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arXiv:physics/0303023 · Physical Review A

Clock transition for a future optical frequency standard with trapped atoms

Irène Ventrillard, Audrey Quessada, Richard P. Kovacich, Anders Brusch, Dmitri Kolker, J.‐J. Zondy, Daniele Rovera, P. Lemonde

Published Sep 9, 2003• 77 citations
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Abstract

We report direct excitation of the strongly forbidden ${5s}^{2}{}^{1}{S}_{0}\ensuremath{-}5s5p{}^{3}{P}_{0}$ transition in ${}^{87}\mathrm{Sr}.$ Its frequency is 429 228 004 235(20) kHz. A resonant laser creates a small leak in a magneto-optical trap (MOT): atoms build up to the metastable ${}^{3}{P}_{0}$ state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of ${10}^{\ensuremath{-}3}\mathrm{Hz}$ and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.

DOI
10.1103/physreva.68.030501

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We report direct excitation of the strongly forbidden 5s21S0\ensuremath−5s5p3P0{5s}^{2}{}^{1}{S}_{0}\ensuremath{-}5s5p{}^{3}{P}_{0}5s21S0​\ensuremath−5s5p3P0​ transition in 87Sr.{}^{87}\mathrm{Sr}.87Sr. Its frequency is 429 228 004 235(20) kHz. A resonant laser creates a small leak in a magneto-optical trap (MOT): atoms build up to the metastable 3P0{}^{3}{P}_{0}3P0​ state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of 10\ensuremath−3Hz{10}^{\ensuremath{-}3}\mathrm{Hz}10\ensuremath−3Hz and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.
Direct observation and frequency measurement of /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0 /line within an accuracy of 20 kHz is reported. The resonance frequency with an accuracy of /spl sim/ 100 kHz is reported by indirect measurement of the energy difference between /sup 1/S/sub 0/ and /sup 3/P/sub 0/ states. It is performed by successive frequency measurements of atomic transitions with higher linewidth, easier to observe: /sup 1/S/sub 0/ - /sup 3/P/sub 1/, /sup 3/P/sub 1/ - /sup 3/S/sub 1/ and /sup 3/S/sub 1/ - /sup 3/P/sub 0/ transitions. To perform direct observation of the clock transition, atoms are cooled to a temperature of /spl sim/ 2 mK in a magneto-optical trap (MOT). Although the saturation broadening of the transition is orders of magnitude smaller than the Doppler width (1.5 MHz), a detectable signal is achieved by inducing a small leak in the MOT with a laser tuned to resonance. Atoms accumulate in the metastable /sup 3/P/sub 0 /state where they escape the trap. The resonance is detected by a decrease in the MOT fluorescence signal with a contrast of 1%.
We report an experimental observation of the weak 'SO-~PO transition in 171,173Ybl and new approaches for qubits and optical clocks with this single-photon resonance and a three-photon resonance in an even isotope, respectively. Introduction - Optical clocks show great promise as the next generation of precision frequency standards due to the high frequency and correspondingly high line-Q of weakly allowed atomic transitions. Thanks to femtosecond comb technology (l), clocks based on optical transitions in both single trapped ions and neutral atom clouds are poised to exceed the already stringent bounds on precision set by microwave frequency standards. A recent proposal combining the Lamb-Dicke confinement of single ion frequency standards with the high signal-tenoise of neutral atom cLouds, and exploiting a highly forbidden transition insensitive to external magnetic fields and trap light polarization has the potential to do even better (2). The scheme makes use of the 'SO-~P~ electric dipole transition in alkaline-earth atoms, weakly allowed in odd isotopes by the hyperfine interaction of the nuclear spin and completely forbidden by a single-photon process in even isotopes. Trapping the atoms in a Stark-free optical httice will allow a large ensemble of atoms to exhibit a spectrum free of Doppler and recoil shifts, leading to an estimated accuracy of a few parts in lo1* (3, 41. As an additional application, the long atomic coherence times in optical lattices also suggest using the 'SO-~PO pairs of states as an array of qubits for quantum information (5, 61. Yb and Sr are both strong candidates for an optical lattice clock as well as for creating useful qubits. Both atoms have experimentally accessible energy level transitions, allowing the atoms to be readily laser-cooled and trapped, and in the case of Yb, Eose-Einstein condensed. While the Sr clock transition has been studied extensively, both in and out of an optical lattice, the same transition in Yb has thus far remained unexplored experimentally. Here we report the direct excitation of the 578 nm 'SO-~P~ transition in odd isotope 171,173Yb. (We understand that a group at NIST has also observed this