Optical clocks and fibre links. Je ro me Lodewyck

Optical clocks and fibre links Je ro me Lodewyck J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 1/34

Content 1 Atomic clocks 2 Optical lattice clocks 3 Clock comparisons 4 Comparison of optical clocks with fibre links J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 2/34

Content 1 Atomic clocks 2 Optical lattice clocks 3 Clock comparisons 4 Comparison of optical clocks with fibre links J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 3/34

A quick overview of atomic clocks Atomic clock: 1: Local oscillator 2: Atomic resonance ω(t) ω 0 e f Linewidth δω 2π 1 Hz J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 4/34

A quick overview of atomic clocks Atomic clock: 1: Local oscillator 2: Atomic resonance ω(t) Lock of the LO on the atom: ω(t) = ω 0 (1 + ɛ + y(t)) ω 0 e f Linewidth δω 2π 1 Hz J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 4/34

A quick overview of atomic clocks Atomic clock: 1: Local oscillator 2: Atomic resonance ω(t) Lock of the LO on the atom: ω(t) = ω 0 (1 + ɛ + y(t)) ω 0 e f Linewidth δω 2π 1 Hz Optical vs. microwave Microwave clocks: ω 0 /2π 10 10 Hz Q = ω 0 /δω 10 10 Optical clocks: ω 0 /2π 10 14 to 10 15 Hz Q = ω 0 /δω 10 15 Optical clocks improves both the accuracy and the frequency stability J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 4/34

History of atomic clocks accuracy Clocks accuracy 10-10 10-11 10-12 10-13 10-14 10-15 10-16 Essen and Parry Redefinition of the SI second H Optical frequency combs Atomic fountains Al + Hg + 10-17 Microwave clocks Al + Sr Optical clocks Sr Yb+ 10-18 1950 1960 1970 1980 1990 2000 2010 2020 Ca H H Sr +, Yb + H Hg +, Yb +, Ca Sr Hg +, Yb + Sr Microwave clocks: 1 order of magnitude every 10 years since 1950 Optical clocks: 2 orders of magnitude every 10 years since 1990 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 5/34

History of atomic clocks accuracy Clocks accuracy 10-10 10-11 10-12 10-13 10-14 10-15 10-16 Essen and Parry Redefinition of the SI second H Optical frequency combs Atomic fountains Al + Hg + 10-17 Microwave clocks Al + Sr Optical clocks Sr Yb+ 10-18 1950 1960 1970 1980 1990 2000 2010 2020 Ca H H Sr +, Yb + H Hg +, Yb +, Ca Sr Hg +, Yb + Sr Microwave clocks: 1 order of magnitude every 10 years since 1950 Optical clocks: 2 orders of magnitude every 10 years since 1990 Microwave Optical Ion low Q, single ion high Q, single ion Neutral low Q, 10 6 atoms high Q, 10 4 atoms J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 5/34

History of atomic clocks accuracy Clocks accuracy 10-10 10-11 10-12 10-13 10-14 10-15 10-16 Essen and Parry Redefinition of the SI second H Optical frequency combs Atomic fountains Al + Hg + 10-17 Microwave clocks Al + Sr Optical clocks Sr Yb+ 10-18 1950 1960 1970 1980 1990 2000 2010 2020 Ca H H Sr +, Yb + H Hg +, Yb +, Ca Sr Hg +, Yb + Sr Microwave clocks: 1 order of magnitude every 10 years since 1950 Optical clocks: 2 orders of magnitude every 10 years since 1990 Microwave Optical Ion low Q, single ion high Q, single ion Neutral low Q, 10 6 atoms high Q, 10 4 atoms Optical lattice clocks J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 5/34

Content 1 Atomic clocks 2 Optical lattice clocks 3 Clock comparisons 4 Comparison of optical clocks with fibre links J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 6/34

Optical lattice clocks Atoms loaded from a MOT to an optical lattice formed by a 1D standing wave Probing a narrow optical resonance with an ultra-stable clock laser Stabilize the clock laser on the narrow resonance Combine several advantages: Optical clock Large number of atoms Lamb-Dicke regime insensitive to motional effects Record frequency stability : a few 10 16 / τ Record accuracy : a few 10 18 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 7/34

