Optical clocks and combs at NMIJ

APMP 2013, TCTF Workshop, Taipei 23 Nov. 2013 Optical clocks and combs at NMIJ F.-L. Hong, D. Akamatsu, M. Yasuda, H. Inaba, K. Hosaka, S. Okubo, T. Tanabe, T. Kohno, Y. Nakajima, K. Iwakuni, T. Suzuyama, M. Amemiya, A. Onae National Metrology Institute of Japan (NMIJ) National Institute of Advanced Industrial Science and Technologies (AIST) This research receives support from the JSPS through its FIRST Program

Collaborators NMIJ, AIST M. Imae Laser center, UEC M. Musha K. Nakagawa (Prof.) K. Ueda (Prof.) The U. of Tokyo H. Katori (Prof.) M. Takamoto R. Higashi

Outline Optical frequency comb Why fiber comb? Narrow linewidth, low phase noise fiber comb Optical lattice clock Measuring the Sr lattice clock at Tokyo (GPS-DO, GPS carrier phase, optical fiber link) Yb lattice clock at NMIJ Dual lattice clock at NMIJ Future comparisons

A precise optical frequency ruler Optical frequency f laser f CEO n=0 n f beat Optical frequency comb rep f Microwave reference frequency f laser = f CEO + n f rep + f beat f An early stage Ti:s comb Pump laser Mode-locked Ti:sapphire laser Photonic crystal fiber Optical beat

A comb based on a Ti:sapphire laser Pump laser Mode-locked Ti:sapphire laser Photonic crystal fiber Beat measurement 60 cm 90 cm

The determination of the 532-nm Nd:YAG/I 2 laser frequency for CIPM At the 2001 MeP working group under the CCL meeting From NMIJ/AIST, we reported the frequency of Y3 ( 15 C) f = 563 260 223 510.1 (3) khz. JILA MPQ-PTB NMIJ/AIST 563 260 223 514.5 khz 563 260 223 515.1 khz 563 260 223 510.1 khz Ave = 563 260 223 513.2 khz Sdev = 2.7 khz CIPM recommendation value 563 260 223 513 (5) khz Refs: K. Sugiyama et al., in Proc. 6 th Symp. Frequency Standards and Metrology, 427 (2002). F.-L. Hong et al., IEEE Trans. Instrum. Meas., 52, 240 (2003).

Frequency measurement of an acetylene-stabilized laser using a comb An optical frequency standard at 1.5 µm for telecommunications Ref.: F.-L. Hong et al., Opt. Lett. 28, 2324 (2003). NMIJ, AIST 194 369 569 383.6 khz NPL 194 369 569 385.9 khz NRC 194 369 569 384 khz International recommendation value 194 369 569 384 (5) khz

From Ti:sapphire combs to fiber combs 1)Coupling the laser light into the PCF is the main issue that limits the reliability. 2)Pump laser is the main issue that limits the cost efficiency and also the compactness. 3)Advantage of frequency metrology at near infrared: It is always easy to start from lower frequencies. (It is easy to double or triple the frequency, but not easy to build a frequency divider.) Mode-locked Er fiber laser at 1.5 µm

Input side Output side

Fiber comb developed @ NMIJ/AIST Pump Isolator Amplifier 1 (To observe f CEO ) WDM coupler Oscillator Coupler 50:50 POL-C Er fiber pump SMF Er:fiber Drum PZT Polarizer +controller POL-C pump Er fiber SMF Amplifier 2 (To observe f beat ) Long-term frequency measurement of an iodine stabilized Nd:YAG laser Continuous measurement over 1 week! H. Inaba et al., Opt. Exp. 14, 5223 (2006).

Phase noise of early-stage fiber combs RBW = 100 khz (sweep time = 12.5 ms) F.-L. Hong et al., Opt. Lett. 28, 1516 (2003). B.R.Washburn et al., Opt. Lett. 29, 250 (2004).

High-speed-controllable and robust fiberbased frequency comb Mode-locked erbium-doped fiber ring laser Intracavity waveguide EOM to control cavity length High speed servo control for the cavity length (f rep ) and f CEO Pulse width: ~ 100 fs Mode-locking mechanism: Nonlinear polarization rotation Repetition rate: ~ 43 MHz Averaging power: ~ 3 mw Spectrum K. Iwakuni et al., Opt. Express 20, 13769 (2012).

Multi-branch amplifiers for beat detections Servo Current f ceo locking electronics EDFA HNLF f - 2f 1. f CEO detection EDFA EDFA HNLF HNLF Application port 1 Application port 2 EDFA HNLF 2. f beat detection (1064 nm laser) TEC Slow servo Fast servo f rep locking electronics InGaAs PIN PD f beat 1064 nm Ultrastable laser (Master laser)

60 cm Amplifier1 Amplifier3 Amplifier4 f-2f interferometer Amplifier2 Mode-locked fiber laser 60 cm

In-loop beat spectra of f ceo and f beat Fast servo is required 900 khz 1.3 MHz In-loop beat spectrum of f ceo Servo bandwidth: 900 khz In-loop beat spectrum of f beat Servo bandwidth: 1.3 MHz Robust phase-locking is obtained by the achieved broad servo bandwidth.

