Towards Polish Optical Clock with Cold Strontium Atoms, present status and performance M. Bober, J. Zachorowski, W. Gawlik M. Smoluchowski Institute of Physics Jagiellonian University Cracow, Poland
[email protected]
P. Morzyński, M. Zawada, D. Lisak, A. Cygan, K. Bielska, M. Piwiński, R. S. Trawiński, and R. Ciuryło Institute of Physics Nicolaus Copernicus University Torun, Poland
F. Ozimek, C. Radzewicz Institute of Experimental Physics, Faculty of Physics University of Warsaw Warsaw, Poland using Sr atoms has been realized by the groups in Tokyo, Paris and Boulder [5–7].
Abstract—We report on the status of the work towards cold strontium clock in the Polish National Laboratory of AMO Physics in Toruń. The system consists of: (1) a Zeeman slower and magneto-optical traps (at 461 nm and 689 nm), (2) a frequency comb, and (3) a narrow-band laser coupled to an ultra-stable optical cavity. So far, all parts of the experiment are working and the whole system is tested at the 1S0–3P1, 689 nm optical transition in 88Sr atoms with 7 kHz linewidth.
I.
In this report we describe the status of the experiment running at the Polish National Laboratory of Atomic, Molecular and Optical Physics in Toruń aiming at the construction of Sr optical lattice clock. At present the first stage has been completed which includes the optical frequency comb, the ultra-narrow band laser stabilized to an optical cavity of high finesse, and the apparatus for cooling and trapping of strontium atoms. All three elements are operating and first spectroscopic measurements have been performed.
INTRODUCTION
Precision timekeeping has been based on atomic clocks for the past half-century. In particular, the SI unit of time, the second, is defined with respect to the frequency of resonance in 133Cs atoms. Techniques of laser cooling and trapping have paved way for much improved atomic clocks: the fountain clocks. A new generation of atomic clocks, using optical transitions instead of the microwave ones offer even higher stability and accuracy(see review [1]). Optical clocks have already surpassed the stability of microwave clocks [2,3] and reached inaccuracies below 10-15 with expected control of systematic effects at the 10-18 level. They are realized as single ion clocks or neutral atoms in an optical lattice [4]. The use of the lattice limits the atom-atom interactions and offers the possibility of simultaneous measurements of large numbers of atoms increasing the S/N ratio while tuning of the lattice laser to the magic wavelength eliminates the light-shift effects.
II.
The 1S0 – 3P0 transition in alkaline-earth atoms has been proposed as the clock transition and, in particular, the clock
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BLUE MOT SETUP
For ultimate laser cooling to temperatures below 10 μK, atoms have to be already precooled down to a few mK temperature in a blue magneto-optical trap (MOT). For this cooling the strongly allowed transition 1S0–1P1 is used at 461 nm. To create MOT with reasonable loading rate in good vacuum conditions an atomic oven and a Zeeman slower have to be used due to low strontium vapour pressure. A vacuum set-up was built with an atomic oven, heated to 500ºC. The set-up is presented in Fig. 1. It is divided into two parts. The first one contains the atomic oven producing collimated strontium beam and laser beam collimation stage. The second part, with vacuum in order of 10-10 mbar consists of a Zeeman slower and main chamber, where MOT is created. The Zeeman slower construction is described in [8]. It has capture velocity of 450 m/s and produces atoms slowed down to
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Figure 3. Power spectrum of the beat note of two independently locked lasers: (a) resolution bandwidth 150 kHz, (b) resolution bandwidth 9 Hz. The width of the peak in (b) is 16.8 Hz.
optical cavity used as short-term frequency reference in the Pound-Drever-Hall configuration and thus an efficient laser line width narrowing is achieved. The cavity mirrors and the 100 mm spacer are made of ultra-low expansion glass. The mirrors are optically contacted to a spacer. The FSR of the cavity is 1,5 GHz and its finesse is F = 62800. Design of the cavity is optimized with respect to insensitivity to vibrations and is adopted from Webster et al. [10]. The horizontal cylindrical shape with cutouts is chosen and the proper choice of the support points assures immunity to vibrations. Fig. 2 presents a photograph of the bare cavity. The cavity is isolated from the laboratory environment, is enclosed in a vacuum chamber placed in another thermal enclosure. The mechanical isolation is provided by vibration isolation platform (Minus-k, BM-1) and a steel chamber lined with acoustic-damping foam (Nova-scan NanoCube). Two identical cavities are constructed.