Which atom for an optical lattice clocks? Sr lattice clock ³S₁ Wavelengths accessible with semi-conductor sources Magic wavelength at 813 nm Implemented in many laboratories good candidate for a new SI second ¹P₁ MOT λ = 461 nm Γ = 32 MHz ¹S₀ ³P₀ Clock ³P₁ ³P₂ λ = 689 nm Γ = 7.6 khz λ = 688 nm J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 8/34

Which atom for an optical lattice clocks? Sr lattice clock ³S₁ Wavelengths accessible with semi-conductor sources Magic wavelength at 813 nm Implemented in many laboratories good candidate for a new SI second ¹P₁ MOT λ = 461 nm Γ = 32 MHz ¹S₀ ³P₀ Clock ³P₁ ³P₂ λ = 689 nm Γ = 7.6 khz λ = 688 nm Hg lattice clock Developed at LNE-SYRTE Low sensitivity to BBR excellent ultimate accuracy Requires UV lasers J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 8/34

Which atom for an optical lattice clocks? Sr lattice clock ³S₁ Wavelengths accessible with semi-conductor sources Magic wavelength at 813 nm Implemented in many laboratories good candidate for a new SI second ¹P₁ MOT λ = 461 nm Γ = 32 MHz ¹S₀ ³P₀ Clock ³P₁ ³P₂ λ = 689 nm Γ = 7.6 khz λ = 688 nm Hg lattice clock Developed at LNE-SYRTE Low sensitivity to BBR excellent ultimate accuracy Requires UV lasers Other candidates Yb: mostly equivalent to Sr Mg, Cd J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 8/34

Two strontium optical lattice clocks Sr1 Sr2 Accuracy : 1.1 10 16 New vacuum system Accuracy : 4.1 10 17 Operational measurements J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 9/34

Sr clock laser 10 cm horizontal ULE spacer and silica mirrors Not at inversion of CTE 3 layers of thermal shielding (short term temperature fluctuations in the nk range, τ = 4 days) Residual drift of a few 10s of mhz/s (feed forward compensation below 1mHz/s) J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 10/34

Locking the clock laser to the atomic transition 0.9 0.8 Transition probability 0.7 0.6 0.5 0.4 0.3 0.2 0.1 3 Hz 0 8 6 4 2 0 2 4 6 8 Frequency (Hz) Resonance of the clock transition Fourier limited at 3 Hz (250 ms) Laser noise dominating J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 11/34

Locking the clock laser to the atomic transition 10-14 Transition probability 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 3 Hz 0 8 6 4 2 0 2 4 6 8 Frequency (Hz) Resonance of the clock transition Fourier limited at 3 Hz (250 ms) Laser noise dominating Fractional Allan deviation 10-15 10-16 10-17 0.1 1 10 100 1000 10000 Time (s) Atoms vs cavity < 5 s : limited by the atoms (Dick effect) 1 10 15 / τ > 5 s : limited by the thermal noise of the cavity 6.5 10 16 Clock comparison 1.5 10 15 / τ resolution in the low 10 17 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 11/34

Accuracy budget for the Sr clocks Main contributions: (Sr 2), in 10 18 Effect Correction Uncertainty Black-body radiation shift 5208 20 Quadratic Zeeman shift 1317 12 Lattice light-shift 30 20 Lattice spectrum 0 1 Density shift 0 8 Line Pulling 0 20 Probe light-shift 0.4 0.4 AOM phase chirp 8 8 Servo error 0 3 Static charges 0 1.5 Black-body radiation oven 0 10 Background collisions 0 8 Total 6487.4 41 The black-body shift remains the most important contribution Other effects mainly limited by statistics J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 12/34

Accuracy budget for the Sr clocks Main contributions: (Sr 2), in 10 18 Effect Correction Uncertainty Black-body radiation shift 5208 20 Quadratic Zeeman shift 1317 12 Lattice light-shift 30 20 Lattice spectrum 0 1 Density shift 0 8 Line Pulling 0 20 Probe light-shift 0.4 0.4 AOM phase chirp 8 8 Servo error 0 3 Static charges 0 1.5 Black-body radiation oven 0 10 Background collisions 0 8 Total 6487.4 41 The black-body shift remains the most important contribution Other effects mainly limited by statistics Particularity of our clocks: Low level of cold collisions. characterization of the static Stark effect Accurate determination of lattice light-shift J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 12/34

Magic wavelength Light-shift Intense trapping laser light-shift of several MHz At the magic wavelength, this huge light-shift is cancelled J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 13/34

