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Symmetry of space-time tested by means of atomic clocks

The comparison of two atomic clocks has confirmed their excellent accuracy as well as a fundamental hypothesis of the theory of relativity.

PTB-News 2.2019
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fundamental research in physics

developers of optical atomic clocks

The first long-term comparison of two optical ytterbium clocks has provided reliable results concerning their accuracy and stability at the limit of what has been measurable to date. At the same time, the Lorentz symmetry was confirmed for electrons in even tighter experimental limits.

A tunable laser excites an extremely narrow-band resonance in an Yb+ ion of an atomic clock. Two ions with wave functions (yellow) that are oriented at right angles are interrogated by means of laser light with an adjustable frequency shift Δf to measure a possible frequency difference. The whole experimental setup rotates together with the Earth once a day relative to the fixed stars.

One of the basic assumptions of Einstein's theory of relativity states that the speed of light is the same in all directions of space. This assumption was demonstrated by Michelson and Morley as early as 1887 by means of a pivot-mounted interferometer comparing the speed of light along two perpendicular optical axes. Now one could ask: Does this symmetry of space (which was named after Hendrik Antoon Lorentz) also apply to the motion of material particles? Or are there any directions along which these particles move faster or more slowly although the energy remains the same? Especially for high energies of the particles, theoretical models of quantum gravitation predict a violation of the Lorentz symmetry.

An experiment has now been carried out with two atomic clocks in order to investigate this question with high accuracy. The frequencies of these atomic clocks are each controlled by the resonance frequency of a single Yb+ ion that is stored in a trap. While the electrons of the Yb+ ions have a spherically symmetric distribution in the ground state, in the excited state they exhibit a distinctly elongated wave function and therefore move mainly along one spatial direction. The orientation of the wave function is determined by a magnetic field applied inside the clock. The field orientation was chosen to be approximately at right angles in the two clocks. The clocks are firmly mounted in a laboratory and rotate together with the Earth once a day (or to be more precise: once in 23.9345 hours) relative to the fixed stars. If the electrons᾽ speed depended on the orientation in space, this would thus result in a frequency difference between the two atomic clocks that would occur periodically, together with the Earth᾽s rotation. To be able to differentiate such an effect clearly from any possible technical influences, the frequencies of the Yb+ clocks were compared for more than 1000 hours. During the experiment, no change between the two clocks was observed for the accessible range of period durations from a few minutes up to 80 hours. For the theoretical interpretation and calculations concerning the atomic structure of the Yb+ ion, PTB's team worked in collaboration with theoretical physicists from the University of Delaware (USA). The recently obtained results have improved those obtained in 2015 by researchers from the University of California, Berkeley with Ca+ ions drastically by a factor of 100.

Averaged over the total measuring time, the two clocks exhibited a relative frequency deviation of less than 3 · 10-18. This confirms the systematic uncertainty of the clocks that had previously been estimated to be 4 · 10-18. Furthermore, it is an important step in the characterization of optical atomic clocks at this level of accuracy. Only after roughly ten billion years will these clocks potentially deviate from each other by one second.


Ekkehard Peik
Department 4.4
Time and Frequency
Phone: +49 531 592-4400
Opens window for sending emailekkehard.peik(at)ptb.de

Scientific publication

C. Sanner, N. Huntemann, R. Lange, C. Tamm, E. Peik, M. S. Safronova, S. G. Porsev: Optical-clock comparison for Lorentz symmetry testing. Nature 567, 204-208 (2019)