce the required volume. WP 2.9 Ultra-high accuracy frequency combs Optical frequency combs generated by ultrafast lasers have revolutionized optical frequency metrology in just a few years and one of the crucial technologies for optical clocks. State of the art and the base line technology for the proposed optical clock demonstrators is a frequency combs based on ultrafast fiber lasers and fiber amplifiers (Figure 1.3.8). They have reached a high degree of reliability and achieve continuous 24 h/7 days per week operation. They are commercially available and come on an easily transportable breadboard, fully automated, hands off and with network control software (MENLO). Specification for the commercial combs is but the systems have been shown in laboratories to perform well down to the measured uncertainties of a few times Accuracy beyond that point has so far not been evaluated and is nontrivial. Since this is necessary for the optical clocks proposed in this project that target below 1x10-16 inaccuracy and instability it is goal of this activity to evaluate the existing frequency combs at PTB and MENLO to determine the operating conditions and components allowing to reach the level and beyond. Changes and refinement of the system hardware is expected to be necessary in order to achieve this accuracy. A second activity concerns a new and very intriguing way to generate frequency combs: the use of micro cavities [Del07] (Figure 1.3.8). These combs are generated using only a low-power continuous-wave telecomtype laser. This new technology is in principle extremely attractive for future use in space since its is potentially capable of being implemented as an integrated-optics element. In this project we intend to make a first thorough phase noise analysis of such combs. ANNEX I 17

18 Figure Commercial fiber-laser-based frequency comb. used as input to the project for further development. Right: miniature (sub-mm) resonators provide a new approach to frequency combs. WP 3 Advanced atomics packages WP 3.1: Improvement and first characterization of transportable source of ultracold Yb The goal of this WP is the improvement of the existing transportable source that has been developed within SOC, in particular with respect to reliability, transportability and power consumption. This will include a full characterization and optimization of the process for production of the ultracold Yb atoms as well as the replacement of parts of the system for which a potential for improvement is identified. The target numbers for this workpackage are the production of 10 7 atoms of the most abundant bosonic ( 174 Yb) and fermionic ( 171 Yb) isotopes at a temperature of µk within 300 ms and a hands-off operation of the system of at least 4 h. A significant part of this workpackage is devoted to the integration of a new 556 nm fiber laser system for the post-cooling stage which will be developed in workpackage WP 2. Due to the intrinsic stability and narrow linewidth of the fiber laser of a few 10 khz, this replacement will allow for a significant simplification of the frequency stabilization system. In addition, the atomic oven will be replaced with an adaptation of the low-power consumption design that has been developed within the previous project SOC for Sr. Another planned improvement will be an upgrade of the precooling laser system at 399 nm to an interference-filter based ECDL that will be developed within WP 2. Finally, the improved clock laser (578 nm) will be integrated. Besides the replacement of these laser subsystems as they become available, a main purpose of this WP is to establish reliable operation routines for the optical clock including calibrations of standard system performance. For this purpose we will also perform an upgrade of the experimental control. WP 3.2: Advanced resonator-based setup for a magic-wavelength optical lattice for Yb This task aims at improving the resonator-based setup for the optical lattice that has been developed within SOC for Yb (Fig ). The main advantage of our design with an enhancement resonator for the optical lattice inside the vacuum system is that a large volume optical lattice can be realized due to the high power circulating inside the resonator. The first version of this design which is currently implemented in the transportable system is a 3D design with a non-adjustable resonator length, which only allows setting the lattice wavelength with an accuracy of several 100 MHz. For the intial studies that are currently under way this accuracy is sufficient, but in order to have the desired performance of the optical lattice clock it will be necessary to control the lattice wavelength at a level of a few 10 MHz. This will require to implement piezotuning of the resonator-length which is complicated by the fact that the resonator is placed inside the vacuum system. Within this workpackage we will develop the corresponding vacuum-compatible design for a 1D as well as a 3D geometry. In addition, a significant activity will be dedicated to optimizing the loading strategies of the optical lattice. Since the temperatures that can be achieved in an Yb postcooling MOT (~ 20 µk) are significantly higher than in the case of Sr (~ 1 µk), this task is more important in the case of Yb than it is for Sr. Particular care will be taken to maximize the atom number in the lattice as the large-volume optical lattice with a diameter of 150 µm holds the promise to be able to trap up to 2 orders of magnitude more atoms (up to 10 5 ) than what is standard in present studies. WP 3.3: Advanced atom chambers Temperature measurement of the clock interrogation region and control to +/- 0.1 K is necessary to reduce variations of the shift from blackbody radiation to below This will be implemented in the 2 nd generation chambers. To this end, we will measure systematic black body radiation shifts in a Sr optical clock vacuum chamber and generate an optimized design to minimize these for breadboard clock chamber. In addition, ANNEX I 18