Magic wavelength Light-shift Intense trapping laser light-shift of several MHz At the magic wavelength, this huge light-shift is cancelled Looking closer... Polarization dependent 1st order light-shifts (vector and tensor shifts) Mostly cancelled for a J = 0 J = 0 transition. Hyperpolarizability (2 photon transitions) Multipolar effects (E2/M1) Need to pay attention to achieve high accuracy P. Westergaard et al., PRL 106 210801 (2011) J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 13/34

Other groups Rapid progress for optical lattice clocks JILA: Sr optical lattice clock with 2 10 18 accuracy NIST: Stability down to 1.6 10 18 after 7 h between 2 Yb clocks PTB: Sr optical lattice clock with 1.9 10 17 accuracy ultra-stable laser with 8 10 17 noise floor Riken: Comparison between to cryogenic Sr clocks with 7.2 10 18 accuracy. J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 14/34

Improving the clock reliability Motivation: Need for operational optical clocks Enhancing the statistical resolution better characterization of systematic effects Enabling clock comparisons Participating to the Pharao/ACES space mission Establishing time scales with optical clocks Goal: Reach a level of maturity equivalent to the Cs based clock architecture possible redefinition of the SI second J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 15/34

Improving the clock reliability Motivation: Need for operational optical clocks Enhancing the statistical resolution better characterization of systematic effects Enabling clock comparisons Participating to the Pharao/ACES space mission Establishing time scales with optical clocks Goal: Reach a level of maturity equivalent to the Cs based clock architecture Results: possible redefinition of the SI second Accumulation of 3 long operations of the Sr2 clock More and more autonomous J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 15/34

Operation of the full metrological chain Feb. 2014 Attended operation of 5 days, 87% uptime Oct. 2014 Unattended operation of 10 days, 92% uptime (ITOC JRP) Maser relative error/10-13 -2.1-2.2-2.3-2.4-2.5-2.6-2.7-2.8-2.9 Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 Jun. 2015 Unattended operation of 31 days, 83% uptime (ITOC JRP) Maser relative error [ 10-13 ] -2.6 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17 D18 D19 D20 D21 D22-2.7-2.8-2.9-3 -3.1 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 16/34

Content 1 Atomic clocks 2 Optical lattice clocks 3 Clock comparisons 4 Comparison of optical clocks with fibre links J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 17/34

Clock comparisons: why and how? Objectives of clock comparisons Prove the reproducibility of optical lattice clocks Determine and track frequency ratios between different atomic species Measure offsets and variations of the geo-potential at country/continental scales Probe fundamental physics Use optical clocks to define international time scales Means of comparison Local or local area comparisons limited range Satellite comparisons (GPS/TWSTFT) limited resolution (fine for microwave clocks, but not optical) Stabilized optical fibre links limited to continental scales and connected places Pharao/ACES space clock on board the ISS (2017) limited in time J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 18/34

Local optical to optical comparison Sr vs. Sr Frequency difference Sr2-Sr1 (Hz) 0,2 0,1 0,0-0,1 Sr2-Sr1 Average Statistic Statistic Systematics Systematics 1 2 3 4 5 6 7 8 Measurement # 4x10-16 2x10-16 0-2x10-16 Sr 2 - Sr 1 = 1.1 10 16 ± 2 10 17 (stat) ± 1.6 10 16 (sys) First agreement between OLCs Enabled the detection of several systematic effects R. Le Targat et al. Nat. Commun. 4 2109 (2013) J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 19/34

Local optical to optical comparison Sr vs. Sr Sr vs. Hg 10-14 Frequency difference Sr2-Sr1 (Hz) 0,2 0,1 0,0-0,1 Sr2-Sr1 Average Statistic Statistic Systematics Systematics 1 2 3 4 5 6 7 8 Measurement # 4x10-16 2x10-16 0-2x10-16 Overlapping Allan variance 10-15 10-16 10-17 10 0 10 1 10 2 10 3 10 4 Time [s] Sr 2 - Sr 1 = 1.1 10 16 ± 2 10 17 (stat) ± 1.6 10 16 (sys) First agreement between OLCs Enabled the detection of several systematic effects Hg/Sr = 2.62931420989890915 ±5 10 17 (stat) ± 1.7 10 16 (sys) Optical to optical frequency measurement Best reproduced frequency ratio (with RIKEN, Tokyo) R. Le Targat et al. Nat. Commun. 4 2109 (2013) R. Tyumenev et al., arxiv:1603.02026 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 19/34