transition (7)). We also discuss the possible application of this type of transitions in making qubits for quantum computation and a clock scheme based on three-photon transition in an even isotope (8, 91. - The level structure of Yb and the experimental apparatus are diagrammed in Fig. 1. The experiment is performed on a sample of cold atoms collected in a magneto- optical trap (MOT). To excite the clock transition, we shut off the MOT beams and irradiate the cloud of atoms with a 578 nm probe. JVe t,hen turn the MOT beams back on, continuously monitoring 399 nm fluorescence from the atom cloud. As the probe laser frequency is scanned over the dock transition, there is an observable decrease in MOT fluorescence as atoms are excited into the metastable 3Po state and escape the trap. By chopping trap and probe beams out of phase, we avoid light shifts of the clock transition due to near-resonant 399 nm light while maintaining a steady-state MOT population. The MOT fluorescence reduction of 171Yb is shown in Fig. 2 (a). Since iodine lines have been well measured in Observation of the 1So-3P, Transition in 1713173Yb
We report the direct excitation of the highly forbidden (6s2) 1S0 <--> (6s6p) 3P0 optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at approximately 70 microK in a magneto-optical trap. The measured frequency in 171Yb (F=1/2) is 518,295,836,591.6 +/- 4.4 kHz. The measured frequency in 173Yb (F=5/2) is 518,294,576,847.6 +/- 4.4 kHz. Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a 10(6)-fold reduction in uncertainty. The natural linewidth of these J=0 to J=0 transitions is calculated to be approximately 10 mHz, making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.
Sr-atoms are of special interest for an optical frequency standard since a large number of atoms can be cooled down efficiently to ultra-low temperatures in a magneto-optical trap (MOT). The ultra-narrow line on the forbidden 1 S 0 -> 3 P 0 transition of 87 Sr ( 3 P 0 natural lifetime is approx. 100 s), and the ability to interrogate Sr atoms in a perturbation free dipole trap promise un-precedented low inaccuracy level and relative frequency instability, well below 10 -15 tau -1/2 . We will show the realization of an efficient atomic source of cold Sr atoms and the construction of an ultra-stable laser for the interrogation of the atoms.
We report on the realization of a magneto-optical trap (MOT) for metastable strontium operating on the 2.92-$\ensuremath{\mu}\mathrm{m}$ transition between the energy levels 5s5p3P25s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{2}5s5p3P2​ and 5s4d3D35s4d{\phantom{\rule{0.16em}{0ex}}}^{3}{D}_{3}5s4d3D3​. The strontium atoms are initially captured in a MOT operating on the 461-nm transition between the energy levels 5s21S05{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}5s21S0​ and 5s5p1P15s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}5s5p1P1​, prior to being transferred into the metastable MOT and cooled to a final temperature of 6 $\ensuremath{\mu}\mathrm{K}$. Challenges arising from aligning the midinfrared and 461-nm light are mitigated by employing the same pyramid reflector to realize both MOTs. Finally, the 2.92-$\ensuremath{\mu}\mathrm{m}$ transition is used to realize a full cooling sequence for an optical lattice clock, in which cold samples of 87Sr^{87}\mathrm{Sr}87Sr are loaded into a magic-wavelength optical lattice and initialized in a spin-polarized state to allow high-precision spectroscopy of the 5s21S05{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}5s21S0​ to 5s5p3P05s5p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}5s5p3P0​ clock transition.
The 1S0 ⁣− ⁣3P0{{^1\mathrm{S}_0}\!-\!{^3\mathrm{P}_0}}1S0​−3P0​ clock transition in strontium serves as the foundation for the world's best atomic clocks and for gravitational wave detector concepts in clock atom interferometry. This transition is weakly allowed in the fermionic isotope 87^{87}87Sr but strongly forbidden in bosonic isotopes. Here, we demonstrate coherent excitation of the clock transition in bosonic 88{}^{88}88Sr using a novel collinear three-photon process in a weak magnetic field. We observe Rabi oscillations with frequencies of up to 50 kHz50~\text{kHz}50 kHz using W/cm2\text{W}/\text{cm}^{2}W/cm2 laser intensities and Gauss-level magnetic field amplitudes. The absence of nuclear spin in bosonic isotopes offers decreased sensitivity to magnetic fields and optical lattice light shifts, enabling atomic clocks with reduced systematic errors. The collinear propagation of the laser fields permits the interrogation of spatially separated atomic ensembles with common laser pulses, a key requirement for dark matter searches and gravitational wave detection with next-generation quantum sensors.