Bright Pulsed Squeezed Light for Quantum-Enhanced Precision Microscopy

TL;DR

This work tackles the need for bright, pulsed squeezed light to surpass the standard quantum limit in nonlinear microscopy. It introduces a compact, single-pass $χ^2$ OPA in a PPLN waveguide with LO co-propagation and post-OPA displacement to produce bright amplitude squeezing suitable for picosecond pulses at biologically compatible powers. Key results include direct-detection squeezing of $-3.2$ dB and homodyne squeezing of $-3.6$ dB (vacuum case $-3.6$ dB), with loss-corrected waveguide squeezing of $-15.4^{+2.7}_{-8.7}$ dB, and near-ideal spatial/temporal mode overlaps ($0.997$ and $0.977$). The work projects substantial gains for quantum-enhanced nonlinear microscopy, estimating up to $-6.2$ dB bright squeezing under practical detector efficiencies, enabling device-independent quantum advantages in SRS and related imaging modalities.

Abstract

Squeezed states of light enable enhanced measurement precision by reducing noise below the standard quantum limit. A key application of squeezed light is nonlinear microscopy, where state-of-the-art performance is limited by photodamage and quantum-limited noise. Such microscopes require bright, pulsed light for optimal operation, yet generating and detecting bright pulsed squeezing at high levels remains challenging. In this work, we present an efficient technique to generate high levels of bright picosecond pulsed squeezed light using a $χ^2$ optical parametric amplification process in a waveguide. We measure $-3.2~\mathrm{dB}$ of bright squeezing with optical power compatible with nonlinear microscopy, as well as $-3.6~\mathrm{dB}$ of vacuum squeezing. Corrected for losses, these squeezing levels correspond to $-15.4^{+2.7}_{-8.7}~\mathrm{dB}$ of squeezing generated in the waveguide. The measured level of bright amplitude pulsed squeezing is to our knowledge the highest reported to date, and will contribute to the broader adoption of quantum-enhanced nonlinear microscopy in biological studies.

Figures (3)

• Figure 1: Experimental setup A dual-head laser generates synchronized 1064nm and 532 nm light pulses with 6 ps and 80 MHz repetition rate. The beams are recombined and focused into a periodically poled MgO-doped lithium niobate (PPLN) waveguide. The 1064 beam is polarized along the ordinary axis of the waveguide and serves as the LO. A weak component polarized along the extraordinary axis seeds the OPA and generates squeezed vacuum. The 532 nm beam is detected and used to lock the relative phase between the 1064 nm and 532 nm fields using a PDH lock scheme. The LO and squeezed vaccum are recombined and mixed at a PBS using three waveplates (WP 1-3). After the PBS, the field is detected either in a balanced homodyne configuration with a 50/50 mixing ratio using two photodetectors (not shown) or with a 90/10 mixing ratio at a single output to generate bright squeezing.
• Figure 2: Spatial and temporal overlap between the LO and squeezed fielda) COMSOL modal analysis of the waveguide modes for S and P polarizations. The top panel shows the mode intensity for the horizontally polarized mode, corresponding to the LO. The bottom panel shows the mode intensity for the vertically polarized mode, corresponding to the squeezed field. b) Experimental measurement of the pulse durations of the LO and OPA pump. Pulse durations are measured using an interferometer by monitoring the interference visibility as the optical path length of one arm is varied. Red markers show the LO data and the solid curve is a Gaussian fit, yielding a pulse duration of $\tau_{\mathrm{LO,exp}} = 6.4 \pm 0.07~\mathrm{ps}$. Green markers correspond to the OPA pump, with a fitted pulse duration of $\tau_{\mathrm{pump,exp}} = 5.17 \pm 0.15~\mathrm{ps}$.
• Figure 3: Squeezed light measurementsa) Squeezing (green) and antisqueezing (orange) detected in homodyne detection, as a function of pump power. Circles indicate experimental data, solid lines show fits to Eq.(1). b) Squeezing (green) and antisqueezing (orange) spectra measured in homodyne detection around $20~\mathrm{MHz}$ for $20~\mathrm{mW}$ of pump power. The black trace is the shotnoise level. A squeezing level of $-3.9~\mathrm{dB}$ is obtained after subtracting electronic noise $13.2~\mathrm{dB}$ below shot noise. c) Bright squeezing (green) and antisqueezing (orange) spectra with $3.2~\mathrm{mW}$ of coherent amplitude, measured in direct detection. Shot noise is the black trace, the pump power is 40 mW. A squeezing level of $-3.35~\mathrm{dB}$ is obtained after subtracting electronic noise $15.3~\mathrm{dB}$ below shot noise.