Optical clock comparison for Lorentz symmetry testing.

Questioning basic assumptions about the structure of space and time has greatly enhanced our understanding of nature. State-of-the-art atomic clocks1-3 make it possible to precisely test fundamental symmetry properties of spacetime and search for physics beyond the standard model at low energies of just a few electronvolts4. Modern tests of Einstein's theory of relativity try to measure so-far-undetected violations of Lorentz symmetry5; accurately comparing the frequencies of optical clocks is a promising route to further improving such tests6. Here we experimentally demonstrate agreement between two single-ion optical clocks at the 10-18 level, directly validating their uncertainty budgets, over a six-month comparison period. The ytterbium ions of the two clocks are confined in separate ion traps with quantization axes aligned along non-parallel directions. Hypothetical Lorentz symmetry violations5-7 would lead to periodic modulations of the frequency offset as the Earth rotates and orbits the Sun. From the absence of such modulations at the 10-19 level we deduce stringent limits of the order of 10-21 on Lorentz symmetry violation parameters for electrons, improving previous limits8-10 by two orders of magnitude. Such levels of precision will be essential for low-energy tests of future quantum gravity theories describing dynamics at the Planck scale4, which are expected to predict the magnitude of residual symmetry violations.

Laser spectroscopic characterization of the nuclear-clock isomer 229mTh.

The isotope 229Th is the only nucleus known to possess an excited state 229mTh in the energy range of a few electronvolts-a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than typical nuclear excitation energies. Of the many applications that have been proposed for this nuclear system, which is accessible by optical methods, the most promising is a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of the doubly charged 229mTh ion and the determination of the fundamental nuclear properties of the isomer, namely, its magnetic dipole and electric quadrupole moments, as well as its nuclear charge radius. Following the recent direct detection of this long-sought isomer, we provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.

Autobalanced Ramsey Spectroscopy.

We devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. Spectroscopy of an atomic clock transition without compromising the clock's accuracy is accomplished by actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations. Our simple and universal approach eliminates a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate autobalanced Ramsey spectroscopy on the light shift prone ^{171}Yb^{+} electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the 10^{-18} level for the Yb^{+} system and for other types of optical clocks.