Evaluation of narrow linewidth combs A 1064 nm ultrastable laser (Master laser) Phase-locking Phase-locking Fiber comb #1 Fiber comb #2 Observing out-of-loop beat Relative linewidth of the comb How well the comb can follow the master laser

Relative linewidth of the fiber comb (Laser linewidth transfer) Phase-locking 1064 nm Ultrastable laser (Master laser) Phase-locking 1064 nm ν 0 f 1064 nm 1 st fiber comb (f rep : 43 MHz) 2 nd fiber comb (f rep : 43 MHz) f ceo f ceo Phase-locking 1542 nm Phase-locking Buffer laser (1542 nm) Out-of-loop beat PLL

Beat spectrum 75 db/hzrbw (S/N of coherent peak) Phase noise 0.34 rad (Integrated RMS phase) Energy concentration of ~ 94%. Beat spectrum (Enlarged) < 30 mhz (linewidth) Frequency stability ~10-19 @10 5 s (Allan deviation) K. Iwakuni et al., Opt. Express 20, 13769 (2012).

Useful fiber combs Turnkey device Long-term operation Low phase noise Useful to measure, generate and control microwave frequencies Can be low cost Can be very compact

Development of optical lattice clocks 2012 2009 2008 2007 2006 2005 2003 2001 Yb lattice clock being the secondary representation of the second Making a Sr/Yb dual lattice clock (NMIJ) Realization of the 171 Yb lattice clock (NMIJ, NIST) Measuring the Sr lattice clock frequency using an optical fiber link (NMIJ,UEC,Tokyo) Determination of the frequency of the Sr lattice clock ~10-15 (JILA, SYRTE) Sr lattice clock being the secondary representation of the second First absolute frequency measurement of the optical lattice clock (NMIJ & U. of Tokyo) Demonstration of the optical lattice clock (U. of Tokyo) Proposal (Prof. Katori, U. of Tokyo) Optical lattice clock around the world Sr: Tokyo, JILA, SYRTE, PTB, NICT, LENS, NIM, NPL, NMIJ Yb: NIST, NMIJ, ECNU, KRISS, INRIM, Washington, Wuhan Hg: Tokyo, SYRTE

How to measure an optical clock 50 km away (in 2004) Tsukuba (NMIJ/AIST) Sr lattice clock Tokyo ~ 50 km Combs & Atomic clocks

Instruments moved to Tokyo from NMIJ TAI Optical comb Joint project between Tokyo Univ. and NMIJ/AIST Cs clock Iodine-stabilized Nd:YAG laser Optical comb HP 5071A (highperformance tube) F.-L. Hong et al., IEEE Trans. Instrum. Meas. Vol. 50, 486 (2001). NMIJ modified MenloSystem

First absolute frequency measurement of Sr lattice clock in 2005 Uncertainty 3 10-14 Linewidth: 27 Hz (Limited by the probe laser) M. Takamoto, F.-L. Hong, R. Higashi and H. Katori, Nature 435, 321 (2005).

Sr frequency measurement of 2006 1) Introduction of an H-maser in the University of Tokyo. 2) GPS carrier phase common view technique

GPS carrier phase common view technique Imae & Fujii 10.6 [m] Temperature stabilization PC for data acquisition PC for data acquisition Z12-T receiver 2 Javad receiver 2 20 MHz 1 pps 10 MHz Doubler 1 pps 10 MHz 10 MHz Distribution Amp. 5 MHz To optical frequency comb H-Maser Data exchange PC for data acquisition 20 MHz Doubler 10 MHz Z12-T receiver 1 Javad receiver 1 AOG 1 pps 5 MHz Distribution Amp. 10 MHz 10 MHz H-Maser UTC(NMIJ) Katori s Lab in the Univ. of Tokyo 50km baseline NMIJ/AIST(Tsukuba) M. Takamoto, F.-L. Hong, R. Higashi, Y. Fujii, M. Imae, and H. Katori, J. Phys. Soc. Jpn. 75, 104302 (2006).

