1. Optical Atomic Clocks for Space
Optical Atomic Clocks for Space? Leo Hollberg National Institute of Standards and Technology (NIST) , Boulder CO
Yb Oven
Optical Atomic
Frequency Reference fr
I(f)
m-wave out
optical out f
fn = nfr
Optical Synthesizer – Divider / Counter

2. Optical Frequency Measurements Group NIST, Boulder
Chip Scale Atomic Devices
clocks, magnetometers …
John Kitching
Svenja Knappe (Germany)
Peter Schwindt (Sandia)
Vishal Shah  (Princeton)
Vladi Gerginov (Bulgaria, N.D....PTB)
Ying-Ju Wang (Taiwan)
Clark Griffith
Andy Geraci
Hugh Robinson
Liz Donley
Eleanor Hodby (England)
Alan Brannon (CU)  industry
Matt Eardley (CU)
Ricardo Jimenez (CU, Mexico)
Susan Schima
Lucas Willis (LSU, SURF)
Nicolas VanMeter (SURF)
Tara Cubel-Liebisch
fs Frequency Combs
Scott Diddams
Tara Fortier (LANL)
Jason Stalnaker  (Oberlin)
Qudsia Quraishi (CU)
Stephanie Meyer (CU)
Albrecht Bartels  (Konstance)
L-S Ma, Z. Bi, (ECNU-BIPM)
Y. Kobayashi (AIST Japan)
Vela Mbele (South Africa)
Matt Kirchner (CU)
Andy Weiner* (Purdue)
Danielle Braje
Vladi Gerginov (Bulgaria, N.D....PTB)
Optical Length Metrology
Richard Fox
Optical Clocks
Chris Oates
Cold Ca
Yann Le Coq  (SYRTE, Paris)
Jason Stalnaker  (Oberlin)
Guido Wilpers (Germany/NPL-UK)
Anne Curtis (CU  NPL-UK)
Kristin Beck (Rochester, SURF)
Cold Yb
Chad Hoyt ( Bethel College)
Zeb Barber (CU)
Valeriey Yudin (Russia)
Aleksei Taichanachev (Russia)
Nathan Lemke (CU)
Nicola Poli (LENS, Italy)
& many others at NIST and JILA
Carol Tanner (Notre Dame)
$$ NIST, DARPA-MTO, ONR-CU-MURI, NASA, LANL

3. GPS (Global Positioning System)
What is the most significant achievement resulting from Atomic Clocks ?
Rubidium Atomic clocks
Array of 24 orbiting GPS satellites
≈ 4 Rb atomic clocks per satellite
Clocks in Space !
GPS (Global Positioning System)

4. Highest Stability demonstrated --
Timing signals and CLOCKS require Very Stable Frequency Reference(s)
Highest Stability demonstrated --
Fabry-Perot Optical Reference cavities for short times
Laser stabilization methods well established, Pound-Drever-Hall … Narrow linewidths (and good short-term stability   1s)
Avoid index of refraction (i.e. use vacuum)
Good materials: Stiff, low Temp-co and low Aging
Environment !
vibration, temp variations, laser heating …
f/f ≈ 5x10-16 for short times
Atoms for longer times
Longer-term Stability and Accuracy
Cross over time from cavities to atoms depends on which atoms and what environmental effects perturb the cavity
Can trade performance for size, power, wavelength …

5. K. Numata et al., Phys. Rev. Lett. 82, 3799 (1999)
Cavity Thermal Noise
K. Numata et al., Phys. Rev. Lett. 82, 3799 (1999)
Suggests that using concepts from P. Saulson and others that thermal noise in cavities limits stability
Reproduces Data from Virgo and NIST Hg+ cavity
B. C. Young et al., Phys. Rev. Lett. 82, 3799 (1999).
F. Bondu et al., Opt. Lett. 21, 582 (1996).
Mirror and spacer thermally induced displacement noise due to finite mechanical Q
6 x (1/f)-1/2 m/Hz kT

6. Oscillator Instability
H-maser Cs
Hg+
projected
Ca vs
Cavity
1 day
1 month
Yb projected
Oscillator Instability
GPS
Optical
Cavities
THZ
1 ns
1 fs
Plot of instability of various atomic frequency standards and references. Frequency instability is typicaly measured in terms of the Allan Deviation which gives the fractional frequency fluctuations as a function of how long you average. Typically atomic frequency standards improve with averaging time as 1/sqrt(averaging time). The best microwave atomic standards are shown (H-Maser, and the best Cs fountain results reported to date from LPTF Paris). We note that the new optical standards have already demonstrated a factor of about 100 better short term stability than the best microwave atomic standards. The doted lines show the stability that that should be possible with the Hg+ and Ca optical standards in the near future.

