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arXiv:2507.23617 · Physical Review Applied

Quasicontinuous sub- μ K strontium source without a high-finesse cavity-stabilized laser

Sana Boughdachi, Benedikt Heizenreder, Ananya Sitaram, Erik F. Dierikx, Yan Xie, Sander Klemann, Paul Klop, Jeroen C. J. Koelemeij, Rafał Wilk, Florian Schreck, Andreas Brodschelm

Published Feb 6, 2026• 1 citation
arXivHTMLPDF

Abstract

We demonstrate a quasicontinuous sub- μ K strontium source achieved without the use of a high-finesse cavity-locked laser. Our frequency reference is based on a dispersion-optimized, fiber-based frequency comb that enables sub-kHz linewidths. The long-term stability of the comb is defined by an external rf reference: either a 10 MHz rf signal from the Dutch Metrology Institute (VSL) or a tunable rf source whose long-term stability is maintained by monitoring and stabilizing the position of a narrow-line magneto-optical trap (MOT) [Sillus , Rev. Sci. Instrum., 92, 033204 (2021); Heizenreder , Continuous cloud position spectroscopy using a magneto-optical trap, Phys. Rev. Appl. (2026)]. The comb-stabilized system is benchmarked against a conventional cavity-locked laser and achieves comparable performance in broadband and single-frequency MOTs using the narrow 1 S 0 → 3 P 1 laser-cooling transition. We generate high-flux, sub- μ K samples of all three bosonic strontium isotopes and demonstrate quasicontinuous outcoupling from the MOT. These results highlight the system’s suitability for compact, robust, and field-deployable continuous cold-atom devices.