19 Figure 1.3.9: (left) Schematic drawing of the resonator setup for the 3D optical lattice for Yb. (right) Light path inside the resonator (made visible by light scattering from water vapour). different methods of minimizing atom-source related blackbody radiation will be investigated, e.g. the use of shutters in vacuum, the implementation of additional MOT stages or the implementation of a 2D MOT. An evaluation will allow an optimized choice for the source in the final advanced atom chamber. We also aim at a design for the final chamber that may house both Yb and Sr. Furthermore the vacuum system and the magnetic field generation for slowing and trapping shall be significantly reduced in size, weight and power consumption by the use of advanced vacuum and coil/permanent magnet technology (see Figure ). The current boundaries of standard (CF) vacuum technology shall be overcome by using ultra-low outgassing glue or glue assisted lead sealing technologies, which in combination with the adapted thermal expansion materials for chamber frame and windows allows baking temperatures in excess of 200 degrees Celsius and ultra low vacuum pressures in the region of mbar. The main advantage will be a very compact system size of e.g. ~10 cm diameter 3cm thickness for the science chamber. A consequence for magnetic gradient scaling as 1/size 2, smaller coils and an order of magnitude lower power consumption than in standard chambers will be needed to achieve the same magnetic field gradient. In addition, we will investigate copper foil coils to minimize size and inductance, while allowing efficient cooling from one side. Similarly, also for Yb a new chamber will be developed in order to significantly increase the number of loaded atoms in the no-zeeman slower configuration (see WP 3.4). The delivery of the advanced chambers is planned for month 24 of the project Figure : Examples of new technology for vacuum cells and coils. Left: concept design of a atom trapping chamber made from expansion-coefficient matched Titanium with YAG windows. Right: highdensity coil made from copper foil WP 3.4 Advanced slowing and trapping methods The highest loading rate of the MOT used for first-stage cooling of the atoms in an optical lattice clock is achieved by decelerating the thermal atomic beam from the oven using a Zeeman slower. A standard Zeeman slower uses a specially designed current-carrying coil to create the required spatial distribution of the magnetic field along the axis of the atomic beam. At NPL a novel design of Zeeman slower based on permanent magnets has been developed [Ovc07], with the advantage for a space clock that it has zero power consumption since there is no need for high currents and corresponding cooling of its coils. In this workpackage, the design of this permanent magnet Zeeman slower will be reviewed with a view to reducing its mass and volume, and with the aim of producing a single slower that works for both Sr and Yb atoms. A new Zeeman slower that is compatible with the existing SOC neutral atom breadboard design will then be built and evaluated. ANNEX I 19

