Optical Atomic Clocks for Space?Leo Hollberg

National Institute of Standards and Technology (NIST) , Boulder CO

Optical Synthesizer – Divider / Counter

Optical Atomic Frequency Reference

-wave out

optical out

fr

0

I(f)

f

Yb Oven

fn = nfr

Optical Frequency Measurements Group NIST, Boulder

Optical ClocksChris OatesCold 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)

fs Frequency CombsScott 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 BrajeVladi Gerginov (Bulgaria, N.D....PTB)

Optical Length MetrologyRichard Fox

•$$ NIST, DARPA-MTO, ONR-CU-MURI, NASA, LANL

Chip Scale Atomic Devicesclocks, 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 RobinsonLiz Donley

Eleanor Hodby (England)Alan Brannon (CU) industry

Matt Eardley (CU)Ricardo Jimenez (CU, Mexico)

Susan SchimaLucas Willis (LSU, SURF)Nicolas VanMeter (SURF)

Tara Cubel-Liebisch

Carol Tanner (Notre Dame)

& many others at NIST and JILA

http://tf.nist.gov/ofm/

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)

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 …

Mirror and spacer thermally induced displacement noise due to finite mechanical Q

6 x 10-17 (1/f)-1/2 m/Hz

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+ cavityB. C. Young et al., Phys. Rev. Lett. 82, 3799 (1999).F. Bondu et al., Opt. Lett. 21, 582 (1996).

Cavity Thermal Noise

kT

10-17

10-16

10-15

10-14

10-13

Alla

n D

evi

atio

n -

- In

sta

bili

ty

10-2 10

0 102 10

4 106

Averaging Time (s)

()

H-maser

Cs

Hg+

projected

Ca vsCavity

1 day 1 month

Yb projected

Oscillator Instability

GPS

Optical Cavities

0.5 Hz @ 500 THZ

1 fs

1 ns

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

RIPPARCS 1997-2002

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

Fra

ctio

nal

Fre

qu

ency

Un

cert

ain

tyAccuracy of Atomic Frequency Standards - History

1.0E-18

1.0E-17

1.0E-16

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

Year

state-of-the-art Cs microwave

Ion

InfraredVisible

Alk. Earth

Ca

Detector

Local Oscillator

High-Q resonatorQuartz

Fabry Perot cavity

Feedback SystemLocks LO to

atomic resonance

Microwave SynthesizerLaser

456 986 240 494 158

Counter

υ

Generic Atomic Clock

Atoms

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

Opticalapprox. uncertainty ?

633nm, 532nm Iodine stabilized HeNe, Nd-YAG 10-11

657 nm Ca atomic beam, PTB, Germany 10-12

1550 nmC2H2 Japan, UK, Germany, US 10-11 3394 nmRussia, CH4 – HeNe 10-13

Cold Calcium optical atomic clock

Relative 657 nm Probe Detuning (MHz)

423 nm cooling = 34 MHz

657 nm clock

1S0 (4s2) m=0

1P1 (4s4p)

= 400 Hz

3P1 (4s4p) m=0

0

10

20

30

40

Per

cent

of

Ato

ms

Exc

ited

0 2 4 6 8 10 12 0 1000 2000 3000 4000Relative Probe Frequency (Hz)

-0.2

-0.1

0.0

0.1

0.2

Dem

odul

ated

Sig

nal (

V)

60 seconds data acquisition 400 Hz linewidth

423 nm MOT

5x106 atoms, ≈5ms

Ca

Ytterbium optical atomic clock

Chad Hoyt Zeb BarberChris OatesJason StalnakerNathan Lemke

398.9 nm,28 MHz

1P1 (6s6p)

3P1: 555.8 nm, 182 kHz

3P0,1,2 (6s6p)

- Excellent prospects for high stability and small absolute uncertainty

Lattice759 nm

Ene

rgy

λ = 578 nmΔν = ~0 174Yb

15 mHz 171Yb, 173Yb

(171Yb, 173Yb I=1/2,5/2)1S0

-20 -15 -10 -5 0 5 10 15 200.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Ato

m n

umbe

r [a

.u.]

