Directly measured squeeze factors over GHz bandwidth from monolithic PPKTP resonators

Abstract Squeezed vacuum states of light with bandwidths in the gigahertz range are required for ultrafast quantum sensors, for high-speed continuous-variable (CV) one-sided device independent QKD and for CV optical quantum computers. Here we report on the detailed investigation of the current performance limits of the established method for the generation and direct measurement of squeezed light with GHz bandwidth. We present squeeze factors of diffraction limited laser beams from monolithic periodically poled KTP resonators measured with two laboratory-built balanced homodyne detectors (BHDs) with gigahertz bandwidth. We realize two complete systems without selection of optical or electronic hardware components to test the reproducibility without rejects. As expected, the systems show clear spectral differences. However, both achieve directly measured squeeze factors in the order of 3 dB over a GHz bandwidth, which is achieved here for the first time. Since the speed of CV quantum protocols scales proportionally to the bandwidth used, our layout for BHDs is suitable for increasing the speed of all these protocols.