Ghost imaging with entangled photon pairs in three experiments

Short answer: Ghost imaging forms an image from photons that never touched the object, by correlating them with partner photons recorded elsewhere. Three experiments cover compressive ghost imaging with entangled photons in 2011, ghost imaging with paired x-ray photons in 2018, and ghost imaging built on entanglement swapping in 2019. All are laboratory demonstrations.

What is ghost imaging?

The 2019 npj Quantum Information abstract describes it this way: traditional ghost imaging requires correlated but spatially separated photons, and has been observed in many physical situations spanning both the quantum and classical regimes. The image comes from correlations between two light paths, with only one path interacting with the object.

What did the 2011 compressive experiment show?

The 2011 Physical Review A abstract reports high-resolution compressive ghost imaging at the single-photon level, using entangled photons from a spontaneous parametric down-conversion source and single-pixel detectors. For a given mean-squared error, the number of photons needed to reconstruct a two-dimensional image was found to be much smaller than in quantum ghost imaging experiments that use a raster scan. The authors say this shortens the data acquisition time and suggests a more economical use of photons for low-light-level and quantum image formation. The abstract gives no photon counts.

What did the x-ray experiment show?

The 2018 Physical Review A paper reports the experimental observation of ghost imaging with paired x-ray photons generated by parametric downconversion. The authors use the one-to-one relation between photon energies and emission angles, and the anticorrelation between the k-vectors of the signal and idler photons, to reconstruct images of slits with nominally zero background levels. They say a further extension of the procedure can be used to observe various quantum phenomena at x-ray wavelengths. That is a stated prospect and no such observation is reported in the abstract.

What did the entanglement-swapping experiment show?

The 2019 paper observes ghost imaging in a new system based on entanglement swapping, which the authors call the key feature of a quantum network. They detail how the exact form of quantum interference between independent photons sets the nature of the ghost imaging. For an anti-symmetric projection, the recorded image is the contrast-reversed version of the object, so that where the object is bright the image is dark, and vice versa. They say the results highlight the importance of state projection in the process and provide a pathway for teleportation of two-dimensional spatial states across a quantum network. They also say ghost images with new properties could be achieved in conventional settings through new signal processing procedures.

How to read this

The three experiments use different photon sources and wavelengths, so the results answer different questions. This post is based on abstracts and visible page text, and full texts were not checked. Any comparison of image quality would need the papers. It makes no claim about products or imaging systems in use.

FAQ

Do the photons that form the image touch the object?

In ghost imaging the image is built from correlations with photons that did not interact with the object, per the 2019 abstract's description of correlated, spatially separated photons.

What did compressive sensing change?

The 2011 authors report that far fewer photons were needed for a given mean-squared error than with raster scanning.

Can it work with x-rays?

The 2018 paper reports ghost imaging with paired x-ray photons of slit images.

What is contrast reversal?

In the entanglement-swapping experiment, an anti-symmetric projection gave an image where bright and dark were swapped.