Violation of local realism by spatially separated quantum optical modes plays an essential role in implementing various quantum information protocols including quantum cryptography. In this talk, I will demonstrate the non-local features of a vacuum-single-photon excitation of a pair of spatially separated modes by robustly violating the Clauser-Horne inequality via weak field homodyne measurements. We prove that it is important to exploit the particle and wave aspects of the single-photon excitation as there exists a local hidden variable model for the phase-sensitive constant field homodyning. I will also describe how to quantify genuine multimode quantum entanglement in the continuous variable system by exploring the geometry of the state space for both Gaussian and non-Gaussian states. I will finally talk about the advantage of using entangled signal-idler modes in detecting a light-absorbing unknown target of low reflectivity, which is embedded in a noisy environment.