DOI
10.1103/3gnx-4s97

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We demonstrate a quasicontinuous sub- μ K strontium source achieved without the use of a high-finesse cavity-locked laser. Our frequency reference is based on a dispersion-optimized, fiber-based frequency comb that enables sub-kHz linewidths. The long-term stability of the comb is defined by an external rf reference: either a 10 MHz rf signal from the Dutch Metrology Institute (VSL) or a tunable rf source whose long-term stability is maintained by monitoring and stabilizing the position of a narrow-line magneto-optical trap (MOT) [Sillus , Rev. Sci. Instrum., 92, 033204 (2021); Heizenreder , Continuous cloud position spectroscopy using a magneto-optical trap, Phys. Rev. Appl. (2026)]. The comb-stabilized system is benchmarked against a conventional cavity-locked laser and achieves comparable performance in broadband and single-frequency MOTs using the narrow 1 S 0 → 3 P 1 laser-cooling transition. We generate high-flux, sub- μ K samples of all three bosonic strontium isotopes and demonstrate quasicontinuous outcoupling from the MOT. These results highlight the system’s suitability for compact, robust, and field-deployable continuous cold-atom devices.
We demonstrate a continuous spectroscopy technique with frequency sensitivity well below the natural transition linewidth, while maintaining a locking range hundreds of times larger. The method exploits the position dependence of a continuous, broadband magneto-optical trap (MOT) operating on the 7.5-kHz-wide intercombination line of strontium. We show that the frequency sensitivity is fundamentally insensitive to the effective MOT laser linewidth. By applying active feedback on the MOT position to a dispersion-optimized frequency comb, which serves as the reference for stabilizing the MOT laser (Boughdachi , arXiv:2507.23617), we achieve a frequency instability below 4.4 × 10 − 13 after 400 s of averaging in both the optical and radio-frequency domains, surpassing the performance of conventional hot-vapor modulation transfer spectroscopy. Our method is a broadly applicable alternative route to frequency references in the high 10 − 14 range around 100 s.
We report on our progress toward the realization of an optical frequency standard referenced to strontium intercombination lines. While first frequency measurement of the weakly allowed 1S 0- 3P 1 transition was done with saturation spectroscopy on a thermal beam, we are preparing the experimental setup for high resolution spectroscopy on ultracold atoms of the doubly forbidden 1S 0- 3P 0 line. Our current setup allows the capture more than 10 7 atoms at 1 μK in 500 ms in two stages of magneto-optical trapping. We also demonstrated the dipole trapping of ultracold strontium isotopes for accurate spectroscopy and collisional measurements. A semiconductor laser source at 698 nm has already been locked to a pre-stabilization cavity obtaining a fast linewidth of less than 1 kHz. Second stage stabilization of the clock laser to ultra-high finesse cavity is in development. Recent measurement of elastic and inelastic cross-sections of the two most abundant 88Sr and 86Sr isotopes are also presented.
We will describe a next-generation active atomic frequency reference based on super-radiant pulses of laser light from the ultra-narrow, 1 mHz linewidth, optical clock transition in an ensemble of cold 87Sr atoms. Light is stimulated from the millihertz linewidth transition by confining an ensemble of laser cooled atoms inside of a high finesse optical cavity. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. We achieve a remarkable short term fractional frequency stability, 6.7 × 1016 at 1 s of averaging, absolute accuracy, 2 Hz (4 × 1015 fractional frequency), and high insensitivity to changes in the cavity length that limits the performance of todays more stable lasers. We will also discuss current work on cavity-enhanced dispersive measurements to perform high resolution spectroscopy and atom counting.
Quantum technologies are nowadays emerging as enabling tools for practical applications, such as quantum sensing, quantum computing and quantum metrology. Lasers play a central role in many of these technological platforms, e.g. for atomic clocks, ion-based or neutral atom-based quantum computers or atom interferometers. Here we present a complete laser system to cool, trap and control strontium atoms in an optical lattice or in tweezer arrays. A sub-Hz linewidth master laser, locked to a high-finesse optical cavity provides the frequency reference for an ultra-low noise comb. The rack-mounted laser system consists of all cooling, repumping, and clock lasers stabilized to the optical frequency comb. Each of the involved laser frequencies can therefore be tuned and mapped in the frequency domain with a high degree of stability. The system is controlled via a software interface, allowing to operate the cold-atom-based physics package autonomously. The system is tailored for the operation of 88Sr or 87Sr optical lattice clocks, or for quantum computing applications, but other sub-Hz lasers could be obtained by phase locking additional clock laser frequencies to the ultra-stable comb, enabling convenient and accurate optical frequency ratio measurements. The laser system architecture and the relevant characterization measurements will be presented, proposing some user-cases such as quantum computing and atom interferometry on strontium atoms. This represents a technological leap for quantum optics, allowing to explore further applications of quantum sensors outside a traditional lab.
The diode laser system for laser cooling on the 1S 0 → 3P1 intercombination line of strontium discussed in this thesis allows us to cool and trap an atomic strontium sample to 15 muK. Samples in this temperature range are useful for the development of the next generation of atomic frequency standards, cold collision studies, and as a step towards quantum degeneracy. This laser system consists of a Littrow configuration external-cavity diode laser, which is frequency locked to a high finesse cavity. The cavity is subsequently locked to an absorption feature to produce a few mW of tunable, stabilized, single mode power at the target wavelength. We present the design and characterization of this system, including a linewidth measurement of 70 kHz, as well as preliminary results from cooling and trapping.
We report recent progress in sub-kHz-linewidth operation of AlGaInP-based SDL systems designed for neutral Strontium optical clocks. Overall system performance is presented, including intensity, frequency and phase noise characterisation. Two almost identical SDLs, each with >150 mW output at 689 nm, were frequency locked and an optical beat note measurement produced a linewidth of 160 Hz with fractional frequency stability reaching below 2×〖10〗^(-15) for averaging times from 60-400 s. Spectroscopic measurements of a blue magneto-optical trap (MOT) of Strontium with one SDL resulted in a reduction of the atomic sample fluorescence and revealed Zeeman splitting of the 3P1 electronic level.