20 For the most compact optical clock it would be preferable to eliminate the Zeeman slower completely, and routes whereby this may be achieved without reducing the number of trapped atoms to unacceptable levels will also be explored for both Sr and Yb atoms. For Sr, dispensers offer a compact alternative to conventional ovens and will require significantly less thermal shielding. A new compact arrangement using Sr dispensers and a 2D MOT to provide a cold beam to load the 3D science chamber MOT will be implemented and analyzed with respect to key parameters such as the atom flux, background gas pressure, laser power and magnetic field requirements. In such a setup the dispensers will be much closer to the science chamber and so care will be taken to avoid a direct line-of-sight between them so that the blackbody radiation shift is minimized. For Yb, the possibility of a MOT without Zeeman slower (with corresponding volume, mass advantage) has already been demonstrated at INRIM; a new chamber will be developed in order to significantly increase the number of loaded atoms in the no-zeeman slower configuration; in particular a new low divergence, high performance Yb oven will be designed and its loading capability characterized in terms of loading time and number of loaded atoms. WP 4 Integrations and Characterizations WP 4.1: Clock Characterization I of transportable Yb optical lattice clock A first characterization of the transportable Yb clock will be performed at the University of Düsseldorf (UDUS) by comparison of the Yb clock with a GPS-steered hydrogen maser at the level. A second possibility is a comparison with an ultrastable cryogenic optical resonator, which is under development in an ESA project, where long-term stability at the level is projected in ~2012. The main aspects that will be investigated in this first characterization sequence are the performance of bosonic and fermionic species in different lattice geometries and isotope-dependent density shifts. WP 4.2: Advanced Sr clock integration and optimization In this work package the compact laser distribution breadboards will be combined to the atomics package. At the atomics package all the light is brought in by fibers connected to collimation packages. For the MOT 3 dichroic couplers including quarter-wave plates suitable for first and second stage cooling at 461 nm and 689 nm will be employed. Dichroic cat eyes with integrated quarter-wave plates produce the reflected beams with low sensitivity to alignment and vibrations. For the slower, repumpers, spin polarization and detection and lattice laser beams similar couplers will be directly attached to the vacuum system for highest stability. Magnetic field control for the MOT quadrupole fields, the homogeneous field during the probing of the clock transition and 3 Helmholtz coils for compensation of surrounding fields will be implemented. The measurement and control signals of all the laser breadboards and of the frequency stabilization system will be connected to the experimental control computer that provides the timing for the laser sources and the magnetic fields. Here software will be developed that will allow mostly unattended operation of the clock for several days, necessary to perform an evaluation with respect to a Cs fountain.. The integration shall start in month 24. The MOT operation and the transfer to the lattice will be optimized and initial tests of the excitation of the clock transition and low noise detection will be performed. Afterwards, the setup will be transported to PTB to integrate the 2nd generation reference cavity from WP 2. WP 4.3: Clock optimization II and characterization with respect to stationary clocks After finalizing the Sr and Yb clock demonstrators and having optimized the laser cooling to µk temperatures and the loading of the lattice, the clock performance will be evaluated. This characterization and optimization of both optical lattice clock demonstrators will be performed by comparison to state-of-the-art stationary lattice clocks at national metrology labs. A fractional uncertainty of the clocks below is to be reached, which requires detailed characterization of e.g. blackbody radiation shifts, magnetic field shifts, lattice-polarization influences, density shift, etc. (see Table in Appendix). The currently largest contribution from black-body shift will be reduced by a well defined thermal environment (+/- 0.1 K) of the second generation atomics package. The correction to zero temperature can be performed using the comparison with stationary cryogenic Sr lattice clocks that are currently developed at PTB or Tokyo. With the high-performance reference cavities of WP 2, an instability below at one second averaging time is aimed at, which will be characterized with stationary ultrastable lasers. Using an ensemble of several high-performance lasers available at the metrology institutes it will be possible to characterize each laser independently by the so called three cornered hat method [eks02, liu09]. ANNEX I 20

21 At the national metrology labs infrastructure for the comparison of the transportable clock with stationary lattice clocks (Sr at PTB, Yb at INRIM) will be provided. Both institutes operate state-of-the art Cs fountain clocks that will allow the absolute frequency measurement of both clock prototypes. Depending on the progress of ACES, with the ground terminal at PTB even comparisons to remote clocks could be performed starting around WP 4.4 Completion and full characterization of a first generation breadboard lattice clock The cold Sr breadboard with 1 st stage cooling from UNIFI will be completed with the modular laser system of WP 1. The integration will lead to a clock with a 1D lattice geometry., The clock will be tested, optimized and characterized by comparison with the stationary Sr clock at UNIFI until the end of the second year. During this work the gained data and experience contibute to the desig of the 2 nd generation breadboard, in order to ensure optimum outcome of the latter. WP 5 Robustness and lifetime tests of critical components WP 5 is a relatively small WP devoted to qualification of selected components. While electro-optical components and some continuous-wave laser sources have been developed for space use (e.g. for the missions ACES, LISA, PROBA-2), many components and sub-systems of optical clocks have not, due to their novelty. It is not possible in this project to perform complete space-qualification, which is an extensive activity that must be done within technology programs of ESA or national agencies. Nevertheless, for some central lasers (1st stage cooling lasers) which have TRL 4, we believe it to be important to perform first tests under conditions of space, especially radiation. Thus, the laser chips identified in WP 1 and WP 2 as suitable for reliable clock operation will undergo characterization before and after exposure to radiation and thermal vacuum to identify suitability. WP 6 Assessment of components, preliminary design of a space optical clock, roadmap In laboratory activities, the main technological and scientific efforts are addressed to the optimization of the physics package composing the clock while the power distribution unit, the control system and the data acquisition equipments are, generally, less emphasized. For all subsystems, power consumption and dimensions are only weakly relevant. This project will make a strong effort in reducing the size, weight, and power consumption of a number of subsystems, both on the lasers and the atomics package side, but not on the electronics side. Thus, significant additional efforts are required in order to make optical clock technology suitable for space applications, and with only moderate performance reduction compared to laboratory clocks. This work WP is intended as a step in this direction (other activities will be performed in ESA programs, and coordination with them will be ensured, see Sec.3.1). It is dedicated to highlight and treat design and technology aspects stemming from specific space related requirements and their impacts on system and subsystem level. The activities are aimed towards the preliminary identification/characterization of critical areas/components and desirable improvements concerning all subsystem areas such as physics package, lasers or electronics and control equipment to be addressed during the breadboarding phase to resolve the problems related mainly to budgets (mass, weight, power consumption). It consists of the preliminary assessment of the power distribution, data acquisition & control system. Following the laboratory activities and development efforts from the WPs right from the start of the project, the system requirements, received as input, shall be elaborated to derive the preliminary requirements for the various subsystems such as power distribution, data acquisition & control system. Once the requirements have been identified, preliminary assessments for the future design of the subsystems will be made.the analysis of the system concept as well as the subsystems under investigation will be based on a preliminarily derived example for environmental conditions and requirements for clock systems on characteristic space missions. On system level the activities will review the previous and on-going developments in order to identify possible improvements both on system and subsystem design. This will include the identification of required interfaces and the establishment and maintenance of system budgets for mass, power etc. An asset will also be the derivation of specific top level operation and calibration routines for clock systems in space which may impact the system design. The elaborated results will serve as basis for the elaboration of a sound roadmap towards the development and qualification of optical clocks for space. A major subsystem task of the WP shall be the assessment of a self-containing module providing power to all the clock sub-systems and of the self-containing module managing the commands and the control functions. ANNEX I 21

