Miniaturized Strontium Atomic Clock using an Integrated Photonics Package

Atomic clocks are crucial for various applications, including navigation, communication, and fundamental physics research. However, their size and complexity often limit their portability and integration into miniaturized systems. Current strontium atomic clocks utilize bulky discrete optical components for laser cooling and trapping, which include multiple mirrors, lenses, and optical elements for beam shaping and polarization control. This work aims to overcome these limitations by developing an integrated photonics solution for a miniaturized strontium atomic clock. The motivation stems from the need for compact, robust, and cost-effective atomic clocks that can be readily integrated into various platforms. Miniaturization offers several advantages, including reduced size, weight, and power consumption, enabling deployment in space, mobile devices, and other resource-constrained environments. The integration of all optical components onto a chip promises enhanced stability and reduced susceptibility to environmental factors, leading to improved clock performance and reliability. This approach uses planar photonic chips to generate the various laser beams required for cooling and trapping strontium atoms, significantly simplifying the overall system architecture and reducing its footprint. The success of this project would demonstrate a crucial step towards the realization of manufacturable photonic integrated circuits (PICs) for advanced atomic technologies.

Previous work on strontium atomic clocks primarily relied on bulky discrete optical components for laser cooling and trapping. These systems typically involved multiple mirrors, lenses, and other optical elements to generate and shape the various laser beams necessary for different stages of the cooling and trapping process. While these systems have achieved high precision, their complexity and size hinder miniaturization and integration efforts. Recent advances in integrated photonics have shown the potential for creating compact and efficient optical systems on a chip. Several groups have demonstrated integrated photonic devices for generating and manipulating laser beams, but their application to atomic clocks, particularly strontium atomic clocks, has been limited. This work leverages advances in metasurfaces and grating outcouplers to overcome challenges associated with beam shaping and polarization control in integrated photonics for atomic clock applications.

The authors designed and fabricated an integrated photonics package consisting of two planar photonic chips. These chips generate all the necessary beams for three stages of laser cooling and trapping: two magneto-optical traps (MOTs) and an optical lattice. The blue MOT (461 nm) is used for initial cooling, followed by the red MOT (689 nm), and finally the optical lattice (813 nm). A clock beam (698 nm) copropagates with the lattice beam for clock transition measurement. The photonic chips are arranged vertically, with one on top and the other at the bottom of a 25 mm³ vacuum chamber containing strontium vapor. The design employs an unconventional beam arrangement to achieve 3D cooling and trapping in a compact geometry. The beams are generated using a three-stage emitter design consisting of an evanescent coupler (EVC), a meta-grating (MG), and a metasurface (MS). The EVC converts the waveguide mode into a collimated dielectric slab mode beam, while the MG controls the outcoupling strength and beam shaping. The MS modifies the phase fronts, polarizations, and shapes of the beams, acting as an optical wedge, lens, and waveplate. The chips are fabricated using silicon nitride and fused silica, bonded together using SU8 photoresist. The system uses commercial V-groove fiber arrays for independent power control of each beam, although future designs will incorporate on-chip beam splitters. The authors characterized the performance of the generated beams using various techniques, including measuring beam profiles, divergence, polarization, and power. The alignment precision between the PIC and MS chips was also measured. The entire system is designed to be compact, robust, and suitable for integration into a complete atomic clock.

The authors successfully demonstrated a compact integrated photonics system capable of generating all the necessary beams for a strontium atomic clock within a 25 mm³ volume. Twelve circularly polarized MOT beams (six blue and six red) were generated with beam diameters up to 10 mm (461 nm). The collinear combination of lattice and clock beams was achieved with a collimation within 0.1°. The measured polarization fidelities were >75%. The blue MOT beams exhibited minimal losses (around 15dB), while red MOT beams showed slightly higher losses (around 10dB). The beam profiles closely matched the designed elliptical Gaussian profiles. The measured beam angles deviated slightly from the designed values, with discrepancies attributable to fabrication tolerances. Unwanted diffraction orders from the metasurfaces were observed, but their impact on system performance was minimized by design. The flip-chip bonding process yielded acceptable alignment precision (7 µm translational and 3 µm rotational misalignment). These results demonstrate the feasibility of using integrated photonics to miniaturize atomic clocks and open avenues for developing more complex and integrated atomic technologies.

The successful demonstration of this integrated photonics system for a strontium atomic clock represents a significant advance in miniaturizing atomic timekeeping devices. The compact design and simultaneous generation of multiple beams with precisely controlled parameters significantly simplify the system architecture and improve its robustness compared to traditional approaches using discrete optical components. The achieved performance is promising, although further optimization is possible. The relatively higher loss in the red beams compared to blue beams highlights areas for future improvement in the fabrication process. The presence of unwanted diffraction orders, though mitigated by design, suggests opportunities for enhanced metasurface design and fabrication techniques. The current system's reliance on external fiber arrays for individual beam power control could be addressed by integrating on-chip beam splitters in future iterations. Despite these limitations, the key contribution of this work lies in demonstrating the integration of various optical functionalities onto a single chip for generating complex beam patterns crucial for advanced atomic technologies. This integrated approach promises significant cost reduction, enhanced stability, and miniaturization, thereby enabling new applications for atomic clocks in various fields.

This research successfully demonstrated a miniaturized strontium atomic clock using an integrated photonics package. The system generated all necessary beams for laser cooling and trapping within a compact volume (25 mm)³. This significantly reduces the size and complexity compared to traditional systems. While further optimization is needed to reduce losses and improve the performance of certain components, the results showcase the potential of integrated photonics for miniaturizing and improving atomic clocks and other advanced atomic technologies. Future work will focus on improving the fabrication process, integrating on-chip beam splitters, and integrating the entire atomic clock system onto a single chip.

The study's limitations include slightly higher-than-ideal losses in the red beams and the presence of unwanted diffraction orders from the metasurfaces. The current design also utilizes external fiber arrays for individual beam power control, adding complexity. The alignment precision between the PIC and MS chips, while acceptable, could be further improved. The reported performance is based on a small number of fabricated devices, and larger-scale studies are needed to fully assess yield and reproducibility. While the compact design represents a significant improvement, the system still requires external components such as the vacuum chamber and magnetic field coils.