Bright Pulsed Squeezed Light for Quantum-Enhanced Precision Microscopy

Technical Summary: Bright Pulsed Squeezed Light for Quantum-Enhanced Precision Microscopy

Problem Statement
Stimulated Raman scattering (SRS) microscopy is a vital tool for label-free, quantitative biological imaging. However, its performance is fundamentally constrained by two factors: photodamage to biological samples, which limits the usable optical power, and the standard quantum limit (shot noise), which restricts signal-to-noise ratios. While squeezed light offers a pathway to sub-shot-noise sensitivity, generating and detecting high levels of bright (high optical power) pulsed squeezing at the picosecond (ps) timescales required for SRS remains a significant experimental challenge. Previous attempts to generate bright ps pulsed squeezing have been limited by parasitic multimode effects, imperfect mode matching, or low detected squeezing levels (e.g., previous work by the authors achieved only −1.8 dB).

Methodology
The authors present an experimental setup utilizing a single-pass optical parametric amplifier (OPA) based on a 5-mm-long periodically poled MgO-doped lithium niobate (PPLN) ridge waveguide. The system employs a dual-head laser providing synchronized 1064 nm (seed/local oscillator) and 532 nm (pump) pulses with 6 ps duration and an 80 MHz repetition rate.

Key methodological features include:

• Co-propagation: The 1064 nm local oscillator (LO) and the weak seed for the OPA co-propagate through the waveguide. This ensures high spatial mode overlap between the LO and the generated squeezed field without requiring complex external spatial shaping.
• Displacement Scheme: Instead of seeding the OPA with a strong field (which introduces mode-matching requirements between seed and pump), the authors use a weak seed to generate squeezed vacuum. The strong LO is then co-propagated and recombined with the squeezed field after the waveguide using waveplates and a polarizing beam splitter (PBS). This "displacement" creates a bright squeezed beam.
• Detection: The system supports two detection modes:
  1. Balanced Homodyne Detection: Used to measure vacuum squeezing and characterize the squeezing parameter by varying the relative phase.
  2. Direct Detection: Configured with a 90/10 splitting ratio to measure bright amplitude squeezing at optical powers (3.2 mW) compatible with nonlinear microscopy.
• Characterization: Spatial overlap was simulated using COMSOL (99.7%), and temporal overlap was measured via interferometric autocorrelation (97.7%).

Key Results
The study reports the following quantitative achievements:

• Measured Squeezing: The system achieved −3.6 dB of vacuum squeezing via homodyne detection and −3.2 dB of bright amplitude squeezing via direct detection at a coherent power of 3.2 mW. After correcting for electronic noise, the direct detection result was −3.35 dB.
• Inferred Generation Level: By modeling the total detection efficiency (accounting for waveguide losses, propagation losses, mode overlap, and detector quantum efficiency of 0.75), the authors infer that the squeezing generated within the waveguide is −15.4 +2.7/−8.7 dB.
• Mode Overlap: The experimental setup achieved a spatial mode overlap of 99.7% and a temporal mode overlap of 97.7%, minimizing losses associated with imperfect mode matching.
• Limitations: The observed squeezing levels are primarily limited by optical losses, specifically the quantum efficiency of the photodetectors and propagation losses, rather than phase noise.

Significance and Claims
The authors claim that the measured level of bright amplitude pulsed squeezing (−3.2 dB) is, to their knowledge, the highest reported to date. They emphasize that this level of squeezing is achieved at optical power levels compatible with state-of-the-art SRS microscopes.

The paper asserts that this work opens a pathway toward the broader adoption of quantum-enhanced nonlinear microscopy in biological studies. By demonstrating a robust method to generate bright pulsed squeezing with high spatial and temporal mode overlap, the authors suggest that future improvements in detector quantum efficiency could enable apparatus-independent quantum advantages in microscopy, potentially allowing for high-sensitivity imaging at the photodamage limits of biological samples. The authors modestly note that practical implementations remain subject to waveguide and displacement losses, which currently limit the achievable bright squeezing to approximately −6.2 dB in a realistic SRS setup, but they identify this as a reachable target with existing high-quantum-efficiency photodiodes.