Laser Cooling of Thulium Atoms

Transcription

1 Laser Cooling of Thulium Atoms N. Kolachevsky P.N. Lebedev Physical Institute Moscow Institute of Physics and Technology Russian Quantum Center D. Sukachev E. Kalganova G.Vishnyakova A. Sokolov A. Akimov V. Sorokin

2 optical atomic clocks a new era of clocks

3 Laboratory frequency measurements relative uncertainty Cs Essen Re-definition of the second Cs beam iodine stabilized He-Ne laser Cs fountain year H Ca H H Hg + vs Al + Hg + Yb + Al + vs Al + Sr + Sr Hg + Sr

4 Laser cooling of Lanthanides Electronic structure of lanthanides (Yb, Dy, Er, Tm) is similar to alkali-earth elements (Ca, Sr, Mg) due to a closed outer electronic s-shell. Laser cooling of lanthanides is more challenging because of the absence of closed strong cooling transitions. All of strong transitions possess decay channels. Requirements for an efficient laser cooling transition: strong rate > 10 7 s -1 cycled accessible for laser sources with a power of > 1 mw

5 Редкоземельные элементы (группа лнтаноидов)

6 Applications Very recently a significant progress in laser cooling and trapping of some lanthanides, including hollow-shell ones, is achieved. They are intensively studied and successfully implemented in precision spectroscopy and optical frequency metrology study of interactions in quantum regime, study of quantum gases

7 Ytterbium optical lattice clock are unprecedentedly stable Comparison of two identical Yb clocks is performed at NIST, 2013 in the group of Andrew Ludlow N. Hinkley at al., arxiv: v1

8 Magnetic gases Cr: dipole-dipole interactions T. Lahaye,et al., Phys Rev Lett (2008) Optical lattice+microscope for degenerate gases? Rb W. S. Bakr, J. I. Gillen, A. Peng, S. Foelling, M. Greiner Nature 462, (2009)

9 Hollow-shell Lanthanides Vacancies in the hollow 4f-shell (e.g. Er, Dy, Tm) provide big magnetic moment in the ground state. Feshbach resonance in Er Magnetic moment of Dy equals 10 m B, of Er - 6m B. Strong dipole-dipole interactions between ground state atoms. Dipoleinteracting condensates and quantum simulators. M. Lu, N.Q. Burdick, S.H. Youn, and B.L. Lev Phys. Rev. Lett (2011) K. Aikawa et al. Phys. Rev. Lett (2012)

10 Thulium electronic structure Tm: 4 f s - one vacancy in the 4 f shell - relatively simple level structure - fine splitting of the ground state m ground 4 m B shell s p d f L L S Large J causes large magnetic moments of the ground state 3 12

11 Similar polarizabilities of the groundstate fine structure components Optical lattice M1 transition = 1.14 mm, ~ 1 Hz LS J 5/ 2 f 2 Ry LS J 7 / 2 (V.D. Ovsyannikov, 2010 ) To what extent the transition frequency remains unperturbed? Calculations needed!

12 Shielding of the 4f shell levels 2 6s Tm 4 n f Because of the closed 6s 2 shell, the inner shells are shielded to the external perturbations. The shileding was first demonstrated experimentally by E.B. Alexandrov in 1983 collisions He J. Doyle at al. measured for He-Tm collisions (Nature, 2004) in el For Tm-Tm collisions in specific magnetic state the shielding disappears (PRA, 2010) E.B.Aleksandrov et al., Opt. Spektrosk., 54, 3, (1983) C.I. Hancox et al. Nature 431, 281 (2004) C.B.Connolly et al., Phys. Rev. A 81, (2010) 5

13 The M1 transition in Tm atom Spectroscopy on the ground state sublevels in lanthanides is not yet performed Thulium: = 1.14 mm, ~ 1 Hz - suppression of the external electric fields perturbations - small black-body shift - loading in the optical lattice with small perturbation of - strong -dependency f 2 Ry the clock transition

14 Laser cooling of Thulium

15 Cooling transitions in Tm 4f 5d 6s /2 J 9/ 2 decay { 5d 6s, J=9/2 5d 5/26s 2, J=7/ cm cm -1 M1 transition 410 nm 17ns 5d 6s, J=7/2 5d 6s, J=11/2 6s 2 6p, =11/2 1/2 J 5d 6s, J=9/2 4 f 5d 6s / cm cm cm cm f 6s J 5/ mm 531 nm 440 ns f 6s J 7/ 2 Cooling transitions

16 Oven (1100 K) MOT chamber Zeeman slower

17 410 nm slower Zeeman slowing 2D molasses Slower operation vapor pressure, mbar melt boil temperature o C oven Oven design 6 counts [10 s -1 ] initial beam ~ 10 at/s slowing 1% atoms beam size 1 cm flux (typ) 10 at/s cm slowed atoms 25 m/s 4-5 ~v beam 45 Doppler profile without slowing velocity [m/s]