Sr vs microwave atomic fountains: stability Absolute frequency measurements Fractional Allan deviation 10-13 10-14 10-15 10-16 10-17 Sr vs Cs and Rb 3.4e-14/ τ 3.9e-14/ τ 0.1 1 10 100 1000 10000 100000 1x10 6 Time (s) Sr vs Rb (best µwave vs optical stability) Fractional Allan deviation 10-13 10-14 10-15 10-16 2.8e-14/ τ 1 10 100 1000 10000 Time (s) Frequency stability limited by the QPN of the microwave fountains 10 16 resolution after 12 h mid 10 17 resolution after 7 days J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 20/34

Sr vs microwave atomic fountains: accuracy 5x10-16 FO1 FO2-Cs FO2-Rb 3.4 4 Absolute frequency of the Sr clock transition (ν Sr/νX)/(νSr 0 /νx 0 ) - 1 0-5x10-16 -1x10-15 -1.5x10-15 SI recommendation 3.2 3 2.8 2.6 ν Sr/Hz - 429228004229870 ν Sr/Hz - 429228004229870 3.5 3 2.5 2-2x10-15 Sept. 2014 Oct. 2014 Mar. 2015 Jun. 2015 2.4 54000 54500 55000 55500 56000 56500 57000 MJD 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 International agreement, limited by the accuracy of the fountains SYRTE, PTB, JILA, Tokyo University, NICT, NMIJ, NIM Track variations of fundamental constants: d ln(ν Sr /ν Cs ) = 1.6 10 16 ± 6.5 10 17 /year dt Annual variation of ν Sr /ν Cs with relative amplitude: 5.5 10 17 ± 1.8 10 16 J. Lodewyck et al., Metrologia (2016), arxiv:1605.03878 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 21/34

International clock comparisons: TWSTFT Comparison Sr vs. Yb + (PTB, NPL) via TWSTFT ITOC JRP project 10-10 10-11 OP Maser vs. PTB Maser Yb + vs. PTB Maser Sr vs. OP Maser + CSO Fractional Allan deviation 10-12 10-13 10-14 10-15 10-16 1 10 100 1000 10000 100000 1x10 6 Time (s) Results Statistical resolution in the mid 10 16 after 7 days of measurement Frequency ratio compatible with independent local measurements 3 weeks comparison achieved in June 2015 with many more clocks! J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 22/34

Content 1 Atomic clocks 2 Optical lattice clocks 3 Clock comparisons 4 Comparison of optical clocks with fibre links J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 23/34

International clock comparisons: fibre links Goal: high resolution comparison Direct comparison of optical clocks over continental scale Pure optical comparison, not limited by Microwave transfer methods Microwave oscillators Preserve the frequency stability over long distances Implementation Disseminate an IR (1542 nm) vector narrow laser through phase-compensated optical fibres Optical frequency combs to measure ν IR /ν clock on both sides J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 24/34

International clock comparisons: fibre links Goal: high resolution comparison Direct comparison of optical clocks over continental scale Pure optical comparison, not limited by Microwave transfer methods Microwave oscillators Preserve the frequency stability over long distances Implementation Disseminate an IR (1542 nm) vector narrow laser through phase-compensated optical fibres Optical frequency combs to measure ν IR /ν clock on both sides Challenges Fibre attenuation (e.g. 450 db for 1500 km) Availability of fibres (dark channel or dark fibre) Propagation delays (cascaded links) Power limits (non-linear effects, disturbance of telecom networks) J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 24/34

SYRTE PTB link, via LPL PTB, LPL and SYRTE established a 1415 km long optical fibre link and performed in 2015 the first direct comparison of optical clocks at continental scale J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 25/34

Challenge 1: Amplification and signal regeneration Paris Strasbourg Signal along internet traffic (RENATER) Bidirectional amplifiers Regeneration stations Poster by A. Amy-Klein J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 26/34

Challenge 1: Amplification and signal regeneration Paris Strasbourg Signal along internet traffic (RENATER) Bidirectional amplifiers Regeneration stations Braunschweig Strasbourg Dark fibre Fibre Brillouin amplification Poster by A. Amy-Klein J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 26/34

Challenge 2: Fibre noise compensation J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 27/34