22 WP 7 Administrative Management Activities here will include preparation of reports and meetings for REA, information of partners, interaction with advisory board on the administrative side, etc. This workpackage does not include any technical management. WP 8 Dissemination and Exploitation A crucial aspect is dissemination. We already have a website ( which will be further developed to include sections on technology (for reaching out to interested external space and non-space industries) animations, extensive references, and status reports. Furthermore, we will establish a presence on the CORDIS platform of the EU. Dissemination to other scientific communities such as geophysics and the general public is taken very seriously (see below in the proposal). The project will be reported on by the team members and the coordinator at numerous relevant conferences (Europ. Time and Freq. Forum, Intl. Conf. On Space Optics, Conf. On Lasers and Electro-Optics, COSPAR, ESA Workshop on Optical Clocks, Intl. Conf. on Atomic Physics, etc.), and at university seminars. We expect to have some 30 presentations over the duration of the project We have also developed a draft preliminary plan for exploitation (see Sec. 3.2) which is already quite extensive. This will be further developed and the means to implement it (obtaining additional funding) worked out. Our contact with potential industrial users of the know-how (spin-off) to be developed in this project is already extremely strong in this project, since we have both non-space as well as space industries as team members (MENLO, TOPTICA, ASD, KT, KI). Nevertheless, we will approach other companies that could also be interested in the technologies we will develop (e.g. TESAT, TERAXION, SIRAH, SODERN). ANNEX I 22

23 1.3.2 Timing of the different WPs and their components and interdependencies ANNEX I 23

24 Appendix 2 Introduction to optical clocks and state-of-the-art for laboratory clocks A2.1 Optical clocks: Introduction Optical atomic clocks use optical transitions in laser cooled neutral atoms or ions as quantum frequency reference (QFR) (see Figure A1). The invention of the femtosecond frequency comb has made it possible to precisely count frequencies in the optical domain, and to transform them into the radiofrequency domain, where they can be used by traditional techniques. The scientific challenges for optical atomic clocks are the establishment of techniques for reliable and simple preparation of suitable QFRs, the control of systematic effects to a high degree of accuracy, and the development of the required components, in particular ultrastable laser sources at the frequencies corresponding to the clock transitions. From an application point of view, the technological challenge lies in developing a system that is robust and whose electronic control is sufficiently sophisticated that unattended, automatic operation is possible. Two approaches towards optical clocks are pursued in the field of time metrology at present. The first is based on a single ion trapped in an electrodynamic trap, where the storage time can exceed many weeks. The second is based on using ensembles of tens of thousand neutral atoms trapped for a relatively short time (seconds) in a trap formed by standing optical waves (optical lattice) delivered by a laser. Here, a new ensemble of atoms is periodically reloaded into the trap. Cold atoms Laser Detector Optical frequency ~ GHz Femtosecond frequency comb Feedback control electronics Error signal l ds dν ν 0 S ν 0 ν Absorption signal ν Radio-frequency signal, 200 MHz Figure A1 Prinicple of an optical atomic clock. A laser, the local oscillator, interrogates an ensemble of ultracold atoms, the QFR. In the case of a lattice optical clock (shown) the atoms are at µkelvin temperature and trapped by laser waves The interrogation by the local oscillator results in a signal proportional to the absorption of the laser light (frequency ν), which is maximum for a light frequency ν 0 corresponding to the center of the atomic resonance. With a feedback control, the laser frequency ν is continuously kept tuned on the atomic resonance frequency. The resulting ultra-stable optical frequency can be converted to an equally stable radio-frequency by means of a femtosecond laser frequency comb. ANNEX I 74

