10 dB interferometer enhancement by squeezing of photon shot noise with sub-femtometer resolution and eye-safe optical power

Abstract Quantum technologies that exploit quantum-correlated resources offer advantages over quantum technologies that just exploit coherent states. Here we propose and demonstrate the exploitation of squeezed light to enhance laser interferometers being at the sensitivity limit of eye-safe optical powers into a new regime otherwise impossible without sacrificing eye-safety. We realised a homodyne Mach–Zehnder interferometer that in its arms used an eye-safe combined optical power of only 10 mW at the wavelength of 1550 nm, and nevertheless achieved a displacement noise spectral density of as low as 0.26 fm Hz −1/2 . This value is more than a factor of 10 below what is possible with 10 mW in ideal coherent states at 1550 nm and slightly lower than what is possible with 100 mW at the same wavelength. We envision a new user application for squeezed light in avoiding costs for laser safety measures.