Need and objectives


The EURAMET European Metrology Network for Quantum Technologies (EMN-Q) has formulated common metrology strategies and a roadmap for Quantum clocks and atomic sensors. There, ultrastable lasers, and low perturbation cryogenic systems were identified as essential enabling technology and their laser noise characterisation as important metrological validation. They are also described as central enabling technology for quantum sensors in the fields of spacetime references, geodetic references, navigation, deep-space tracking/positioning, monitoring of key variables for climate, geoscience, monitoring of underground resources, defence, and health. The research in this project on an ultrastable cavity design optimised for low frequency gravitational wave (GW) detection may also lead to future applications in these fields.

As one of the most advanced quantum sensors, optical clocks will immediately benefit from improved ultrastable lasers that interrogate atomic transition, as this fundamentally limits the measurement resolution that can be reached in finite measurement times. High resolution optical clock comparisons are necessary for the evaluation of their accuracy towards a redefinition of the SI second in terms of optical transitions. The improved ultrastable lasers developed in this project will also further enable clocks to become effective tools for tests of fundamental physics, chronometric geodesy, gravitational waves and dark matter detection.

Ultrastable frequencies are needed for many high-tech industrial applications: Optical telecommunication, bistatic radar, long distance fibre links, synchronisation of telecommunication networks, satellite navigation and communication systems, and optical sensing. Here, the uptake of the project results by laser producers will make ultrastable frequency sources available to these communities, leading to improved quality and performance in these applications


This project will address the current limitations of ultrastable lasers, aiming to set the ground for the next generation of these devices with frequency instabilities at and below 1×10-17.

The specific objectives of the project are:

  1. To apply novel low thermal noise designs. New high-reflectivity low thermal noise mirror coatings such as single crystalline Bragg reflectors, micro-structured surfaces or dielectric coatings optimised for higher mechanical quality factor will be investigated. Furthermore, highly dispersive cavities based on the slow light effect aim at further reducing the thermal noise sensitivity. Spacer materials with high specific stiffness and thus lower sensitivity to vibrations will be addressed for the next generation of ultrastable cavities. Frequency references based on spectral hole burning (SHB) in rare-earth ion doped crystals as a promising alternative for frequency stabilisation of ultrastable lasers will also be investigated. Mechanisms (temperature, pressure, vibration, etc.) that can perturb the frequency stability of the spectral holes will be studied in order to understand the fundamental limits of such systems.
  2. To demonstrate improved vibration isolation systems at low frequency, by using state-of-the-art seismometers, tiltmeters and interferometric levelling systems, involving new materials, multi-degree of freedom servo control, and suspension systems. Their performance will be optimised for the frequency of interest of ultrastable lasers (≈ 1 mHz – 100 Hz).
  3. To integrate closed cycle cooling for continuous cryogenic operation of SHB and optical cavities at 124 K, 4 K and even below. Novel approaches will be investigated to more efficiently decouple temperature fluctuations and vibrations intrinsic of the closed cycle operation, from the optical cavity or the SHB setup.
  4. To make the ultrahigh stability available to optical clocks. As the ultrastable lasers operate at wavelengths optimised for stability of the references (spectral holes in crystals or low-noise mirrors) different from the wavelengths needed to interrogate clock transitions in atoms or ions, the stability must be transferred to other wavelengths. The noise sources in this spectral transfer will be investigated and fundamentally limiting noise processes will be identified, amongst other techniques by using advanced digital signal processing.
  5. To apply tests of fundamental physics. Local and remote cavity-vs-atom and cavity-vs-cavity frequency data will be correlated to perform new tests of fundamental physics such as fine structure constant variations. The prospects of specially designed optical cavities for future low-frequency gravitational waves detection will be analysed.
  6. To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (NMIs, research laboratories), and potential end users (e.g. geodesy, quantum technologies). The developments will enable robust, reliable ultrastable frequency sources with a large range of applications that will likely lead to commercial products.