Quantum Noise Locking of Squeezed Light Generated in Periodically Poled LiNbO₃ Waveguides

Squeezed states of light produced by nonlinear interactions in periodically poled crystals are specially engineered quantum states where the uncertainties in the phase or amplitude quadrature can be traded against each other. These low-noise states have applications in quantum communication, sensing, and computing [1]. To fully exploit these applications, precise phase locking of the squeezed light to a local oscillator (LO) phase is essential. For single-pass waveguides the used locking technique has relied on the injection of a frequency-shifted input seed to generate the error signal [1]. Here we assess the advantages and challenges of implementing a technique originally devised for cavity-based squeezed light sources called quantum noise locking (QNL) [2], which features a simplified feedback structure, eliminating the need for an additional frequency shifted field, thereby avoiding seeding and extra optical components. This work experimentally extends QNL to periodically poled LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> waveguides for the first time, which allows for phase controlled squeezing, with less than 10 mrad phase noise, over a 50 GHz bandwidth.