Absolute frequency of 87 Sr clock transition (September 2006) Uncertainty of U. Tokyo & NMIJ 9.8 10-15 Agreement of 3 institutes 7.5 10-15 JILA: A.D. Ludlow, M.M. Boyd, T. Zelevinsky, S.M. Foreman, S. Blatt, M. Notcutt, T. Ido, and J. Ye, PRL 96, 033003 (2006) Tokyo-NMIJ: M. Takamoto, F.-L. Hong, R. Higashi, Y. Fujii, M. Imae, and H. Katori, J. Phys. Soc. Jpn. 75, 104302 (2006). SYRTE: R.L. Targat, X. Baillard, M. Fouche, A. Brusch, O. Tcherbakoff, G. D. Rovera, and P. Lemonde, PRL 97, 130801 (2006).

CCTF/CCL JWG @ BIPM (September 2006) CIPM: International Committee for Weights and Measures CCL: Consultative Committee for Length WG-MeP: Frequency stabilized laser for the definition of the meter. CCTF: Consultative Committee for Time and Frequency The secondary representations of the second

CIPM recommendation In Sep. 2006, the CCTF has adopted 4 kinds of optical clocks as secondary representations of the second for the first time. Microwave region Optical region

Stability of clocks and time transfer

Remote frequency measurement with an optical fiber link 1. MW frequency transfer Optical + Amplitude modulation MW signal transfer, SYRTE-LPL, 43 km fiber, CO 2 /OsO 4 frequency measurement C. Daussy et al., Phy. Rev. Lett. 94, 203904 (2005). 2. Optical carrier transfer Optical carrier Optical signal transfer, NIST, 251 km fiber, stability 6x10-19 @ 100 s N.R. Newbury et al., Opt. Lett. 32, 3056 (2007). Highly sensitive Highly stable First inter-city fiber link with optical carrier transfer distance ~ 50 km Tokyo Tsukuba (NMIJ/AIST) fiber length 120 km 100 km fiber from JGN, Japan + 20 km local fiber at Tsukuba and Tokyo Total loss 52 db Optical frequency transfer @ 1.5 µm

Fiber link between Tsukuba & Tokyo

Stability of fiber length control Free running Estimated achieved stability with fiber length control 8x10-16 @ 1 s (theoretically-limited) System noise Evaluation of inloop signals M. Musha, F.-L. Hong, K. Nakagawa, K. Ueda, Opt. Express 16, 16459 (2008).

Schematic diagram of experimental setup F.-L. Hong et al., Opt. Lett. 34, 692 (2009).

Sr frequency measurement in 2008 Agreement 6x10-16 Measured in only 3 hours! Tokyo-NMIJ (2006): J. Phys. Soc. Jpn. 75, 104302 (2006). SYRTE (2008): Eur. Phys. J. D, 48, 11-17 (2008) JILA (2008): Metrologia 45, 539 (2008) Tokyo-NMIJ-UEC (2008): Opt. Lett. 34, 692 (2009).

Secondary representations of the second (Candidates of the redefinition of the second) Reference Frequency(Hz) Uncertainty Microwave 87 Rb, grand state hyperfine 6834682610.904324 3 10-15 Optical 88 Sr +, 5s 2 S 1/2 4d 2 D 5/2 444779044095484 7 10 15 Optical Optical 199 Hg +, 5d 10 6s 2 S 1/2 (F=0) 5d 9 6s 2 2 D 5/2 (F=2), m F =0 171 Yb +, 6s 2 S 1/2 (F=0, m F =0) 5d 2 D 3/2 (F=2, m F =0) 1064721609899145 3 10 15 688358979309308 9 10 15 Optical 87 Sr, 5s 2 1 S 0 5s5p 3 P 0 429228004229873.7 1 10 15 The frequency value of 87 Sr was update in 2009 by CIPM

Yb lattice clock at NMIJ, AIST Clock transition 1 S 0 3 P 0 Wavelength : 578 nm Lifetime : ~ 20 s Natural Linewidth : ~ 44 mhz Cooling transitions 1 S 0 1 P 1 Wavelength : 399 nm Natural Linewidth : 28 MHz 1 S 0 3 P 1 Wavelength : 556 nm Natural Linewidth : 182 khz Yb Term Scheme Yb Blue MOT Yb Green MOT M. Yasuda, F.-L. Hong, T. Kohno, H. Inaba, K. Hosaka, C. Willis, T. Kurosu, A. Onae, S. Ohshima, SPIE Vol. 6673, 66730D.

Effect Blackbody radiation shift 1 st phase of research and results Correction (Hz) (2009) Uncertainty (Hz) + 1.32 0.13 Gravitational shift - 1.19 0.03 2nd order Zeeman shift + 0.4 0.05 Scalar light shift 0 14 Clock laser light shift - 0.04 < 0.01 Frequency scan step 0 23 UTC (NMIJ) 0 5 Total + 0.49 27 1 S 0 (F = 1/2)- 3 P 0 (F = 1/2) transition in 171 Yb f = 518 295 836 590 864 (28) Hz (Fractional uncertainty 5.4 10-14 ) T. Kohno et al., Appl. Phys. Express vol. 2, 072501, June 2009. CIPM Recommended frequency list (June, 2009) cf. NIST group s GREAT result: N. D. Lemke et al., Spin-1/2 Optical Lattice Clock Phys. Rev. Lett., vol. 103, pp. 063001, August 2009 f = 518 295 836 590 865.2(0.7) Hz (Fractional uncertainty 1.4 x 10-15 ) 171 Yb clock can be so good!

