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Reducing frequency instabilities to unlock development of stable higher-resolution quantum technologies

Quantum technologies are of utmost importance for the European community in many different fields, from computing and communications to health, finance, transport and national security. For the exploitation of quantum effects in sensing, computing and communication, coherence is the most important parameter, and highly coherent laser sources are required both for the development of more advanced devices and for the widespread applications of quantum sensors outside specialist laboratories.

The EURAMET European Metrology Network for Quantum Technologies (EMN Quantum) has formulated common metrology strategies and a roadmap for Quantum clocks and atomic sensors. Ultra-stable 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 a 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, and health.

Completed EMPIR project Next generation ultrastable lasers: reducing thermal noise limit and overcoming technical limitations with new materials and technologies (20FUN08, NEXTLASERS) has laid the groundwork for developing quantum devices with frequency instabilities below 1×10^-17 Hz – which were not previously available. Novel methods have been used to optimise laser sources to reduce thermal noise and improve vibration isolation. Closed-cycle cooling for enabling cryogenic operation was investigated, as well as digital signal processes for optimising laser wavelength for stability.

Project achievements

Specific project outputs include:

• Project participant FEMTO-ST demonstrated cooling of a single-crystal silicon cavity in a dilution cryocooler. The lowest temperature measured with the closest sensor to the cavity is of 360 mK. This sub-Kelvin temperature was later confirmed by adding a sensor directly glued to the cavity, reaching a lowest temperature of 585 mK. As far as the consortium is aware, this is ls the lowest temperature ever used for operating an ultra-stable optical resonator.
• Mona Kempkes from PTB was awarded with the 2024 European Time and Frequency Forum Student Poster Award - for investigations on unexpected, novel noise sources and birefringence of AlGaAs crystalline coatings and their effects on ultra-stable lasers investigated in the course of the project.
• INRiM has designed and set-up an FPGA (field programmable gate array) board for transferring the frequency stability of a superior laser to many regions of the optical spectrum simultaneously. The board was successfully used in the framework of Metrology Partnership project Primary spectrometric thermometry for gases (22IEM03, PriSpecTemp) in which PTB's portable Sr clock was brought to INRIM and compared to the local Yb clock and other EU optical clocks via fiber links. With the FPGA board, the stability of PTB's clock laser was transferred to the Yb clock, which improved the stability of the latter by a four-fold factor.
• The successful implementation of a closed cycle cooling of a cryogenic silicon cavity at PTB now enables long-term reliable use of these cavities for the most stable laser sources currently available worldwide and opens opportunities for their widespread application in research and technology by planned commercialisation.
• Project work related to the use of an optical frequency comb as a transfer oscillator required an extended and comprehensive characterisation of the uncertainty contributed by all steps of the metrological chain. This enabled scientists at INRiM, LNE-SYRTE and PTB to improve the accuracy and stability of the setup used for international clock comparisons within other EURAMET projects including 18SIB05-ROCIT and 22IEM01-TOCK. With these new results, the uncertainty budget was upgraded and possible sources of discrepancies in ongoing and planned clock comparisons could be ruled out.
• Project work related to the use of rare-earth ion-based frequency references as alternatives to cavities at LNE_SYRTE could set new limits on temperature sensitivity and thermal noise, and promise instabilities below 10⁻¹⁷.

Project coordinator Uwe Sterr from PTB said

‘The joint efforts by the project partners have provided important insights into methods for reducing the frequency noise of lasers to a new level. Our findings make an important contribution to improving these sources and we expect that more stable laser sources based on these methods will soon emerge to benefit quantum technologies.’

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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