Search

Questioning basic assumptions about the structure of space and time has greatly enhanced our understanding of nature. State-of-the-art atomic clocks1–3make 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 10−18level, 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 violations5–7would lead to periodic modulations of the frequency offset as the Earth rotates and orbits the Sun. From the absence of such modulations at the 10−19level we deduce stringent limits of the order of 10−21on Lorentz symmetry violation parameters for electrons, improving previous limits8–10by 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.

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.

This article reviews two aspects of the present research on fundamental constants: their use in a universal and precisely realizable system of units for metrology and the search for a conceivable temporal drift of the constants that may open an experimental window to new physics. [Copyright &y& Elsevier]

Optische Uhren nutzen die Anregung optischer Übergänge in neutralen Atomen oder Ionen. Deren Übergangsfrequenz liegt bei etwa 1015Hz, also fünf Größenordnungen oberhalb der GigahertzSchwingungen heutiger Atomuhren. Das erlaubt eine bis zu tausendmal präzisere Zeitmessung. Für die Genauigkeit ist es entscheidend, dass der Uhrenübergang eine schmale Linienbreite besitzt und seine Frequenz nicht verschoben wird. Dazu müssen die Atome oder Ionen möglichst ungestört und genau lokalisiert sein. Deshalb basiert ein Uhrentyp auf einem kalten Ion in einer PaulFalle, ein anderer Uhrentyp arbeitet mit vielen kalten Neutralatomen in optischen Gittern. Der Metrologie sollen optische Uhren zunächst als sekundäre Realisierung der Sekunde dienen. Derzeit ermöglichen sie bereits Frequenzmessungen mit 17 Stellen Genauigkeit.

A fundamental limit to the stability of a single-ion optical frequency standard is set by quantum noise in the measurement of the internal state of the ion. We discuss how the interrogation sequence and the processing of the atomic resonance signal can be optimized in order to obtain the highest possible stability under realistic experimental conditions. A servo algorithm is presented that stabilizes a laser frequency to the single-ion signal and that eliminates errors due to laser frequency drift. Numerical simulations of the servo characteristics are compared to experimental data from a frequency comparison of two single-ion standards based on a transition at 688 THz in 171Yb+. Experimentally, an instability ?y(100 s) = 9 × 10?16is obtained in the frequency difference between both standards.