Passive optical time-of-flight for Non line-of-sight localization
Optical imaging through diffusive, visually-opaque barriers, and around corners is an important challenge in many fields, ranging from defense to medical applications. Recently, novel techniques that combine time-of-flight (TOF) measurements with computational reconstruction, have allowed breakthrough imaging and tracking of objects hidden from view. These light detection and ranging (LiDAR)-based approaches, however, require active short-pulsed illumination and ultrafast time-resolved detection. Here, bringing notions from passive RADAR and passive geophysical mapping approaches, we present an optical TOF technique that allows to passively localize light sources and reflective objects through diffusive barriers and around corners. Our approach retrieves TOF information from temporal cross-correlations of scattered light, providing temporal resolution that surpasses the state-of-the-art ultrafast detectors by three orders of magnitude. We demonstrate passive localization of multiple white-light sources and reflective objects hidden from view, using a simple setup, with interesting potential for covert imaging.