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Register a new userNew Limits on Coupling of Fundamental Constants to Gravity Using $^{87}$Sr Optical Lattice Clocks
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The $^1mathrm{S}_0$-$^3mathrm{P}_0$ clock transition frequency $ u_text{Sr}$ in neutral $^{87}$Sr has been measured relative to the Cs standard by three independent laboratories in Boulder, Paris, and Tokyo over the last three years. The agreement on the $1times 10^{-15}$ level makes $ u_text{Sr}$ the best agreed-upon optical atomic frequency. We combine periodic variations in the $^{87}$Sr clock frequency with $^{199}$Hg$^+$ and H-maser data to test Local Position Invariance by obtaining the strongest limits to date on gravitational-coupling coefficients for the fine-structure constant $alpha$, electron-proton mass ratio $mu$ and light quark mass. Furthermore, after $^{199}$Hg$^+$, $^{171}$Yb$^+$ and H, we add $^{87}$Sr as the fourth optical atomic clock species to enhance constraints on yearly drifts of $alpha$ and $mu$.
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We demonstrate photoassociation (PA) of ultracold fermionic $^{87}$Sr atoms. The binding energies of a series of molecular states on the $^1Sigma^+_u$ $5s^2,^1$S$_0+5s5p,^1$P$_1$ molecular potential are fit with the semiclassical LeRoy-Bernstein model, and PA resonance strengths are compared to predictions based on the known $^1$S$_0+^1$S$_0$ ground state potential. Similar measurements and analysis were performed for the bosonic isotopes $^{84}$Sr and $^{86}$Sr, allowing a combined analysis of the long-range portion of the excited-state potential and determination of the $5s5p,^1$P$_1$ atomic state lifetime of $5.20 pm 0.02$ ns. The results enable prediction of PA rates across a wide range of experimental conditions.
Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over km-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1x10-16 fractional level, surpassing the best current evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.
Electrometry is performed using Rydberg states to evaluate the quadratic Stark shift of the $5s^2$ $^1textrm{S}_0-5s5p$ $^3textrm{P}_0$ clock transition in strontium. By measuring the Stark shift of the highly excited $5s75d;^1textrm{D}_2$ state using electromagnetically induced transparency, we characterize the electric field with sufficient precision to provide tight constraints on the systematic shift to the clock transition. Using the theoretically derived, and experimentally verified, polarizability for this Rydberg state we can measure the residual field with an uncertainty well below $1 textrm{V} textrm{m}^{-1}$. This resolution allows us to constrain the fractional frequency uncertainty of the quadratic Stark shift of the clock transition to $2times10^{-20}$.
We report on a series of 42 measurements of the transition frequency of the 429~THz (5s$^2$)~$^1$S$_0$--(5s5p)~$^3$P$_0$ line in $^{87}$Sr taken over three years from 2017 to 2019. They have been performed at the Physikalisch-Technische Bundesanstalt (PTB) between the laboratory strontium lattice clock and the primary caesium fountain clocks CSF1 and CSF2. The length of each individual measurement run has been extended by use of a hydrogen maser as flywheel to improve the statistical uncertainty given by the Cs clocks. We determine an averaged transition frequency of $429:228:004:229:873.00(0.07)$~Hz with $1.5times10^{-16}$ fractional uncertainty, at the limit of the current realization of the unit hertz. Analysis of the data provides an improved limit on the coupling of the gravitational potential of the Sun to the proton--electron mass ratio $mu$, and confirms the limits on its temporal drift.
We demonstrate a one-dimensional optical lattice clock with a spin-polarized fermionic isotope designed to realize a collision-shift-free atomic clock with neutral atom ensembles. To reduce systematic uncertainties, we developed both Zeeman shift and vector light-shift cancellation techniques. By introducing both an H-maser and a Global Positioning System (GPS) carrier phase link, the absolute frequency of the $^1S_0(F=9/2) - {}^3P_0(F=9/2)$ clock transition of the $^{87}$Sr optical lattice clock is determined as 429,228,004,229,875(4) Hz, where the uncertainty is mainly limited by that of the frequency link. The result indicates that the Sr lattice clock will play an important role in the scope of the redefinition of the ``second by optical frequency standards.
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