Silicon nitride photonic integrated circuit for controlling chip-based cold-atom inertial sensors (2509.03279v1)

Abstract: We have developed a silicon nitride based photonic integrated circuit (PIC) that is responsible for the cooling, pumping and imaging of an on-chip based cold atom inertial sensor. The photonic integrated circuit consists of two chips placed next to each other and has a total area of 2x2~cm$^2$. This greatly minimizes the area needed while still having all the optical control functions to create, control and measure a magneto-optical trap (MOT). The piezo electric material Lead Zirconate Titanate (PZT) on the PIC is employed for phase shifting a Mach-Zehnder type configuration where extinction ratios up to 50 dB and switching speeds of 1 MHz are achieved. For the first time two and three dimensional optical traps for rubidium 87 atoms are realized using an active photonic integrated circuit. For the three dimensional optical trap, we measure $7\times 10^7$ atoms with a temperature of 270~$\mu$K.

• The paper demonstrates a silicon nitride PIC integrating PZT actuators and extended TBS designs to achieve MHz-rate switching with 40–50 dB extinction ratios.
• It details on-chip optical functions for rubidium-87 MOTs, drastically reducing the system's size compared to conventional free-space optical setups.
• Experimental results show 3D MOT loading of 7×10⁷ atoms at 270 μK, underscoring the potential for scalable, miniaturized inertial sensor systems.

Silicon Nitride Photonic Integrated Circuit for Chip-Based Cold-Atom Inertial Sensors

Introduction

This paper presents the design, fabrication, and experimental validation of a silicon nitride photonic integrated circuit (PIC) for the control of chip-based cold-atom inertial sensors. The PIC is engineered to perform all essential optical functions—cooling, pumping, and imaging—required for the operation of a magneto-optical trap (MOT) using rubidium-87 atoms. The integration of piezoelectric phase shifters (PZT) enables MHz-rate switching and high extinction ratios, facilitating precise temporal and spatial control of laser beams in a compact footprint. The work demonstrates the feasibility of using PICs to drastically reduce the size and complexity of cold-atom sensor systems, with direct implications for deployable inertial navigation units.

Photonic Integrated Circuit Architecture

The PIC comprises two identically designed chips, each with a silicon base and a single-stripe silicon nitride waveguide (0.9 μm × 75 nm) embedded in silicon oxide, optimized for single-mode TE operation at 780 nm. The chips are arranged side-by-side, occupying a total area of 2 × 2 cm². The architecture incorporates three primary building blocks:

• Tunable Beam Splitters (TBS): Mach-Zehnder interferometer configurations with integrated heaters for phase shifting.
• Extended TBS: Redundant phase shifter and directional coupler sections, enabling high extinction ratios even in the presence of fabrication imperfections.
• Switches: Extended TBS with dual PZT actuators for redundancy and MHz-rate switching.

Each chip contains 9 TBSs, 1 extended TBS, 5 switches, 10 PZT actuators, and 21 thermal heaters. The chips are fiber-coupled, with measured fiber-chip coupling losses of 1–2 dB per facet. The assembly is thermally stabilized via a peltier element.

Piezoelectric Phase Shifting and Switching

The integration of PZT actuators (PbZrₓTi₁₋ₓO₃) on the waveguides enables rapid phase modulation via the stress-optic effect. The dome-like PZT structure maximizes the y-stress tensor in silicon nitride, achieving a π phase shift with V_π ≈ 16 V at 1550 nm. Experimentally, switching speeds of 1–2 μs and extinction ratios up to 50 dB are demonstrated. The extended TBS architecture is shown to be robust against directional coupler errors, with analytical and numerical results confirming that perfect extinction is achievable for coupling coefficients in the range 0.25 ≤ k ≤ 0.75, provided both phase shifters are tuned.

Experimental Implementation

The PIC is integrated into a cold-atom sensor setup for rubidium-87. The laser system utilizes frequency-doubled telecom lasers at 1560 nm, producing cooling and pumping beams at 780 nm. The PIC routes and shapes the beams for 2D and 3D MOT configurations, replacing bulk optics and minimizing free-space alignment. The system achieves:

• 3D MOT Loading: $7 \times 10^7$ atoms at 270 μK in ~100 s.
• Switching Performance: MHz-rate switching and 40–50 dB extinction ratios for MOT beams.
• Volume Reduction: Optical system volume of $4 \times 10^{-4}$ L, several orders of magnitude smaller than conventional optical benches (4.3 L) or free-space tables (600 L).

The PIC is not fully reconfigurable, which reduces complexity and optical loss. Average transmission through both chips is 27% for the cooler beam and 33% for the pumper beam, with additional losses incurred to achieve high extinction ratios during switching.

Performance Analysis

Atom number and temperature are characterized via fluorescence and time-of-flight (TOF) measurements. The MOT loading curve saturates at $7 \times 10^7$ atoms for a 3D MOT cooler power of 3.7 mW. TOF analysis yields a temperature of 270 μK in the x-direction and 1 mK in the y-direction, attributed to anisotropic magnetic field turn-off dynamics. The atom number and loading rate are lower than previous miniaturized optical bench implementations, primarily due to reduced laser power; however, the results are sufficient for high-stability atomic clocks ($10^{-13}$ relative stability) and μg-level acceleration sensitivity.

Theoretical and Practical Implications

The demonstrated PIC architecture provides a scalable pathway for the miniaturization of cold-atom inertial sensors. The use of extended TBS and PZT actuators addresses key challenges in photonic circuit fabrication, enabling high extinction ratios and fast switching in the presence of process variations. The integration of all optical control functions on-chip, with fiber-based input/output, eliminates free-space alignment and dramatically reduces system volume. This approach is directly applicable to the development of deployable inertial measurement units for navigation in aerospace, marine, and space platforms.

Future work will focus on further integration, including on-chip beam collimators and grating couplers for direct emission of MOT beams with controlled diameter and intersection geometry. This will enable fully chip-based and fiber-integrated cold-atom systems, further reducing size, weight, and power requirements.

Conclusion

This work establishes the viability of silicon nitride PICs with integrated PZT phase shifters for the control of chip-based cold-atom inertial sensors. The architecture achieves MHz-rate switching, high extinction ratios, and substantial volume reduction, with demonstrated MOT loading and temperature performance suitable for precision sensing applications. The results have significant implications for the miniaturization and deployment of cold-atom sensors in real-world navigation systems, and the approach is extensible to fully integrated photonic-atomic platforms.