18 Magneto-optical trap (2010) 10 6 atoms Tm-169

19 The life time of Tm atoms in the MOT -1-1 decay rate t [s ] G 0 [ s -1 ] G1 18 s-1 24 s s red detuning I 0 [mw/cm 2 ] G 1 = 22(6) s -1 Binary collisions in the MOT Dark MOT implemented t, seconds 3(2) cm s 5 times more atoms Temperature of atoms

20 Temperature measurements 0 5 Ballistic expansion of the atomic cloud to measure temperature time, ms

21 Temperature mk Temperature mk Temperature in Tm MOT T D = 240 мкк T TD min 240mK 25mK T min = 25 mk I T F Detuning T I F Saturation parameter

22 Magnetic field Due to specific level structure of Tm atom (degeneracy of the Landé g-factors) sub- Doppler mechanism IS EFFICIENT even in the presence of magnetic field v Doppler: D g e mbb k sub-doppler v S g g mbb k

23 Role of Landé g-factors Tm: g g = 1.14, g e = 1.12 Rb: g g = 1/2, g e = 2/3 (2% difference) (30% difference) M. Walhout, J. Dalibard, S.L.Rolston, and W.D. Phillips, J. Opt.Soc. Am. 9, 1997 (1992)

24 Magnetic trap for Tm MOT B = t, ms MT B 0 20 G/cm atoms, life time 0.5-1s, T~ 40 mk t, ms

25 z Magnetic trap (in presence of gravitation) I F = 4 B m F = 4 I m F = 4 U effective potential E -mb z

26 z Magnetic trap (in presence of gravitation) B I F = 4 m F = 2 I m F = 2 U effective potential E -mb z

27 Magnetic trap profile Only atoms in m F =+2,3,4 states are trapped 5 g Temperature Vertical coordinate, mm temperature, m K time, ms density [arb. un.]

28 Second stage cooling First stage cooling at 410 nm T D =240 mk Second stage cooling at nm T D =9 mk Frequency-doubled laser diode radiation is used

29 Second stage cooling ms Efficient cooling recapture directly from Zeeman slower Number of atoms similar to blue MOT due to Zeeman slower design Recapture efficiency from blue MOT 100% temperature, m K To reach lower temperatures we need to narrow the diode laser line width (lower than 100 khz) red detuning,

30 trapping laser Optical trapping w 0 =30 mm MOT mirror Red detuning Blue detuning

31 Spectroscopy of Tm clock transition in the optical lattice Dipole optical trap with a standing wave Excitation of the clock transition in trapped atoms

32 One trapping beam nm 4 f 5d 6s nm / 2 trapping f 6s J 7/ 2 Loading from MOT at 100 mk Laser Verdi G-12 blue detuned! Optical trap depth 1 mk Strong blue transitions mainly contribute to the polarizability About 1% of atoms is recaptured Optical lattice nm 4 f 5d 6s nm / 2 trapping f 6s J 7/ 2 Loading from MOT at 100 mk Laser Verdi V-8 red detuned! Optical trap depth 1 mk Optical trapping is more efficient for red detuning Trapping depends on polarization => lattice effect!

33 Intermediate conclusions - Tm atoms are trapped in an optical lattice and prepared for spectroscopy of clock transition at 1.14 mm - Temperature of atoms is still too high for efficient recapture into the shallow lattice => further cooling is necessary - Narrow line lasers for second stage cooling (530.7 nm) and studying of metrological transition (1.14 mm) - Increasing of the number of atoms

34 Stabilized laser systems at Lebedev Institute Vertical cavity F=60000 Allan deviation averaging time [s] Comparison of two systems designed for 698 nm

35 Laser systems for optical clocks at Lebedev Institute Transportable setup Vertical cavity

36 Beatnote frequency MHz] Cavity length variation [nm] Compensation of temperature fluctuations ULE thermal expansion coefficient -9 2 l l 10 ( T -T ) c Частота сигнала биений [ МГц ] T c Temperature [ o C] o Температура [ C] Изменение длины резонатора [ нм ]

37 Spectral power density [arb. un.] Spectral line width Beatnote between two independent cavities (972 nm, for hydrogen spectroscopy) 0.5 Hz Fourier frequency [Hz] J. Alnis, A. Matveev, N. Kolachevsky, Th. Udem, and T. W. Hänsch, Phys. Rev. A 77, (2008)

38 Signal GaN diode nm Diode PHR-803T (HD-DVD, Blue-Ray) +70 o C Shuji Nakamura Fraction of power in a single frequency >95% Saturation absorption signal in Tm time [s] Tm cloud trapped by diode laser radiation Injection locking gives up to nm

39 2D molasses for Tm beam collimation 410 nm slower 2D molasses oven Slave diode SF-BW512P Max power 500 mw Central wavelength: C C Seed power < 1 mw Output power 120 mw number of atoms, arb.un. up to threefold increase in the number of atoms in the MOT Power in 2D molasses, mw

40 Thank you for attention!

41 Cooling transitions in Tm