Figure 1. Vacuum set-up. The inset shows the experimental chamber.
30 m/s. The flux at 460ºC is 3.5×109 s-1. Blue MOT beams are detuned 40 MHz below strontium 1S0–1P1 transition and have 23 mm in diameter. During the blue MOT stage two repumping lasers are used: 679 and 707 nm. 6–8×108 atoms are loaded into blue MOT and cooled down to 2–3 mK. At the present stage the apparatus is optimized for cooling 88Sr but it can be rearranged for cooling 87Sr. III.
The laser system is based on commercially available external cavity diode lasers, ECDL (Toptica DLpro and DL100) which both provide about 20 mW output power at wavelength 687–693 nm. Each laser is independently locked to its own identical optical cavity with use of the PDH locking method. With this setup optical beat note of two independently locked lasers can be measured. In another configuration (master-slave scheme) one of the lasers is locked to the cavity and the other is phase-locked to the first one with the wellcontrolled offset. This configuration allows for wide frequency tuning with narrow line width.
NARROW BAND LASER
For ultimate precision spectroscopy on doubly forbidden clock transition an ultra-narrow line width, tunable diode laser system is used as a local oscillator. The details of the system can be found in [9]. The laser is locked to a high-finesse
We achieved laser line width of about 8 Hz, measured by comparison of two identical systems (see Fig. 3). The relative phase lock of two lasers is better than 150 mHz. The main limitation in the current state of this setup is instability of temperature of the optical cavity. Its active stabilization to a few mK should result in the drift rate of the order of 1 Hz/s. IV.
SPECTROSCOPY AND LASER STABILIZATION ON 689 NM TRANSITION
The simplified apparatus used for spectroscopy of 689 nm transition is presented in Fig. 4. Laser beam from one of the ultra-narrow lasers has been sent to room with strontium setup. Cavity used for ultra-narrow laser was not temperature
Figure 2. High finesse optical cavity with the support structure.
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Figure 4. Scheme of the spectroscopy of 689 nm transition. The whole red laser set-up with offset lock, and locking to atomic transition scheme is presented. The absolute laser frequency is referenced to the optical frequency comb.
commercial Rb 10 MHz reference. This measurement, done as a test, was highly limited by the Rb reference accuracy. In the next step cavity drift cancellation system has been build and the laser frequency was locked to atomic transition frequency. Fig. 4 shows the whole red laser set-up with offset lock, and locking to atomic transition scheme. Fig. 6 presents the locking scheme. AOM2 used for creating the beam for spectroscopy is modulated. Level of fluorescence signal from both sides of the atomic resonance are compared and correction is calculated for AOM1 in the ultra-narrow laser path, which cancels the cavity drift. This locking set-up can be also applied, after a few modifications, in future for locking of 698 nm laser to the strontium clock transition.
stabilized and laser frequency was drifting. Five-meter long, single mode, polarization maintaining fiber was used and a second Toptica DLpro laser was locked to the master laser using offset lock technique. This second laser was used for spectroscopy and for creating red MOT. Spectroscopy was done using the laser beam passing twice through AOM (which produced modulation required for spectroscopy). The beam was than expanded to 5 mm diameter, it was crossing the atomic beam perpendicularity and was precisely retroreflected. Laser beam had π polarization with respect to the magnetic field of 1 mT, directed parallel to the atomic beam. Fluorescence signal was detected with a photomultiplier. Exemplary signal, with narrow sub-Doppler structure of 88Sr 1 S0(m=0)–3P1(m=0) line at 689 nm, taken at oven temperature of 500ºC is presented in Fig. 5. Using a fiber frequency comb absolute transition frequency was measured with respect to a
V.
