Squeezed light reduces the quantum noise in one observable below the shot-noise limit, at the cost of increasing noise in the conjugate observable. This is not a violation of the uncertainty principle — it's a redistribution. The total uncertainty product still satisfies Heisenberg's bound, but one quadrature gets quieter while the other gets noisier. Gravitational wave detectors use squeezed light to push the shot-noise limit below what coherent laser light can achieve. Quantum sensors and secure communication protocols use it similarly.
Generating squeezed light requires nonlinear optical processes — parametric down-conversion, four-wave mixing, or optical parametric amplification — where the nonlinear interaction correlates photon pairs in a way that reduces the noise in one quadrature. The best squeezing results (~15 dB below shot noise) come from bulk optical parametric oscillators: free-space cavities with nonlinear crystals, mirrors on optical mounts, and table-filling setups.
Ren, Kopparapu, Karnik, and collaborators (arXiv 2602.22693, February 2026) demonstrate -7.52 dB of on-chip squeezing in a periodically poled lithium niobate microresonator — the highest squeezing ratio achieved on any integrated χ² cavity platform. The chip fits on a fingernail.
The key technical achievement is the escape efficiency. Squeezing is generated inside the resonator, but it must escape through the output coupler to be useful. Any loss between generation and detection degrades the measured squeezing. The authors achieve greater than 90% escape efficiency through highly over-coupled resonances — the coupling to the output waveguide far exceeds the internal losses, so nearly all the intracavity squeezed light exits through the intended channel.
The dual-resonant design resonates both the pump and the signal wavelengths simultaneously, enhancing the parametric interaction without requiring high pump power. Continuous-wave operation at only 27 mW produces the measured squeezing — orders of magnitude less power than bulk OPO systems. The near-complete domain inversion in the periodic poling ensures efficient quasi-phase matching across the full resonator length.
The squeezing bandwidth exceeds 10.3 THz — the squeezed quadrature stays below shot noise across a spectrum spanning more than 10 terahertz. This broadband performance arises from the microresonator's free spectral range and the parametric gain bandwidth, both of which benefit from the small cavity size.
The measured squeezing at the detector is -0.81 dB — modest by bulk-optics standards. The difference between -0.81 dB measured and -7.52 dB inferred on-chip quantifies the loss in the measurement chain: fiber coupling, waveguide propagation, and detector efficiency each degrade the signal. The on-chip squeezing is strong; the bottleneck is getting it out and detecting it. Each improvement in packaging, coupling, and detector efficiency translates directly to more usable squeezing.