![]() Interestingly, time-bin encoding 1, 18, 19, – although widely used for single-mode optical fibers 19, 20, 21, 22, 23 – has only been recently demonstrated for free-space channels 24, 25, 26, 27, 28, 29, 30 due to the problem of atmospheric turbulence and scattering. Another approach to encode free-space channels is the use of higher-order spatial modes, recently utilized for intracity quantum key distribution 15, yet these photon states are directly impacted by wavefront distortion and are expected to completely vanish upon random scattering from a surface 16, 17. A previous study 14 showed that the observed polarization visibility depends on the scattering surface material, and even the best material (cinematic silver screen) showed a strong dependence on the photon scattering angles with only a total angle of <45° was suitable for quantum communications. ![]() When photons are scattered, however, their polarization states are inherently disturbed, and the quantum encoding is degraded. Furthermore, the photon coherence recovered from scattered light could be utilized to improve noise performance in low-light and 3D imaging, non-line-of-sight imaging 10, 11, velocity measurement 12, light detection and ranging (LIDAR), surface characterization, or biomedical sample identification.Ĭurrently, the predominant photon encoding used on free-space quantum channels is polarization, because it is not impacted by turbulent atmosphere for clear line-of-sight transmission 13. For instance, quantum communication capable of operating over a scattering channel could accommodate free space communication with non-line-of-sight between multiple users such as indoors around corners, or with short range links with moving systems. The ability to transfer quantum coherence via scattering surfaces and its successful recovery from scattered photons enhances several applications of quantum technologies. Quantum coherence is a key ingredient in many fundamental tests and applications of quantum mechanics including quantum communication 1, characterization of single-photon sources 2, generation of non-classical states 3, quantum metrology 4, quantum teleportation 5, quantum fingerprinting 6, quantum cloning 7, demonstrating quantum optical phenomena 8, and quantum computing 9 etc. We believe our method will instigate new lines for research and development on applying photon coherence from scattered signals to quantum sensing, imaging, and communication in free-space environments. Firstly, using scattered photons as an indirect channel for quantum communication thereby enabling non-line-of-sight quantum communication with background suppression, and secondly, using the combined arrival time and quantum coherence to enhance the contrast of low-light imaging and laser ranging under high background light. Using our method, we demonstrate the viability of two novel applications. The observed time-bin visibility for scattered photons remained at a high 95% over a wide scattering angle range of −45 0 to +45 0, while the individual pixels in the detector array resolve or track an image in its field of view of ca. ![]() Here we demonstrate a novel approach to transfer and recover quantum coherence from scattered, non-line-of-sight photons analyzed in a multimode and imaging interferometer for time-bins, combined with photon detection based on a 8 × 8 single-photon-detector-array. Quantum channels in free-space, an essential prerequisite for fundamental tests of quantum mechanics and quantum technologies in open space, have so far been based on direct line-of-sight because the predominant approaches for photon-encoding, including polarization and spatial modes, are not compatible with randomly scattered photons. ![]()
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