Comparing remote optical clocks Completely independent Sr clocks: PTB SYRTE Loading of the atoms Blue MOT-Red MOT Blue+atomic drain Lattice light TiSa pumped by a multimode pump TiSa pumped by a monomode pump BBR Shield from oven No direct line of sight Deflected atomic beam Lattice orientation Horizontal Vertical Lattice effect Retroreflected light Cavity-formed + PDH lock Clock laser 48 cm long cavity, flickering at 8 10-17 10 cm long cavity, flickering at 5 10-16 Density of atoms 1-2 atoms/site 5-10 atoms/site Coils In-vacuum MOT coils MOT coils outside of vacuum Gravitational redshift -247.4 ( 0.4) 10-17 Uncertainty budgets 1.9 10-17 4.1 10-17 Only agreement between completely independent optical clocks Second to best comparison of optical lattice clocks J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 28/34

Results 2 Measurement campaigns Total clocks 650.3 4.1 496.3 1.9 Ratio SrPTB/SrSYRTE Campaign I Campaign II Unc. (10 17 ) Unc. (10 17 ) Systematics SrSYRTE 4.1 4.1 Systematics SrPTB 2.1 1.9 Statistical uncertainty 2 2 fs combs 0.1 0.1 Link uncertainty < 0.1 0.03 Counter synchronization * 10 < 0.01 Gravity potential correction ** 0.4 0.4 Total clock comparison 11.2 5.0 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 29/34

Results 2 Measurement campaigns Total clocks 650.3 4.1 496.3 1.9 Ratio SrPTB/SrSYRTE Campaign I Campaign II Unc. (10 17 ) Unc. (10 17 ) Systematics SrSYRTE 4.1 4.1 Systematics SrPTB 2.1 1.9 Statistical uncertainty 2 2 fs combs 0.1 0.1 Link uncertainty < 0.1 0.03 Counter synchronization * 10 < 0.01 Gravity potential correction ** 0.4 0.4 Total clock comparison 11.2 5.0 Statistical uncertainty 2 10 17 after 1 hour Sr PTB /Sr SYRTE 1 = (4.7 ± 5.0) 10 17 150 hours of data C. Lisdat et al., arxiv:1511.07735 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 29/34

Applications Gravitation Correction for the gravitational redshift: ( 247.4 ± 0.4) 10 17 4 mm uncertainty of the (geodetic) height of the clocks Work by P. Delva (SYRTE), H. Denker (LUH) The next generation of remote clocks comparison will improve our knowledge of the gravitational potential of the Earth Presentation by P. Delva Fundamental sciences Precise measurement of frequency ratios Search for variation of fundamental constant, detection of dark matter J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 30/34

Extensions of the fibre networks Link SYRTE NPL J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 31/34

Extensions of the fibre networks Link SYRTE NPL National and international fibre networks J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 31/34

Conclusion GRAM 2010: Horloges optiques local Sr/Cs comparisons at 10 15 J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 32/34

Conclusion GRAM 2010: Horloges optiques local Sr/Cs comparisons at 10 15 Perspectives Improved stability and accuracy of OLCs 10 19 feasible Contribution of optical clocks to international time-scales redefinition of the SI second Complete European fibre network for comparison of optical clocks J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 32/34

Clock ensemble at SYRTE 2 strontium lattice clocks J. Lodewyck, R. Le Targat, S. Bilicki, E Bookjans, G. Vallet 1 mercury lattice clock S. Bize, L. De Sarlo, M. Favier, R. Tyumenev Frequency combs (Ti-Sa, fiber) Y. Lecoq, R. Le Targat, D. Nicolodi 3 atomic fountains (1 Cs, 1 Cs mobile, 1 Cs/Rb) J. Guéna, P. Rosenbusch, M. Abgrall, D. Rovera, S. Bize, P. Laurent J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 33/34

Comparison with fiber links LPL N. Quintin, F. Wiotte, E. Camisard, C. Chardonnet, A. Amy-Klein, O. Lopez SYRTE C. Shi, F. Stefani, J.-L. Robyr, N. Chiodo, P. Delva, F. Meynadier, M. Lours, G. Santarelli, P.-E. Pottie PTB C. Lisdat, G. Grosche, S.M.F. Raupach, C. Grebing, A. Al-Masoudi, S. Dörscher, S. Häfner, A. Koczwara, S. Koke, A. Kuhl, T. Legero, H. Schnatz, U. Sterr LUH H. Denker, L. Timmen, C. Voigt J. Lodewyck Optical clocks and fibre links GRAM, Juin 2016 34/34