RED MOT
With 689 nm laser stabilized to strontium transition we created red MOT. The red MOT beams had 8 mm diameter and were superimposed on the blue MOT beams. Magnetic
Figure 5. Spectroscopy of the of 88Sr 1S0(m=0)–3P1(m=0) line (689 nm). The lower trace shows the wide scan, the inset shows detail of the Doppler-free feature in the center.
Figure 6. The algorithm of the locking scheme.
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Polish MNiSW project No. N N202 1489 33 and the Foundation for Polish Science TEAM Project co-financed by the EU European Regional Development Fund.
field gradient was lowered from around 0.68 T/m during the blue MOT phase to 0.03 T/m at the beginning of the red MOT phase and then the cloud was compressed by linearly ramping the field to 0.10 T/m. In order to transfer as many atoms as possible from the blue trap the narrow band red laser beams were modulated with frequency 16 kHz and modulation depth 1 MHz. Up to 107 88Sr atoms was stored with temperature below 15 μK. Cooled down atoms in red MOT are detected with probe 461 nm laser beam, resonant to 1S0–1P1 transition. VI.
REFERENCES [1]
S. A. Diddams, J. C. Bergquist, S. R. Jefferts, and C. W. Oates, “Standards of time and frequency at the outset of the 21st century,” Science, vol. 306, pp. 1318–1324, 2004. [2] A. D. Ludlow et at., “Sr lattice clock at 1×10-16 fractional uncertainty by remote optical evaluation with a Ca clock,” Science, vol. 319, pp. 1805-1808, 2008. [3] T. Rosenband et al., “Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place,” Science, vol. 319, pp. 1808–1808, 2008. [4] H. Katori, T. Ido, and M. Kuwata-Gonokami, “Optimal design of dipole potentials for efficient loading of Sr atoms,” J. Phys. Soc. Jpn. vol. 68, pp. 2479–2482, 1999. [5] M. Takamoto, F. L. Hong, R.Higashi, and H. Katori, “An optical lattice clock,” Nature (London), vol. 435, pp. 321-324, 2005. [6] R. Le Targat et al., “Accurate optical lattice clock with 87Sr atoms,” Phys. Rev. Lett., vol. 97, 130801, 2006. [7] A. D. Ludlow et al., “Systematic study of the 87Sr clock transition in an optical lattice,” Phys. Rev. Lett., vol. 96, 033003, 2006. [8] M. Bober, J. Zachorowski, and W. Gawlik, “Designing Zeeman slower for strontium atoms - towards optical atomic clock,” Optica Applicata, vol. 40, pp. 547–555, 2010. [9] D. Lisak et al., “Ultra narrow laser for optical frequency reference,” Acta Phys. Pol. A, vol. 121, pp. 614–621, 2012. [10] S. A. Webster, M. Oxborrow, P. Gill, “Vibration insensitive optical cavity,” Phys. Rev. A, vol. 75, 011801(R), 2007.
CONCLUSION AND OUTLOOK
In the first stage of the project the main elements of the apparatus for an optical atomic clock with Sr atoms have been built: the optical frequency comb, the ultra-narrow band laser stabilized to an optical cavity of high finesse, and the apparatus for cooling and trapping of strontium atoms. First spectroscopic measurements have been performed as a test of the system integration. The system is oriented at bosonic 88Sr atoms, but operation with fermionic 87Sr atoms can be achieved after minor modifications. In a second stage of the project the clock transition 1S0— P0 (698 nm) will be investigated. To this end the atoms will be captured in the magic wavelength optical lattice. The ultranarrow band laser will be tuned to the 698 nm transition and its bandwidth further decreased. The active stabilization of the optical cavity will be implemented assuring low drift of the laser. 1
ACKNOWLEDGMENT The research is part of the program of the National Laboratory FAMO in Torun, Poland and was supported by the
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