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Physical Review Applied

Laser-cooling 88 Sr to microkelvin temperature with an integrated-photonics system

Andrew R. Ferdinand, Zheng Luo, Sindhu Jammi, Zachary L. Newman, Grisha Spektor, Okan Koksal, Parth Patel, Daniel Sheredy, William D. M. Lunden, Akash Rakholia, Travis C. Briles, Wenqi Zhu, Martin M. Boyd, Amit Kumar Agrawal, Scott B. Papp

Published Mar 21, 2025• 8 citations
Publisher

Abstract

Optical lattice clocks generate pristine timing signals, which advance understanding of physics and open new application opportunities. However, the existing experimental apparatus for such clocks consists of numerous complicated subsystems, and are laborious to assemble. This Letter demonstrates the use of integrated photonics technologies to generate the free-space optical configuration needed to laser cool and trap atomic samples for a lattice clock and to create a frequency-comb supercontinuum to stabilize the lasers for the lattice clock. The authors assemble integrated photonics devices without active alignment, highlighting the potential for scalability in lattice-clock systems.

DOI
10.1103/physrevapplied.23.l031002

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Optical lattice clocks generate pristine timing signals, which advance understanding of physics and open new application opportunities. However, the existing experimental apparatus for such clocks consists of numerous complicated subsystems, and are laborious to assemble. This Letter demonstrates the use of integrated photonics technologies to generate the free-space optical configuration needed to laser cool and trap atomic samples for a lattice clock and to create a frequency-comb supercontinuum to stabilize the lasers for the lattice clock. The authors assemble integrated photonics devices without active alignment, highlighting the potential for scalability in lattice-clock systems.
Optical atomic clocks provide exceptionally accurate and precise signals for timekeeping and precision measurements, but they require high-power, free-space laser configurations that limit scalability. We introduce and explore a scalable infrastructure for strontium (Sr) optical-lattice clocks that incorporates co-design of atomic-beam slowing and a magneto-optical trap (MOT) from an effusion source, generation of complex, three-dimensional free-space laser configurations with a photonic integrated circuit (PIC) and metasurface (MS) optics, and laser stabilization to a frequency-comb supercontinuum generated with integrated nonlinear photonics. With these elements, we realize MOTs of all stable strontium isotopes (84^{84}84Sr, 86^{86}86Sr, 87^{87}87Sr, 88^{88}88Sr) with populations commensurate with natural abundances, demonstrating precise beam control and robustness. Access to laser-cooled alkaline-earth atoms with scalable integrated photonics enables system engineering for optical clocks, quantum sensing, and quantum information, and our experiments demonstrate extensible technologies that advance toward a Sr optical clock largely free of bulk optics.
Laser cooling 88^{88}88Sr to microkelvin temperature with an integrated-photonics system
We report on experiments generating a magneto-optical trap (MOT) of 88-strontium (88^{88}88Sr) atoms at microkelvin temperature, using integrated-photonics devices. With metasurface optics integrated on a fused-silica substrate, we generate six-beam, circularly polarized, counter-propagating MOTs on the blue broad-line, 461 nm, and red narrow-line, 689 nm, Sr cooling transitions without bulk optics. By use of a diverging beam configuration, we create up to 10 mm diameter MOT beams at the trapping location. To frequency stabilize and linewidth narrow the cooling lasers, we use fiber-packaged, integrated nonlinear waveguides to spectrally broaden a frequency comb. The ultra-coherent supercontinuum of the waveguides covers 650 nm to 2500 nm, enabling phase locks of the cooling lasers to hertz level linewidth. Our work highlights the possibility to simplify the preparation of an ultracold 88Sr gas for an optical-lattice clock with photonic devices. By implementing a timing sequence for control of the MOT lasers and the quadrupole magnetic-field gradient, we collect atoms directly from a thermal beam into the blue MOT and continuously cool into a red MOT with dynamic detuning and intensity control. There, the red MOT temperature is as low as 2 μ2~μ2 μK and the overall transfer efficiency up to 16%. We characterize this sequence, including an intermediate red MOT with modulated detuning. Our experiments demonstrate an integrated photonics system capable of cooling alkaline-earth gases to microkelvin temperature with sufficient transfer efficiencies for adoption in scalable optical clocks and quantum sensors.
Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities to explore fundamental physics and enable new measurements. However, their size and use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 10^13. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.
Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities to explore fundamental physics and enable new measurements. However, their size and use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 10^(13). These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.
We present experimental results for an optical lattice clock operating on the1S0harr3P0transition in88Sr, which is excited with the technique of magnetic field-induced spectroscopy To reduce the complexity of the setup we have developed a set of new experimental techniques that greatly simplifies the clock spectroscopy in Sr atoms. First, we developed a method for finding the clock transition that removes the need for extensive frequency metrology hardware and optical frequency combs. This technique exploits a near coincidence in the atomic wavelengths of the1S0harr3P0clock and1S0harr3P1second stage cooling transitions in Sr, which are only 5 THz far apart. This coincidence enables the use of an optical (transfer) cavity to reference the frequency of the clock transition relative to that of the much stronger coolign transition Second, all laser sources in the experimental setup are based on semiconductor technology, which greatly reduces the complexity of the apparatus. With this setup, about 10488Sr atoms are trapped in a 1D lattice formed by 200 mW of radiation tuned near the magic wavelength at 813 nm. Preliminar uncertainty budget for our optical lattice clock is also presented, with particular attention to density dependent collisions, which led to an unexpectedly high signal contrast for long interaction times.