Real-Time Monitoring of Multimode Squeezing

Abstract: Multimode squeezed light is a key resource for high-dimensional quantum technologies, enabling applications in metrology, secure communication, and measurement-based quantum computing. So far, its full practical potential remains unrealized due to the limitations of homodyne detection, which is inherently restricted to single-mode operation and, at best, retrieves multimode data through post-processing. Here, we employ multimode optical parametric amplification (MOPA) to, for the first time to the best of our knowledge, achieve real-time monitoring of co-propagating modes of multimode squeezed light. By leveraging simultaneous multimode phase-sensitive amplification followed by mode sorting, we enable real-time detection of nine spatial modes and report high-purity squeezing of up to $7.9 \pm 0.6$ dB, which, as far as we are aware, is the highest squeezing recorded for pulsed light. Furthermore, we propose MOPA for the efficient characterization of cluster states, enabling real-time monitoring of all cluster links. This work completes the set of capabilities needed for robust multimode squeezed light detection and significantly expands its applicability in quantum technologies.