Frequency offset [Hz]

High resolution spectroscopy with lattice-trapped 174Yb atoms

full width ~ 4 Hz (Q >1014)

171Yb, Fermion, I = 1/2Optical pumping easy

• B = 4G splits 3P1 by 6 MHz/mf

• Green pulse: 1mS, 7uW, σ+ & σ-

1S0

3P1

mf=-3/2

mf=-1/2

mf=-1/2 mf=+1/2

mf=+1/2

mf=+3/2

Magic Wavelength

IncreasingWavelength0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-40

-30

-20

-10

0

10

20

30

40

Power [Watts]

Shi

ft [

Hz]

λ = 759.3480 nm

λ = 759.3597 nm

λmag = 759.3537 nm

2-photon resonances

6s2 1S0

6s6p 1P1

399nm

3P0

3P23P1

556nm

578nm Clock

6s8p 3P0,1,2

6s6p

6s5f 3F2

2 x

759.

3537

nm

3F2

3P0

3P2

3P1 (not allowed)

+2.7 THz

-2.9 THz

-186 GHz

2 x 759.3537nm

2-photon resonance

100 150 200 250 300 350 400 450 500 550-15

-10

-5

0

5

10

152 AC Stark Shift at 6s6p3P

0 6s8p3P

0 resonance

f2-

- 394 615 000 MHz

Sh

ift

[kH

z] 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

PDcomb

stabilizationCa, Hg+, Al+, Srstabilized laser

PDto counter

578 nm Ybstabilized laser

f = N x frep + f0

f0 lock

T. FortierS. Diddams …

Current instabilities of clock comparisons with NIST frequency comb

10-19

10-18

10-17

10-16

10-15

10-14

10-13

Fra

ctio

na

l In

sta

bili

ty

100

101

102

103

104

Averaging Time (s)

Optical- Cs

Optical-optical

Optical comb instability

Current optical clock accuracy

Cs clock accuracy

2

3

4567

10-15

2

Alla

n D

evia

tion

12 3 4 5 6 7 8 9

102 3 4 5 6

Averaging Time (s)

Allan deviation between clock lasers (Yb vs. Al+)

10-16

2.2 x 10-15 @ 1 s

3

171Yb vs Ca

Stability of 174Yb vs. Ca

1 10 1001E-16

1E-15

1E-14

0.1

1

All

an d

evia

tion

y()

Averaging time (s)

Fre

quen

cy p

reci

sion

[H

z]

Instability: Sr v. Yb

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

f-wave= fopt/N-wave out

Optical reference

0

I(f)

f

f-wave

fopt

PUMP

OCM3

M1 M2

Optical Clock, fs frequency combs as Optical Frequency DividerGeneration of microwaves with low phase noise

1 ns

time

Optical outputs

30 ps

Microwave pulses

20 fs

I(f)

f

Microwave frequency

Microwave comb w/ 1 GHz mode spacing

10 GHz Output

100 101 102 103 104 105 106

-160

-140

-120

-100

-80

-60

-40

L(f

) (d

Bc/

Hz)

frequency (Hz)

region where OFD combs’performance excels

Poseidon sapphire oscillator

10 GHz synthesizer Agilent 8257D + Wenzel quartz

Two Optical Frequency Dividers Phase noise for 10 GHz output

April 2008

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

10-4

10-3

10-2

10-1

fs/s

qrt

(Hz)

100

101

102

103

104

105

106

Frequency (Hz)

0.01

0.1

1

10

nm/sq

rt(Hz)

Integrated timing jitter: 400 as Integrated displacement: 120 nm

4x10-6 of 10 GHz period and wavelength

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 forOptical 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