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Exploiting optical clocks for time measurement and sensor development

From advanced navigation and telecommunications to radio astronomy and the testing of fundamental physical laws, the most precise and accurate time measurements are required. To meet these technical challenges, optical clock development is at the forefront of research in this field. The state-of-the-art technology, however, has its limits: frequency stability of the lasers used in such systems is affected by thermal noise from mirror coatings and more fundamentally clocks are limited by so-called quantum projection noise (a noise related to the number of atoms and the probability to detect them in a given quantum state). Even more advanced quantum-based sensors could also be realised if current fundamental constraints could be surpassed.

EMPIR project Ultra-stable optical oscillators from quantum coherent and entangled systems (17FUN03, USOQS) is working to implement, study and characterise both established and brand-new methods for the development of optical clocks. To go beyond noise limits and increase frequency stability, multi-particle entanglement of atoms/ions will be investigated; this phenomenon could also be exploited for a variety of measurements with enhanced sensitivity. The project results will support future realisations of the SI second through new frequency standards, and significantly impact many fields requiring ultra-precise time measurement.

The stability of an optical clock is limited by two noise processes: the quantum projection noise due to the number of atoms and the noise due to the laser used to interrogate them. Work performed at PTB (Germany’s National Metrology Institute) addresses the latter. To overcome the limits posed by the laser, its phase noise is measured several time during a Ramsey excitation, a form of interferometry between the phase of a quantum state and the phase of an electromagnetic field used to measure transition frequencies of particles. This technique can be exploited to improve the clock stability and accuracy allowing more accurate and faster frequency measurements.

Project Coordinator Filippo Levi from INRIM (the Italian National Metrology Institute) said

“Finding the way to overcome a fundamental physical limit is always a great achievement: to extend the interrogation time above the coherence time of the laser is one of these cases. The technique, invented and successfully tested by PTB scientists, represents a novel approach to beat the laser noise, one of the most fundamental limits of optical clocks.”

This work is described in the open access paper Dynamical decoupling of laser phase noise in compound atomic clocks in the journal Communications Physics.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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