1. Jeroen Koelemeij LaserLaB VU University j.c.j.koelemeij@vu.nl PartnersFunding Putting optical-fiber frequency links to work

2. Atomic clocks & network synchronization Atomic clocks: – Define International Atomic Time (TAI) and UTC – Stratum 1 frequency reference in telecom networks – Global Positioning System (GPS) – Scientific clocks Important: network synchronization – Telecom, power grid, electronic financial transactions, …

3. Outline 1.Atomic clock – Principle of operation – State of the art atomic clocks 2.(Optical) time and frequency transfer – Clock comparisons through optical networks – Applications Optical network synchronization Geodesy Positioning

4. 3. Counter (convert oscillator frequency to useful quantity) 3. Counter (convert oscillator frequency to useful quantity) 1. Stable oscillator (microwave source) 2. Atoms (fixed oscillation frequency) 2. Atoms (fixed oscillation frequency) Atomic clock: principle of operation Traditional (cesium) atomic clocks: based on microwaves Accuracy 15-16 digits (commercially 10-14 digits) NEW: ‘optical’ clocks based on lasers

5. Optical clock: principle of operation Oscillator: ultrastable laser (<1 Hz linewidth) Image courtesy W. Oskay (NIST) Atoms: single-ion trap (NIST) CCD camera image  10  m *NIST Al + clock: 17+ digits accuracy C.W. Chou, D.B. Hume, J.C.J. Koelemeij, D.J. Wineland, T. Rosenband, PRL 104, 070802 (2010)

6. How accurate are 17 digits accuracy? Relativistic effects at ‘human scale’ are  10 -17 =Time dilation running @ 2 m/s =Gravitational redshift over 10 cm height difference Effects visible when moving/lifting two Al + clocks!* * C.W. Chou, D.B. Hume, T. Rosenband, D.J. Wineland, Science 329, 1630 (2010) 10 cm 2 m/s Optical clocks = ‘Einstein sensors’

7. Clock comparisons: scientific value Atomic clock frequency depends on value of fundamental constants Example: fine structure constant (electromagnetic coupling strength) Atomic clocks based on different elements depend differently on  If value of  is changing over time(!), clocks based on different elements drift apart over time

8. Amsterdam ( 27 Al + ) Groningen ( 223 Ra + ) 150 km Current best test of variation  Comparison of Al + and Hg + optical clocks at NIST Boulder, CO (USA) – 27 Al + : little dependence on  – 199 Hg + : large dependence on  Constraint on variation  : (  1.6 ± 2.3) × 10  17 /year T. Rosenband et al., Science 319 1808 (2008) Europe: number of optical clocks (under development), but located in laboratories >>100 km apart Example: Netherlands

9. Comparing clocks at >100 km distance ‘Long-haul time and frequency transfer’: GPS (traditionally state of the art) Works well for traditional atomic clocks, but orders of magnitude insufficient for optical clocks… ‘traditional’ atomic clocks Digits of accuracy Year GPS accuracy Development of atomic clock accuracy over time ‘optical’ atomic clocks ?

10. Comparing clocks at >100 km distance ‘Long-haul time and frequency transfer’: GPS (traditionally state of the art) Works well for traditional atomic clocks, but orders of magnitude insufficient for optical clocks… ‘traditional’ atomic clocks Digits of accuracy Year GPS accuracy Development of atomic clock accuracy over time ‘optical’ atomic clocks ?

11. Optical clock through optical fiber? ‘Ticking’ signal of optical clock is a laser Use optical fiber for T&F transfer?

12. Telecom-wavelength optical clock ‘Copy’ optical clock laser to frequency comb (PLL)  all comb teeth become replicas of the clock laser! frequency intensity 1 GHz Telecom wavelengths (1.5  m) This example: f clock laser = 1 121 015 × (1 GHz) 123 … Clock laser 1 121 015 Frequency comb laser spectrum

13. Clock comparisons through fiber frequency Telecom wavelengths (1.5  m) Clock laser A frequency Telecom wavelengths (1.5  m) Clock laser B Location A Location B But what about optical path length fluctuations in the fiber? (acoustic, thermal) A B