2 nd phase of research and results 2012 (Metrologia 50, 119) (PRL 103, 063001) NMIJ: (this work) Absolute frequency uncertainty 3.9 10-15 Yb clock uncertainty: 4.1 10-16 USA Korea Japan Appl. Phys. Express 5, 102401 (2012) (Sep. 2012)

CIPM recommendation 2012 Yb optical lattice clock: NMIJ f = 518 295 836 590 863.1(2.0) Hz NIST f = 518 295 836 590 865.2(0.7) Hz NMIJ(2009) f = 518 295 836 590 864 (28) Hz Old Recommendation: 518 295 836 590 864 (1.6 x 10-13 ) Recommendation (weighted mean): f 171Yb = 518 295 836 590 865.0 Hz (2.7 x 10-15 ) A new Secondary Representation of the Second

Sr-Yb dual optical lattice clock at NMIJ/AIST Build up 87 Sr/ 171 Yb optical lattice clocks in a new chamber. Motivation 1) Contribution to the Sr lattice clock community; 2) As a second optical clock to be used for the evaluation of the Yb lattice clock; 3) Measurement of the Sr/Yb frequency ratio with an uncertainty beyond the Cs limit; 4) Contribution to the experimental demonstration of alpha variation. D. Akamatsu, M. Yasuda, T. Kohno, A. Onae, F.-L. Hong, Opt. Express 19, 2046 (2011).

Applications to optical lattice clocks using narrow linewidth combs ν~1 Hz Nd:YAG 1Comb is phase locked to the ultra-stable laser at 1064 nm. ν High-finesse optical cavity at 1064 nm Frequency comb 2Continuous wave lasers are phase locked to the comb. ν Yb lattice clock Clock laser (578 nm) Sr lattice clock 2 nd cooling laser (689 nm) PDH lock Ultra stable laser at 1064 nm The comb transfers the linewidth to other lasers at some wavelengths. H. Inaba et al., Opt. Express 21, 7891 (2013). Sr lattice clock Clock laser (698 nm)

Narrow linewidth optical frequency comb and linewidth transfer Peltier ultranarrow linewidth comb keep locking for weeks Fast servo f - 2f EDFA+HNLF EDFA+HNLF EDFA+HNLF ECDL (698nm) phase lock ECDL (689nm) phase lock clock laser Nd:YAG laser f=1064 nm Δf ~Hz 87 Sr 2 nd cooling laser D. Akamatsu et. al., Opt Express 20, 16016 (2012). Slow servo Y. Nakajima et. al., Opt. Express 19, 2046 (2010). K. Hosaka et. al., IEEE Trans. Ultra. Ferro. Freq. Cont., 57, 606 (2010). ULE cavity (F~735,000)

Sr/Yb Frequency Ratio Measurement beyond Cs limit 87 Sr 171 Yb ν ν Sr Yb = f + q f CEO = f + p f CEO rep rep + Sr + Yb 6 p,q ~ 10 AOM Sr Yb AOM ν ν q q Sr = + CEO Sr 1 + Yb p p ν Yb ν Yb p f q ν Yb Yb Laser (698nm) Nd:YAG laser (1064 nm) Laser (578nm) q p ~ 0.8 f ν CEO ~ 100 MHz 6 = ~ 10 Yb ~ 500THz

Toward the redefinition of the second 1) Realize smaller uncertainty of optical clocks than Cs fountain clocks (uncertainty evaluation) 2) Comparison of optical clocks transportable optical clocks, optical fiber links, etc. 3) Measure the ratio of the optical clocks 4) Operation of optical clocks by multiple institutes 5) Contribute to TAI (operation period of > 5 days)

Advanced frequency links in Japan ISS (ACES) Tsukuba (NMIJ/AIST) Sr & Yb lattice clock Sr lattice clock Koganei (NICT) ~ 20 km ~ 50 km Tokyo Sr & Hg lattice clock

Summary Optical frequency comb Why fiber comb? Narrow linewidth, low phase noise fiber comb Optical lattice clock Measuring the Sr lattice clock at Tokyo (GPS-DO, GPS carrier phase, optical fiber link) Yb lattice clock at NMIJ Dual lattice clock at NMIJ Future comparisons

The NMIJ Optical Clock Group Onae Kohno Hong Yasuda Nakajima Akamatsu Inaba Hosaka Okubo Tanabe