In his Special Theory of Relativity, Einstein formulated the hypothesis according to which the speed of light is always the same, no matter what the conditions are. It may, however, be possible that – according to theoretical models of quantum gravitation – this uniformity of space-time does not apply to particles. Physicists have now tested this hypothesis with a first long-term comparison of two optical ytterbium clocks at the Physikalisch-Technische Bundesanstalt (PTB). With these clocks, whose error amounts to only one second in ten billion years, it should be possible to measure even extremely small deviations of the movement of the electrons in ytterbium. But the scientists did not detect any change when the clocks were oriented differently in space. Due to this result, the current limit for testing the space-time symmetry by means of experiments has been drastically improved by a factor of 100. In addition to this, the extremely small systematic measurement uncertainty of the optical ytterbium clocks of less than 4 × 10-18 has been confirmed. The team consisting of physicists from PTB and from the University of Delaware has published its results in the current issue of Nature.
Optical clocks with trapped ions and the Th-229 nuclear clock will be a topic at several summer schools this year, featuring nuClock partner E. Peik as lecturer:
European Frequency and Time Seminar EFTS 2019
1-5 July 2019, Besancon, France
International School of Physics “Enrico Fermi”
COURSE 206 – NEW FRONTIERS FOR METROLOGY: FROM BIOLOGY AND CHEMISTRY TO QUANTUM AND DATA SCIENCE,
4-13 July 2019, Varenna, Italy
Les Houches predoc school on
Interaction of Light and Cold Atoms
30 September – 11 October 2019, Les Houches, France
Using 2-photon resonant laser spectroscopy, the PTB team identifies 166 previously uncharted electronic states of the Th1+ ion in the energy range between 7.8 and 9.8 eV. The observed levels can be relevant for the excitation or decay of the 229mTh isomeric nuclear state which lies in this energy range. The high density of electronic levels promises a strongly enhanced electronic bridge excitation of the isomer in 229Th. Read more on arXive.
A VUV frequency comb seems to be a promising candidate for performing high-resolution spectroscopy on the Th-229 nucleus. Such a device up-converts the frequency of a stabilized femtosecond laser from the infrared to the ultraviolet using a process called “high-harmonic generation” (HHG). This process is usually performed in noble gas jets. It requires extreme optical field amplitudes, which usually can only be realized when enhancing the laser power in a passive built-up cavity. The Vienna team has now demonstrated that such a process can be realized more efficiently using a solid-state target than a noble gas jet. The all-solid approach furthermore makes the setup simpler (no gas streaming into vacuum!) and compact. We could show that the solid-state HHG process maintains the original comb structure by performing a beating measurement with a UV CW laser at 266 nm, generously borrowed by the TOPTICA team. Further credits got to Erin Young and Jim Speck from University of California for producing the AlN targets. Read the full article HERE.
While several groups make progress on determining the exact energy of the Th-229 isomer, the uncertainty is still too large for a direct optical excitation with a narrow-band source to be successful. Most approaches use the U-233 alpha-decay as a means of populating the isomer with a 2% probability. This alpha decay is however a violent process, transferring >80 keV of recoil energy to the “new-born” Th-229 ion, which makes further manipulation a formidable task. A Japanese consortium, with support by TU Wien and associated Okayama University and TU Wien, has now succeeded in populating the the isomer, starting from the Th-229 ground state, transferring only a negligible amount of recoil energy. The team resonantly excites to the second nuclear level at 29 keV using the SPring-8 synchrotron facility, which then predominantly decays to the Th-229 isomer, realizing highly efficient “x-ray pumping” into the isomer. The energy of the 2nd nuclear level is measured with extreme accuracy (0.07 eV), also the decay branching ratio from the 2nd nuclear state back to ground- and isomeric state respectively, is measured experimentally for the first time. This two numbers allow a re-interpretation of the “historic” gamma measurements on this system which have been performed for over 40 years. The preprint of the article can be found here.
Brenden, Wen-Te, and Adriana of MPIK in Heidelberg have released a new paper investigating collective effects that could occur in Thorium doped crystals when excited by narrow-band coherent pulses. Here’s the link to the paper. A variety of schemes are discussed with the goal of creating unique signatures of excitation which can be used to determine the detuning of the exciting laser pulses from 229Th nuclear transition energy. Other details such as multi pulsed excitation, pulse shape, phase shifting, static magnetic fields and quantization axis are discussed to give a comprehensive understanding of the possibilities when tackling such a project.
A personal note from the nuClock coordinator: This work fulfills the last formal deliverable for EU reporting, rejoice!
Marianna Safronova and colleagues from the Univ. of Delaware and from the Kurchatov Institute in St. Petersburg have performed new atomic structure calculations for Th+and Th2+that are used to relate measured spectroscopic isotope shifts to differences of the nuclear charge radii. Such calculations are notoriously difficult for these thorium ions because of strong configuration mixing of the electrons. Combined with experimental data on isotope shifts in Th+from 227Th to 232Th, measured at KfK Karlsruhe in 1989, and the recent results on isotope shifts of Th2+obtained by PTB and LMU within nuclock, they provide a more reliable picture of the thorium nuclear charge radii, including an improved value for the radius change between 229Th and the229mTh isomer: Excitation to the isomeric state increases the charge radius by less than 0.02%. The work is published in Physical Review Letters.
Starting with October 22., Tomas Sikorsky is enforcing the TU Wien Thorium team. Tomas has worked on NMR during his masters and on ion collisions with ultra cold gasses during his PhD in the team of Roee Ozeri at Weizmann. In Vienna, he will establish detection of the Thorium isomer via nuclear quadrupole resonance spectroscopy (NQRS). Welcome Tomas!
Every year, the German Physics Society (DPG) offers a PhD thesis prize for each of its sections. This year, Lars von der Wense of LMU Munich was awarded the thesis prize in the section “Matter and Cosmos”. Congratulations to Lars, his supervisor Peter G. Thirolf, and the whole team at LMU for this prestigious recognition!