Quantum Ghost Image Identification with Correlated Photon Pairs

Ghost imaging can be performed using either quantum or classical states of light that possess strong spatial correlations. In both cases, the image is formed by averaging over many optical events. Here we show that it is possible to distinguish an object from a preestablished basis set of objects by using a small number of position-correlated photon pairs produced by spontaneous parametric down-conversion. The signal photon is incident on one member of a set of spatially nonoverlapping objects. The “ghost” image information is impressed upon the spatially separated idler photon and is extracted by means of holographic filtering and coincidence detection. We were able to distinguish among sets of two and four spatially nonoverlapping objects with confidence levels higher than 87% and 81%, respectively. This method of ghost imaging can be performed in situations requiring extremely low light levels.

Figure 1

Experimental setup. DM is a dichroic mirror for blocking the pump laser; IF is an interference filter with 10 nm bandwidth, centered at 727.6 nm. The dotted lines indicate the imaging process for a point object.

Figure 2

Image-identification results for the two-object case. (a) Data for each object-detector combination are normalized by the maximum coincidence count for the corresponding object. (b) $T/A$ ratio is calculated by dividing the total coincidences by the accidental coincidences for each object-detector combination.

Figure 3

(a) and (b) Graphs of image-identification results for the four-object case. (c) The four spatially nonoverlapping objects used in our experiment.