[Paper Review] Chip-Scale, Sub-Hz Fundamental Sub-kHz Integral Linewidth 780 nm Laser through Self-Injection-Locking a Fabry-Pérot laser to an Ultra-High Q Integrated Resonator

Andrei Isichenko, Nitesh Chauhan+3 more|arXiv (Cornell University)|Jul 11, 2023

Advanced Frequency and Time Standards9 citations

TL;DR

Demonstrates a chip-scale 780 nm laser with sub-Hz fundamental and sub-kHz integral linewidth by self-injection-locking a Fabry-Pérot laser to an ultra-high-Q integrated resonator.

ABSTRACT

Today's state of the art precision experiments in quantum, gravimetry, navigation, time keeping, and fundamental science have strict requirements on the level and spectral distribution of laser frequency noise. For example, the laser interaction with atoms and qubits requires ultra-low frequency noise at multiple offset frequencies due to hyperfine atomic transitions, motional sidebands, and fast pulse sequencing. Chip-scale integration of lasers that meet these requirements is essential for reliability, low-cost, and weight. Here, we demonstrate a significant advancement in atomic precision light sources by realizing a chip-scale, low-cost, 780 nm laser for rubidium atom applications with record-low 640 mHz (white noise floor at 0.2 Hz$^2$/Hz) fundamental and 732 Hz integral linewidths and a frequency noise that is multiple orders of magnitude lower than previous hybrid and heterogeneous self-injection locked 780 nm lasers and lower noise than bulk microresonator implementations. The laser is a Fabry-Pérot laser diode self-injection locked to an ultra-high Q photonic integrated silicon nitride resonator. This performance is enabled by a 145 million resonator Q with a 30 dB extinction ratio, the highest Q at 780 nm, to the best of our knowledge. We analyze the impact of our frequency noise on specific atomic applications including atomic frequency references, Rydberg quantum gates, and cold atom gravimeters. The photonic integrated resonator is fabricated using a CMOS foundry-compatible, wafer-scale process, with demonstrated integration of other components showing promise for a full system-on-a-chip. This performance is scalable to other visible atomic wavelengths, opening the door to a variety of transitions across many atomic species and enabling low-power, compact, ultra-low noise lasers impacting applications including quantum sensing, computing, clocks and more.

Motivation & Objective

• Demonstrate a chip-scale, low-cost 780 nm laser suitable for rubidium atom applications.
• Achieve record-low fundamental and integral linewidths and ultra-low frequency noise.
• Analyze how frequency noise impacts atomic references, quantum gates, and cold-atom gravimeters.
• Show CMOS-foundry-compatible fabrication of the photonic integrated resonator for potential system-on-a-chip integration.

Proposed method

• Self-injection-locking a Fabry-Pérot laser diode to an ultra-high-Q photonic integrated silicon nitride resonator.
• Utilize a resonator with Q ≈ 145 million and a 30 dB extinction ratio.
• Characterize laser frequency noise: measure fundamental and integral linewidths.
• Analyze frequency noise impact on atomic references, Rydberg gates, and gravimeters.
• Leverage wafer-scale, CMOS-compatible fabrication for integration of components.

Experimental results

Research questions

• RQ1 What fundamental and integral linewidths are achievable for a 780 nm laser when self-injection-locked to an ultra-high-Q integrated resonator?
• RQ2 How does the ultra-high-Q resonator influence the laser’s frequency noise across relevant offset frequencies?
• RQ3 Can CMOS-foundry-compatible photonic integration deliver chip-scale, ultra-low-noise lasers for atomic applications?
• RQ4 What are the implications of the obtained noise performance for atomic references, quantum gates, and cold-atom gravimeters?

Key findings

• Achieves 640 mHz white-noise floor at 0.2 Hz^2/Hz.
• Demonstrates 732 Hz integral linewidth.
• Uses a 145 million Q resonator with 30 dB extinction ratio, the highest Q at 780 nm reported.
• Outperforms prior hybrid/heterogeneous self-injection locked 780 nm lasers and some bulk microresonator implementations in noise performance.
• CMOS-compatible, wafer-scale photonic integration enables potential full system-on-chip solutions.