14. Test: SURFnet optical fiber link VU - KVI Link part of SURFnet DWDM network Length 317 km, round trip 635 km Single -channel (1559.79 nm) Fiber carrying live data traffic Frequency comb Frequency comb

15. Optical link frequency stability test 1 frequency Telecom wavelengths (1.5  m) Clock laser frequency Telecom wavelengths (1.5  m) Clock laser B VU Amsterdam KVI Groningen SURFnet optical fiber link A B GPS comparison GPS comparison

16. Optical link frequency stability test 1 Result: Identical laser frequencies measured in Amsterdam and Groningen (to within GPS accuracy, 12 digits) Stability: Allan deviation vs. averaging time  Combined inaccuracy GPS links

17. Optical link frequency stability test 2 Bypass GPS altogether: compare optical frequency before and after 635 km round trip Signal: optical beat note send and return light

18. Averaging time Stability (ADEV) Optical link frequency stability test 2 Resulting frequency stability (Allan deviation) 6×10  13 @ 1 s 1×10  14 @ 1 200 s (20 min) 2×10  15 @ 20 000 s (5.5 hr) Limiting factor: fiber optical path length fluctuations (likely) Performance better than state-of-the-art GPS!

19. Ultrastable frequency links Standard network fiber links provide 14-15 digits accuracy in frequency transfer BUT optical clocks require 17+ digits! Solution: active fiber length compensation

20. Optical path length stabilization Optical fiber (  100 km) Optical fiber (  100 km) 1.5  m clock laser 1.5  m clock laser Clock laser + noise Partial reflector roundtrip contains 2× noise! Compensation of frequency fluctuations due to length fluctuations*: PLL *L.-S. Ma, P. Jungner, J. Ye, J.L. Hall, Opt. Lett. 19, 1777(1994)

21. Ultrastable fiber frequency transfer Does this work in, say, urban dark telecom fiber? YES - tests on links 50 – 900 km in Germany, France, USA, UK *: 14-15 digits accuracy w/o compensation: 14-15 digits accuracy : 19 digits accuracy! compensated link: 19 digits accuracy! D oes this work in, say, live telecom networks? YES - France, Paris metro area** 13-14 digits accuracy w/o compensation: 13-14 digits accuracy : 19 digits accuracy! compensated link: 19 digits accuracy! Daisy-chaining of  100 km segments demonstrated Can we get rf/microwave instead of optical carrier too? 14 digits18 digits YES – Use AM, 14 digits w/o compensation, 18 digits with* * 2008-2011 PTB, NIST/JILA, NPL ** 2010 Observatoire de Paris/U. Paris Nord GPS accuracy: 12-15 digits

22. Optical T&F transfer activities Europe EMRP Joint Research Program NEAT-FT: “Accurate time/frequency comparison and dissemination through optical telecommunication networks” Coordinator: Harald Schnatz, PTB Braunschweig (D) General issue: Getting access to (dark) fiber is difficult and/or expensive

23. Applications Fundamental scientific research ‘Relativistic geodesy’

24. Clocks & fibers for height measurement Compare clocks through fiber to sense height differences Clocks ‘measure’ geoid (sea level) GPS position w.r.t. to ellipsoid Complementary method to GPS geodesy – Leveling & geodesy: geoid/sea level is relevant – frame transformation ellipsoid  geoid:  2-3 cm inaccuracy – This decade: clocks < 1 cm accuracy Model of the Earth

25. Digging for (black) gold Gravimeters (accelerometers): – Measure g  1/R 2 (R = distance to center of the Earth) – Sensitive to density variations in Earth crust (oil, mineral reserves) AND to height variations R + h Fiber-coupled clocks: – Measure gravitational potential U  1/R – Barely sensitive to density or g variations, measure primarily R + h Combination: improve gravimeter sensitivity to hidden oil and mineral reserves GPS data: inaccuracy 2-3 cm

26. Applications Fundamental scientific research ‘Relativistic geodesy’ Surveying and oil/mineral exploration With accurate time and frequency available everywhere in the network you can do whatever GPS can do – and better!

27. A vision of ‘SuperGPS’ through optical networks Digits of accuracy Year GPS accuracy Development of atomic clock accuracy over time ‘SuperGPS’ accuracy

28. A vision of ‘SuperGPS’ through optical networks

29. Thanks!