7. Myopic history of cold atom clocks for space
≈ 1989 several months at ENS-Paris & already found there discussions, proposal for cold Cs clock in space
19 years later, ACES-PHARAO a reality (but still on the ground)
NASA microgravity program: 1997, PARCS, RACE
NIST-JPL … PARCS hardware built/tested,
SCR, PDR, RDR reviews….
uncert. ≈ 1x10-16
PARCS
RIP
PARCS
Optical Atomic Clocks – Prospects
Improved stability and accuracy
Increased complexity
Optical and microwave connections
uncert. ≈ 1x10-17 feasible?

8. Accuracy of Atomic Frequency Standards - History
Infrared
Visible
1.0E-10
Ion
Alk. Earth
1.0E-11
1.0E-12
1.0E-13
Fractional Frequency Uncertainty
1.0E-14
state-of-the-art Cs microwave
1.0E-15
1.0E-16
1.0E-17
1.0E-18
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
Year

9. Microwave Synthesizer
Generic Atomic Clock
Feedback System
Locks LO to
atomic resonance
Detector Dn Ca υ
Atoms
Local Oscillator
High-Q resonator
Quartz
Fabry Perot cavity
Microwave Synthesizer
Laser
Counter

10. Optical Atomic Clocks for Space
Claims
Exceptionally performance
Could provide exquisite timing and frequency reference for science missions (tests of relativity, precision probes of space-time, searches for new physics, temporal variation of fundamental constants…)
Technological advances: improved time transfer, navigation reference, unprecedented imaging from space …
But NOT yet ready for space :
Would require major investment of people-time and $$ to put optical clocks in space within 10 years
Critical components missing
Complexity, SWAP issues

11. Issue of required lasers
Since 1970 it was clear lasers would enhance clock performance
After almost 40 years, lasers not used in high performance commercial clocks
NO Reliable source of Reliable lasers reaching atomic transitions
Must stay on atomic resonance for years
Space applications require redundancy of key components
Long lived, robust, appropriate Size-Weight and Power = SWAP
solid state, semiconductor …
Is possible w/ engineering and $$$, (e.g. NPRO, telecom DFB lasers)
Issue of Time-Frequency transfer
GPS, TWSTT, limited to about (note 1 day ≈ 105 seconds) 1 ns
Via telecom optical fiber networks, possible, not generally avail. few ps
Dark optical fibers, two-way, not generally avail. fs
Challenge to audience --- try to get freq. 1 x 10-14
Note: 1st order Doppler for clock at 1x10-17 , requires v < 3 x 10-9 m/s

12. High performance Transportable Atomic Clocks
Current State of the Art
Microwave
LITS Hg+ Ion , JPL hot ion cloud few x ?
BNM-SYRTE, Paris cold Cs atomic fountain 1 x 10-15
Optical
approx. uncertainty ?
633nm, 532nm Iodine stabilized HeNe, Nd-YAG
657 nm Ca atomic beam, PTB, Germany
1550 nm C2H2 Japan, UK, Germany, US
3394 nm Russia, CH4 – HeNe

13. Cold Calcium optical atomic clock
1S0 (4s2) m=0
1P1 (4s4p)
Dn = 400 Hz
3P1 (4s4p) m=0
423 nm cooling
Dn = 34 MHz
657 nm clock
423 nm MOT
5x106 atoms, ≈5ms Ca 40
60 seconds data acquisition
400 Hz linewidth
0.2 30
0.1
Percent of Atoms Excited 20
Demodulated Signal (V)
0.0
-0.1 10
-0.2
1000
2000
3000
4000 2 4 6 8 10 12
Relative Probe Frequency (Hz)
Relative 657 nm Probe Detuning (MHz)

14. Ytterbium optical atomic clock
- Excellent prospects for high stability and small absolute uncertainty
1P1 (6s6p)
Lattice
759 nm
398.9 nm,
28 MHz
3P0,1,2 (6s6p)
Energy
Hot topic: evidence
3P1: nm, 182 kHz
Chad Hoyt
Zeb Barber
Chris Oates
Jason Stalnaker
Nathan Lemke
λ = 578 nm
Δν = ~ Yb
15 mHz Yb, 173Yb
1S0
(171Yb, 173Yb I=1/2,5/2)

15. High resolution spectroscopy with lattice-trapped 174Yb atoms
full width ~ 4 Hz (Q >1014)

16. 171Yb, Fermion, I = 1/2 Optical pumping easy
B = 4G splits 3P1 by 6 MHz/mf
Green pulse: 1mS, 7uW, σ+ & σ-
3P1
mf=+3/2
mf=+1/2
mf=-1/2
mf=-3/2
1S0
mf=-1/2
mf=+1/2

