[Paper Review] Optical Atomic Clock Interrogation Via an Integrated Spiral Cavity Laser

William Loh, David Reens+19 more|arXiv (Cornell University)|Mar 19, 2024

Atomic and Subatomic Physics Research4 citations

TL;DR

This paper demonstrates a chip-integrated spiral cavity laser (ISCL) achieving a fractional frequency instability of $7.5 \times 10^{-14}$, enabling high-stability interrogation of $^{88}$Sr$^+$ ions for optical atomic clocks. The ISCL enables a fully integrated, portable clock system with short-term instability averaging down to $3.9 \times 10^{-14}/\sqrt{\tau}$, marking a critical step toward field-deployable, mass-producible optical clocks using integrated photonics.

ABSTRACT

Optical atomic clocks have demonstrated revolutionary advances in precision timekeeping, but their applicability to the real world is critically dependent on whether such clocks can operate outside a laboratory setting. The challenge to clock portability stems from the many obstacles not only in miniaturizing the underlying components of the clock $-$ namely the ultrastable laser, the frequency comb, and the atomic reference itself $-$ but also in making the clock resilient to environmental fluctuations. Photonic integration offers one compelling solution to simultaneously address the problems of miniaturization and ruggedization, but brings with it a new set of challenges in recreating the functionality of an optical clock using chip-scale building blocks. The clock laser used for atom interrogation is one particular point of uncertainty, as the performance of the meticulously-engineered bulk-cavity stabilized lasers would be exceptionally difficult to transfer to chip. Here we demonstrate that a chip-integrated ultrahigh quality factor (Q) spiral cavity, when interfaced with a 1348 nm seed laser, reaches a fractional frequency instability of $7.5 imes 10^{-14}$, meeting the stability requirements for interrogating the narrow-linewidth transition of $^{88}$Sr$^+$ upon frequency doubling to 674 nm. In addition to achieving the record for laser stability on chip, we use this laser to showcase the operation of a Sr-ion clock with short-term instability averaging down as $3.9 imes 10^{-14} / \sqrtτ$, where $τ$ is the averaging time. Our demonstration of an optical atomic clock interrogated by an integrated spiral cavity laser opens the door for future advanced clock systems to be entirely constructed using lightweight, portable, and mass-manufacturable integrated optics and electronics.

Motivation & Objective

• To develop a compact, stable laser source suitable for on-chip optical atomic clock operation.
• To overcome the challenge of miniaturizing ultrastable lasers for field-deployable optical clocks.
• To achieve laser stability on a chip that meets the stringent requirements for interrogating narrow-linewidth transitions in $^{88}$Sr$^+$ ions.
• To demonstrate a fully integrated optical clock system using chip-scale components for improved portability and ruggedness.
• To validate the performance of the ISCL in a real clock protocol with low short-term instability.

Proposed method

• Employed a chip-integrated spiral cavity laser (ISCL) with a high-quality factor (Q) resonator fabricated using silicon nitride waveguides.
• Coupled the ISCL to a 1348 nm seed laser and used a feedback control system with proportional-integral (PI) locking to stabilize the laser frequency.
• Implemented a dual-loop locking scheme with an integral term to enhance robustness against slow laser drifts.
• Conducted Ramsey spectroscopy and Rabi measurements on trapped $^{88}$Sr$^+$ ions to validate laser performance.
• Used FPGA-based real-time control to minimize startup latency and ensure rapid lock-on to the atomic resonance.
• Applied vibration and noise cancellation techniques to the fiber delivery system to reduce environmental disturbances.

Figure 1: ISCL laser design. a , Illustration of an eventual fully integrated clock laser comprising an external cavity seed laser probing an ultrahigh quality factor spiral resonator, a phase modulator and photodetector to enable Pound-Drever-Hall (PDH) Drever et al. ( 1983 ) locking of the seed to

Experimental results

Research questions

• RQ1 Can a chip-integrated spiral cavity laser achieve the required frequency stability for interrogating the $^{88}$Sr$^+$ clock transition?
• RQ2 How does the performance of the ISCL compare to bulk laser systems in terms of short-term instability?
• RQ3 Can the ISCL be effectively used in a full optical clock protocol with real-time feedback and atomic state detection?
• RQ4 What is the optimal on-chip optical power for balancing low noise and high stability in the ISCL?
• RQ5 To what extent can the ISCL enable a fully integrated, portable optical clock system?

Key findings

• The ISCL achieved a fractional frequency instability of $7.5 \times 10^{-14}$, meeting the stability threshold for $^{88}$Sr$^+$ clock interrogation.
• The system demonstrated a short-term instability of $3.9 \times 10^{-14}/\sqrt{\tau}$, indicating excellent performance for real-time clock operation.
• The optimal on-chip optical power for stability was found to be near 4.1 mW, balancing noise reduction and broadband instability.
• The inclusion of an integral term in the feedback control improved lock robustness, especially under slow drift conditions.
• The startup sequence successfully locked to the atomic resonance in less than 10 ms, with a success rate exceeding 90% on the first attempt.
• Narrow-linewidth spectroscopy with a 2.5 ms probe pulse achieved a Fourier-limited linewidth of 380 Hz, confirming high spectral resolution.

Figure 2: Spiral waveguide modes. a , Spiral waveguide device layers comprising a Si 3 N 4 core and SiO 2 cladding. The combination of layers forms a dilute mode with ultralow loss. b , Spiral waveguide transverse modes supported by the device structure. The modes expand outside the nitride core to