The secure transmission of an image can be accomplished by encoding the image information, securely communicating this information, and subsequently reconstructing the image. As an alternative, here we show how the image itself can be directly transmitted while ensuring that the presence of any eavesdropper is revealed in a way akin to quantum key distribution. We achieve this transmission using a photon-pair source with the deliberate addition of a thermal light source as background noise. One photon of the pair illuminates the object, which is masked from an eavesdropper by adding indistinguishable thermal photons, the other photon of the pair acts as a time reference from which the intended recipient can preferentially detect the image carrying photons. These reference photons are themselves made sensitive to the presence of an eavesdropper by traditional polarization-based QKD encoding. Interestingly, the security encoding is performed in the two-dimensional polarization-basis, but the image information is encoded in a much higher-dimensional, hence information-rich, pixel basis. In our example implementation, our images have more than 100 independent pixels. Beyond the secure transmission of images, our approach to the distribution of secure high-dimensional information may create new high-bandwidth approaches to traditional QKD.
A limitation of free-space optical communications is the ease with which the information can be intercepted. Overcoming this limitation is possible by hiding the information within the background optical noise that is present in all real-world situations. We demonstrate the limitations of our experimental system for transferring images over free-space using a photon-pair source emitting two correlated beams. The system uses spontaneous parametric down-conversion to create photon-pairs, where one photon contains the spatial information and the other the heralding information.
We present a method of using a high-flux entangled photon-pair source to improve the signal-to-noise ratio of a single-pixel imaging system. Sensing with single-photon counting detectors will often suffer from measurement noise due to any background light levels. Using a single detector enables a high efficiency of detection and when paired with a variable transmission mask enables full images to be captured. The heralding photon from the source acts as a temporal reference, allowing the signal photons to be distinguished from background noise. This heralding method is key to understanding how quantum measurements can produce higher contrast images than their classical equivalent.
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