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

Optical Frequency Measurements Group NIST, Boulder http://tf.nist.gov/ofm/ 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

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)

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 …

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 10-17 (1/f)-1/2 m/Hz kT

Oscillator Instability H-maser Cs Hg+ projected Ca vs Cavity 1 day 1 month Yb projected Oscillator Instability GPS Optical Cavities 0.5 Hz @ 500 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.

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 1997-2002 Optical Atomic Clocks – Prospects Improved stability and accuracy Increased complexity Optical and microwave connections uncert. ≈ 1x10-17 feasible?

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

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 456 986 240 494 158

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

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

High performance Transportable Atomic Clocks Current State of the Art Microwave LITS Hg+ Ion , JPL hot ion cloud few x 10-14 ? BNM-SYRTE, Paris cold Cs atomic fountain 1 x 10-15 Optical approx. uncertainty ? 633nm, 532nm Iodine stabilized HeNe, Nd-YAG 10-11 657 nm Ca atomic beam, PTB, Germany 10-12 1550 nm C2H2 Japan, UK, Germany, US 10-11 3394 nm Russia, CH4 – HeNe 10-13

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)

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: 555.8 nm, 182 kHz Chad Hoyt Zeb Barber Chris Oates Jason Stalnaker Nathan Lemke λ = 578 nm Δν = ~0 174Yb 15 mHz 171Yb, 173Yb 1S0 (171Yb, 173Yb I=1/2,5/2)

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

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

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

2-photon resonances 6s2 1S0 +2.7 THz -186 GHz 3P0 -2.9 THz 3P2 6s8p 3P0,1,2 6s5f 3F2 2 x 759.3537nm 3P1 (not allowed) 6s6p 1P1 3P0 -186 GHz 6s6p 3P2 399nm 3P1 3P0 2 x 759.3537nm 556nm 3F2 578nm Clock -2.9 THz 6s2 1S0

2-photon resonance 0.200 Hz at Magic Wavelength

Current Systematics & Uncertainties Systematic 174Yb 171Yb Effect Shift Uncertainty Shift Uncertainty 2nd order Zeeman -18 0.2 -.08 .006 Probe Light Shift 7 0.2 .01 ~0 Lattice Polarizability 0 0.3 0 .3 Hyperpolarizability .18 0.04 .18 .04 Density -.1 0.5 -0.52 0.13 Blackbody Shift -1.3 0.13 -1.3 0.13 Present knowledge of total systematics: 174Yb: 1.5 x 10-15 171Yb: 6.8 x 10-16

Optical clock comparisons via optical frequency combs PD 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 …

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

Allan deviation between clock lasers (Yb vs. Al+) 3 2 2.2 x 10-15 @ 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)

171Yb vs Ca

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)

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

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

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

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

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

$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

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.