Optical fiber link to make CERN more on time than ever

A new optical fiber link between CERN and Paris will provide the Laboratory with an accurate frequency reference, enhancing precision and supporting experiments like ALPHA in their search for matter–antimatter differences.

The Standard Model of particle physics is based on the idea that if you simultaneously swap a matter particle for its antimatter version (changing the sign of its charge), flip its spatial coordinates as if viewed through a mirror (parity) and reverse the direction of time, then there should be no difference in the behavior of the two particles.

Due to the central role of fundamental symmetry in quantum field theory, discovering even a small violation of this principle, known as charge-parity-time (CPT) symmetry, would suggest that our understanding is incomplete and could point to new physics beyond the Standard Model.

Experiments at CERN’s Antimatter Factory test fundamental principles such as CPT symmetry by studying the properties and behavior of antimatter and comparing them with normal matter. The ALPHA experiment performs such tests through spectroscopy of antihydrogen—that is, by measuring the frequencies of transitions in the anti-atom using laser light or microwaves. If the results match those of normal hydrogen, the measurement is consistent with CPT symmetry.

These frequencies, measured in units of Hz, equivalent to one per second, correspond to the energy level intervals in atoms and the spectral lines that arise when they make quantum transitions between levels. To accurately compare matter and antimatter, the frequencies must be determined incredibly precisely, requiring ultra-precise clocks.

That’s why a cesium fountain clock was recently installed in ALPHA and a new optical fiber link between the experiment and the French National Metrological Institute in Paris is now online. Both the clock and the optical link will help improve the precision of ALPHA’s antihydrogen measurements by orders of magnitude.

“For our previous measurement of the transition between the ground state and the first excited state of antihydrogen, we used a simpler clock made out of a quartz oscillator referenced via GPS satellite as a frequency reference, and we reached a precision on the transition frequency of two parts per trillion (2×10^-12),” says physicist Janko Nauta from the ALPHA collaboration.

“However, the equivalent measurement on hydrogen, performed a few years before our antihydrogen measurement, has an even higher precision, of four parts per quadrillion (4×10^-15), calling for a better clock to look for potential differences between matter and antimatter.”

“For ALPHA, both the optical fiber link and the cesium fountain clock play important roles in making antihydrogen measurements with a precision that matches that of the hydrogen measurements,” continues Nauta.

“While we rely on the clock, the link helps us to reduce noise in the measurement and to better evaluate the clock in the long term, to verify that it stays accurate. In addition, the link will make it possible to use signals from optical quantum clocks in the future, surpassing the stability of clocks that currently realize the SI second.”

The link is part of the REFIMEVE+ network, a project that distributes an ultra-stable optical frequency reference to research laboratories across France and beyond via existing optical cables on the French internet network. It is a pilot implementation of a new project that aims to connect multiple experiments at CERN to REFIMEVE+.

This has the potential to improve the precision of clocks across CERN and could provide a new way for the Laboratory to access Coordinated Universal Time (UTC)—the global standard for timekeeping. The optical signal from the link can synchronize with UTC more precisely than via GPS satellite, which is currently used across CERN.

“The CERN Quantum Technology Initiative envisages having more precise frequency signals delivered to CERN from other national metrology institutes and distributing them to all the interested experiments at the Laboratory,” says Edoardo Martelli from CERN’s IT department. “Having more sources allows a more precise synchronization of the local clocks and increases the robustness of the service.”

ALPHA’s most recent precise measurement of the transition between the ground state and the first excited state of antihydrogen has placed tighter constraints on violations of CPT symmetry than its previous measurement. With the new optical link, the collaboration hopes to put CPT symmetry to even more stringent tests.