17. Magic Wavelength λmag = 759.3537 nm λ = 759.3597 nm λ = 759.3480 nm
Increasing
Wavelength

18. 2-photon resonances 6s2 1S0 +2.7 THz -186 GHz 3P0 -2.9 THz 3P2
6s8p 3P0,1,2
6s5f 3F2
2 x nm
3P1 (not allowed)
6s6p 1P1
3P0
-186 GHz
6s6p
3P2
399nm
3P1
3P0
2 x nm
556nm
3F2
578nm Clock
-2.9 THz
6s2 1S0

19. 2-photon resonance
0.200 Hz at Magic Wavelength

20. Current Systematics & Uncertainties
Systematic Yb 171Yb
Effect Shift Uncertainty Shift Uncertainty
2nd order Zeeman
Probe Light Shift ~0
Lattice Polarizability
Hyperpolarizability
Density
Blackbody Shift
Present knowledge of total systematics:
174Yb: 1.5 x 10-15
171Yb: 6.8 x 10-16

21. Optical clock comparisons via optical frequency combsPD
to counter
578 nm Yb
stabilized laser
f0 lock
f = N x frep + f0
comb
stabilization
Ca, Hg+, Al+, Sr
stabilized laser PD
T. Fortier
S. Diddams …

22. Current instabilities of clock comparisons with NIST frequency comb
Optical- Cs
Cs clock accuracy
Current optical clock accuracy
Optical-optical
Optical comb instability

23. Allan deviation between clock lasers (Yb vs. Al+)3 2
2.2 x 1 s
-15 10 7
Allan Deviation 6 5 4 3 2
-16 10 2 3 4 5 6 7 8 9 2 3 4 5 6 1 10
Averaging Time (s)

24. 171Yb vs Ca

25. t s ) ( Frequency precision [Hz] Allan deviation Averaging time (s)
Stability of 174Yb vs. Ca
1E-14 ) t ( s y 1
1E-15
Frequency precision [Hz]
Allan deviation
0.1
1E-16 1 10
100
Averaging time (s)

26. Instability: Sr v. Yb Sr at JILA, Ludlow, Ye et al.
vs. Yb at NIST via 3 km optical fiber

27. Optical Clock, fs frequency combs as Optical Frequency Divider
Generation of microwaves with low phase noise
I(f)
Optical
reference
Optical outputs
fopt f
f-wave= fopt/N
m-wave out
f-wave
PUMP OC M3 M1 M2
Microwave pulses
1 ns
20 fs
30 ps
time
Microwave comb w/ 1 GHz mode spacing
I(f) f
Microwave frequency
10 GHz Output

28. Two Optical Frequency Dividers
Phase noise for 10 GHz output
-40
Poseidon sapphire oscillator
-60
10 GHz synthesizer
Agilent 8257D + Wenzel quartz
-80
region where OFD combs’
performance excels
-100
L(f) (dBc/Hz)
-120
-140
-160 1 2 3 4 5 6 10 10 10 10 10 10 10
frequency (Hz)
April 2008

29. Looking at 10 GHz Phase-Noise Data in Terms of Timing Jitter noise and Spatial Displacement

30. Routes for Advanced Clocks to Space ?
National Standards Labs X
(BIPM, NIST, PTB, NPL, SYRTE, IEN … )
Insufficient resources
No commercial industrial users, “customers” requiring very high performance
Space Agencies (NASA, ESA …) ??
Science, exploration
Defense (DOD, …) ??
Security, Imagining, Space Navigation, Spy satellites …
Recall origins of GPS

31. $30,000 mini-prize for Optical Atomic Clock in Space
Achieve 2008 ground-based performance in space
Instability 1x10-15 t -1/2
Frequency uncertainty 1x10-17
Verifiable
And in the Spirit of the Decadal Survey and
Perversity of Policy/Politics
Prize expires one decade from today, on 9 July 2008

32. Applications of Optical Frequency References and Combs
Advanced communication systems (security, autonomous synchronization)
Advanced Navigation (position determination and control)
Precise timing (moving into the fs range)
Tests of fundamental physics (special and general relativity, time variation of fundamental constants)
Sensors (strain, gravity, length metrology ……)
Ultrahigh speed data, multi-channel parallel
broadcast, or receivers, coherent communications
Low noise microwaves, and electronic timing signals
Scientific applications ( precision spectroscopy, chemistry, trace gas detection… )
Quantum information ( Ivan Deutsch …)
Fourier synthesized arbitrary waveform generation
Phase-coherent imaging from independent satellites
These are just some of the obvious applications that one can list. Historically navigation and communication systems have been the most demanding users of precise frequency standards. However, if we succeed in getting the orders of magnitude improvement in short-term stability that we expect the most important applications might not be predictable from what we know today.