25 Figure A2: Schematic of a lattice clock apparatus showing the required elements. An atomic beam is produced by an oven and travels towards the right through a space-varying magnetic field. In it, the atoms are slowed down by a laser beam (blue arrow) that eventually stops the atoms inside the experimental chamber (square). The red double arrows indicate the laser beams for 2 nd stage cooling and trapping in the MOT. Orange-red: lattice laser standing-wave, yellow: clock laser wave. Inset at bottom: variation of the potential felt by the atoms due to the lattice laser. Since the lattice wavelength is longer than the allowed atomic transition wavelength, the atoms are trapped in the potential minima. The fundamental advantage of neutral atom optical clocks is that comparatively large numbers (~10 4 ) of QFRs are used simultaneously, resulting in a high signal-to-noise ratio. This leads, in turn, to a short-term stability which is potentially vastly better (factor 10 2 ) than that already obtained with the single-ion clocks. Lattice optical clock with neutral atoms confine them (for a short time) in a so-called magic-wavelength optical lattice [Kat03], where the wavelength is chosen such that the energy shifts of the lower and upper states of the clock-transition 1 S 0 3 P 0 are exactly equal and thus the trapping potential exerts no shift on the clock transition. The values are 813 nm for Sr and 759 nm for Yb. The clock transition is a singlet-to-triplet transition that is nearly forbidden and therefore exhibits an extremely narrow (theoretical) linewidth << 1 Hz. In practice, due to a finite interrogation time, it is of the order of a 1 10 Hz. The preparation of a sample of ultracold neutral atoms follows the same basic principle for all species currently considered as optical clock candidates (Figs. A2, A3). Efficient precooling to temperatures in the mk range is done on a spectrally broad 1 S 0 1 P 1 (some tens of MHz) cycling transition (refer to Fig. A4 for the atomic level scheme), followed by a postcooling stage, typically on the 1 S 0 3 P 1 intercombination transition, which brings the temperature down into the µk range. The atoms are then transferred into an optical lattice, formed by at least two counterpropagating laser beams of (same) suitable wavelength. Theoretical investigations have shown that it should be possible to control higher order perturbations caused by these lattice laser waves at levels allowing an inaccuracy below one part in [Por08]. ANNEX I 75

26 Figure A3 Basic steps of operation of a lattice optical clock. Top row :(a) precooling and trapping: atoms produced by an oven are slowed down, then cooled and simultaneously trapped in a magneto-optical trap produced by two coils and six counter-propagating 1 st stage cooling laser beams (blue).(b) postcooling and trapping: Once the atoms are cold enough, the 2 nd stage cooling laser beams nearly resonant with the 1 S 0 3 P 1 transition are turned on, forming a 2 nd stage MOT. The atoms are cooled further, because the transition to the 3 P 1 level is spectrally narrow. The counterpropagating optical lattice laser beams (red) are also turned on. (c) intercombination transition interrogation: when the atoms are again cooled sufficiently, the 2 nd stage cooling beams are turned off, leaving a fraction of the atoms trapped in the optical lattice. The clock laser beam is turned on for a short time (not shown), exciting a fraction of the atoms from the 1 S 0 to the 3 P 0 state. Subsequently, a pulse of 1 st stage cooling light (blue) is applied. The ensuing fluorescence of the decay from the 1 P 0 state is measured; its strength is an indication of the number of atoms that was not excited by the clock laser, and represents the spectroscopic (clock absorption) signal also shown in Fig. A1. After interrogation, the atoms are lost and the cycle is repeated, with the clock laser frequency changed by a small amount. In this way, the resonance line is obtained, that also gives an error signal for correction of the laser frequency. Bottom inset: geometry of the lattice laser beams (red), produced by retroreflection, and the superposed probe beam (yellow). A magnetic field is applied to compensate the earth field or, in case of a bosonic atomic species, to make the clock transition an allowed one. Two types of atoms can serve as a QFR. Fermionic isotopes, in which the 1 S 0 3 P 0 transition possesses a finite linewidth of typically a few mhz [Por04] due to hyperfine mixing in the excited state are one choice. However, it is also possible to use the bosonic isotopes where the strongly forbidden transition becomes weakly allowed by admixing some 3 P 1 or 1 P 1 character to the 3 P 0 state, by applying a magnetic field to the atomic sample. The availability of both bosons and fermions opens interesting possibilities and in-depth studies of the respective advantages and disadvantages of the various species, such as density-induced frequency shifts or line broadening, and will be pursued in this project. ANNEX I 76

