The first demonstrations of ghost imaging were based on the quantum nature of light. Specifically, quantum correlations between photon pairs were utilized to build up an image of the unseen object. When one of the photons strikes the object, the other follows a different path to the camera's lens. If the camera is constructed to only record pixels from photons that hit simultaneously at the object and the camera's image plane, an image of the object is reconstructed.
It was soon realized that the correlations between the light beam that hits the camera and the beam that hits the object can be purely classical. If quantum correlations are present, the signal-to-noise ratio of the reconstructed image can be improved. The exact role of quantum and classical correlations in ghost imaging is still controversial.
In 2009 'pseudothermal ghost imaging' and 'ghost diffraction' were demonstrated using only a single single-pixel detector. This was achieved by implementing the 'Computational ghost-imaging' scheme, relaxing the need to evoke quantum correlations arguments for the pseudothermal source case.
Recently, it was shown that the principles of 'Compressed-Sensing' can be directly utilized to reduce the number of measurements required for image reconstruction in GI. This allowed to acquire an N pixel image with much less than N measurements and may have applications in LIDAR and microscopy
A simple example clarifies the basic principle of (classical) ghost imaging. Take two transparent boxes; the left box is empty and the right box has an object in it. A camera is behind the left box, and just a single-pixel detector is behind the right box.
Align a laser and beamsplitter so that the laser light always passes through the same part of both boxes. For example, if one beam from the beamsplitter passes through the bottom-center of one box, then the other beam from the beamsplitter should pass through the bottom-center of the other box.
Now, scan the laser around the box. (It does not need to be raster-scanned; it can be randomly moved around.) But, set up "gating" for the camera on the left, so that the camera only accepts light when the single-pixel detector on the right has a signal.
When the light beam in the right box is blocked by the object, it does not reach the single-pixel detector, so the corresponding light beam in the left box will not contribute to the camera's image. When the light beam in the right box misses the object, and passes through to the single-pixel detector, then the light beam in the left box will contribute to the camera's image.
In this way, the camera will show an image of the object, even though the light going towards the camera passed through the empty left box, and did not touch the object in the right box.
In this example, the boxes are illuminated one pixel at a time. But the correct image will also form if more complex light distributions are used.
- 'Ghost Imaging with a Single Detector' by Y.Bromberg, O.Katz and Y.Silberberg
- 'Computational Ghost Imaging' by J.Shapiro
- 'Compressive Ghost Imaging' by O.Katz, Y.Bromberg and Y.Silberberg
- Ryan S. Bennink, Sean J. Bentley, and Robert W. Boyd (2002). Physical Review Letters 89: 113601. doi:10.1103/PhysRevLett.89.113601.
- Quantum camera snaps objects it cannot 'see' by Belle Dume, New Scientist, 2 May 2008. Accessed July 2008
- Air Force Demonstrates 'Ghost Imaging' By Sharon Weinberger, Wired, 3 June 2008. Accessed July 2008
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