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Physical Review A

Collisional shifts in one- and two-dimensional shallow blue-detuned 87 Sr optical lattice clocks

Deshui Yu

Published Mar 12, 2012• 4 citations
Publisher

Abstract

We study the influence of ultracold collisions induced by the inhomogeneous excitation on the clock-transition frequency in one- and two-dimensional shallow blue-detuned (BD) ${}^{87}$Sr optical lattice clocks with a magic wavelength of ${\ensuremath{\lambda}}_{b}=389.889$ nm. The minimal lattice potential required to tightly confine atoms in each single lattice site is much larger than that required for a red-detuned (RD) lattice operating at ${\ensuremath{\lambda}}_{r}=813.4$ nm because of the relatively smaller magic wavelength. For a deep optical lattice, the two-body interactions of atoms in a single lattice site mainly contribute to the clock-transition frequency shift, while the intersite collisions primarily affect the collisional shift in a shallow optical lattice. For an effective misalignment angle $\ensuremath{\Delta}\ensuremath{\theta}\ensuremath{\sim}10$ mrad of the probe beam with respect to the lattice axis, the fractional collisional shifts in both one- and two-dimensional BD lattices are at the ${10}^{\ensuremath{-}16}$ level.

DOI
10.1103/physreva.85.032705

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We study the influence of ultracold collisions induced by the inhomogeneous excitation on the clock-transition frequency in one- and two-dimensional shallow blue-detuned (BD) 87{}^{87}87Sr optical lattice clocks with a magic wavelength of \ensuremathλb=389.889{\ensuremath{\lambda}}_{b}=389.889\ensuremathλb​=389.889 nm. The minimal lattice potential required to tightly confine atoms in each single lattice site is much larger than that required for a red-detuned (RD) lattice operating at \ensuremathλr=813.4{\ensuremath{\lambda}}_{r}=813.4\ensuremathλr​=813.4 nm because of the relatively smaller magic wavelength. For a deep optical lattice, the two-body interactions of atoms in a single lattice site mainly contribute to the clock-transition frequency shift, while the intersite collisions primarily affect the collisional shift in a shallow optical lattice. For an effective misalignment angle \ensuremathΔ\ensuremathθ\ensuremath∼10\ensuremath{\Delta}\ensuremath{\theta}\ensuremath{\sim}10\ensuremathΔ\ensuremathθ\ensuremath∼10 mrad of the probe beam with respect to the lattice axis, the fractional collisional shifts in both one- and two-dimensional BD lattices are at the 10\ensuremath−16{10}^{\ensuremath{-}16}10\ensuremath−16 level.
We consider the frequency shift in optical lattice clocks which arises from the coupling of the electronic motion to the atomic motion within the lattice. For the simplest of three-dimensional lattice geometries this coupling is shown to affect only clocks based on blue-detuned lattices. We have estimated the size of this shift for the prospective strontium lattice clock operating at the 390-nm blue-detuned magic wavelength. The resulting fractional frequency shift is found to be on the order of ${10}^{\ensuremath{-}18}$ and is largely overshadowed by the electric quadrupole shift. For lattice clocks based on more complex geometries or other atomic systems, this shift could potentially be a limiting factor in clock accuracy.
We investigated the properties of optical lattice clocks operated with a repulsive light-shift potential. The magic wavelength, where light-shift perturbation for the clock transition cancels, was experimentally determined to be 389.889(9) nm for 87Sr. The hyperpolarizability effects on the clock transition were investigated theoretically. With minimal trapping field perturbation provided by the blue-detuned lattice, the fractional uncertainty due to the hyperpolarizability effects was found to be 2x10;{-19} in the relevant clock transition.
We have quantified collisional losses, decoherence and the collision shift in a one-dimensional optical lattice clock on the highly forbidden transition (1)S(0)-(3)P(0) at 698 nm with bosonic (88)Sr. We were able to distinguish two loss channels: inelastic collisions between atoms in the upper and lower clock state and atoms in the upper clock state only. Based on the measured coefficients, we determine the operation parameters at which a 1D-lattice clock with (88)Sr shows no degradation due to collisions on the fractional uncertainty level of 10(-16).
We present detailed analyses of our recent experiment on the three-dimensional (3D) optical lattice clock with bosonic 88Sr^{88}\mathrm{Sr}88Sr atoms in which the collisional frequency shift was suppressed by applying a single-occupancy lattice. Frequency shifts in magnetically induced spectroscopy on the 1S0−−3P0{}^{1}{S}_{0}--{}^{3}{P}_{0}1S0​−−3P0​ clock transition ($\ensuremath{\lambda}=698$ nm) of 88Sr^{88}\mathrm{Sr}88Sr were experimentally investigated by referencing a one-dimensional (1D) lattice clock based on spin-polarized 87Sr^{87}\mathrm{Sr}87Sr atoms. We discuss that the clock stability is limited by the current laser stability as well as the experimental sequence of the clock operation, which may be improved to ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}/\sqrt{\ensuremath{\tau}}$ by optimizing the cycle time of the clock operation.
We have investigated the frequency shifts caused by inhomogeneous excitation in a 171Yb optical lattice clock. The dependences of the inhomogeneity on the temperature of the cold ytterbium atoms and the misaligning angle between the lattice laser and the clock laser are analyzed by numerical calculations. The dependence of the fractional collisional frequency shift on the ground state fraction under different cold atom temperatures, atom numbers, lattice trap depths and unequal transverse and longitudinal temperatures are also shown. The results show that the uncertainty of the ytterbium clocks, contributed by the inhomogeneous excitation, can be reduced to be 10−19 or even lower with certain conditions.
We present the measurements of the photoionization cross sections of the excited 1P1 and 3S1 states of ultracold 88Sr atoms at 389.889 nm wavelength, which is the magic wavelength of the 1S0-3P0 clock transition. The photoionization cross section of the 1P1 state is determined from the measured ionization rates of 88Sr in the magneto-optical trap in the 1P1 state to be 2.20(50)×10−20 m2, while the photoionization cross section of 88Sr in the 3S1 state is inferred from the photoionization-induced reduction in the number of atoms transferred through the 3S1 state in an operating optical lattice clock to be 1.38(66) ×10−18 m2. Furthermore, the resulting limitations of employing a blue-detuned magic wavelength optical lattice in strontium optical lattice clocks are evaluated. We estimated photoionization induced loss rates of atoms at 389.889 nm wavelength under typical experimental conditions and made several suggestions on how to mitigate these losses. In particular, the large photoionization induced losses for the 3S1 state would make the use of the 3S1 state in the optical cycle in a blue-detuned optical lattice unfeasible and would instead require the less commonly used 3D1,2 states during the detection part of the optical clock cycle.