27 Figure A4 shows the relevant energy diagrams of the two atomic species used in this work. Figure A4 Level schemes for Sr (top) and Yb (bottom) showing some of the relevant transitions. Color (grey) double arrows: transitions excited by lasers. Black single arrows: spontaneous emission loss channels. Γ denotes the spontaneous emission rates. The magic wavelengths for the optical lattice are not shown in this diagram. ANNEX I 77

28 A2.2 State-of-the art in stationary optical clocks For completeness we give the state-of-the-art of optical clocks in general. Note, however, that for transportable and compact systems, compromises in the type of employed techniques, components and subsystems have to be made in order to achieve compactness. For comparison with the following optical clocks: the instability of cold atom Cs microwave clocks (defining the unit of time) is at a level of / τ 1/2 and inaccuracy of a several parts in Single-Ion clocks: Single-ion optical clocks are under development since the mid-1980s. Ion clocks operating or under development use the following atom species: Mercury (NIST USA), Aluminium (NIST, PTB (Germany, under construction)), Strontium (NPL UK, NRC Canada), Ytterbium (PTB, NPL), Calcium (U. Marseille, U. Innsbruck, NICT Japan), and Indium (MPI Erlangen Germany). Currently, best performance is from Hg + and Al +, for which the published inaccuracy is at a 2.3 and [Cho09] respectively. Ca +, Yb + and Sr + have reached uncertainties of 18, 9, and , respectively. For two Yb + ion clocks (PTB), a combined instability of at 3000 s was demonstrated and recently an inaccuracy of was reached [Tam09]. The most recent measurement of the instability between the Al + ion and a Yb lattice clock gave at 2000 s [Oat09]. Neutral atom lattice clocks This more recent clock type was first proposed at the beginning of the decade and is meanwhile under study in all major national metrology institutes and and in several university laboratories. The atom species most studied is neutral Strontium (OBSPM, PTB, NPL, JILA, U. Tokyo, UNIFI, VNIIFRI), followed by Ytterbium (NIST, INRIM, NMIJ Japan, U. Tokyo, U. Washington, UDUS). Magnesium (LUH, U. Copenhagen) and, more recently, Mercury (OP Paris, U. Tokyo) are additional clock systems (with UV laser cooling) being investigated. The Sr clock has reached an inaccuracy of [Lud08]. The Yb clock has recently been shown with inaccuracy at , limited by the not yet precisely known black-body shift coefficient [Lem09]. The table summarizes the various contributions. The instability of the same Yb clock has reached at 2000 s, in a comparison measurement with the Al + clock [Oat09]. ANNEX I 78

29 Table : Present state-of-the art error budget of a Sr lattice clock [lud08] and for a Yb lattice clock (in units of ) [Lem09] ANNEX I 79

30 A3 State-of-the-art in transportable optical clocks A description has been given in Sec. 1.2 above. For further detail, the figure below shows a schematic of the components and laser beam paths of the ultracold atom source of one of the transportable clock systems developed in the SOC project [Abo09], which is an input to this project. Fig. A5 : Schematic of the transportable ultracold Yb lattice clock system (current status, Fig ). All components for the ultracold source and the optical lattice (Fig ) are included on one optical table. After replacement of the post cooling laser system (green) with an optimized fiber laser system (WP 2), the optical table will also host the repumping laser at 1389 nm (WP 2). ANNEX I 80