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Lecture notes in physics

Imaging Processes and Coherence in Physics

M. Schlenker, Mathias Fink, J.-P. Goedgebuer, C. Malgrange, Jean-Charles Viénot, Richard H. Wade

Published Jan 1, 1980• 130 citations
Publisher
DOI
10.1007/3-540-09727-9

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Interest in induced coherence [1] has had a resurgence thanks to its demonstration in imaging with undetected photons [2], enabling infrared imaging capability to be achieved without the need for infrared camera technology. More recent implementations of undetected sensing have instead made use of nonlinear interferometers [3], [4], which have allowed simpler experimental designs to achieve similar results. Here we will discuss the differences between the two modes undetected sensing. The primary difference in experimental implementation is that in induced coherence the signals from the two nonlinear processes are mixed on an external beamsplitter, while all fields are instead recombined at the second crystal in a nonlinear interferometer. We have built an experimental setup and accompanying theoretical model, capable of switching between induced coherence and nonlinear inter-ferometry to fully test the fundamental differences in the modalities and the underlying physics. The switch is achieved by altering the distinguishability of the two nonlinear processes both inside and outside of the interferometer. Distinguishability is created and subsequently destroyed through polarisation control of the down-converted signal fields (in a mode akin to a quantum eraser). This allows a simple wave-plate to switch the system between nonlinear interferometer and induced coherence. The mathematical model enables us to fully map out the polarisation space, as well as including the effects of spectral width and coherence lengths. Experimental data shows a very close match to the outcomes predicted by the model, confirming the same underlying principle effect. These tests form the first direct comparison of these interferometric modes; we also further demonstrate the capability to detect both induced coherence and nonlinear interferometry simultaneously - as shown modelled in Fig. 1-opening the pathway to novel imaging modalities as well as further tests of the underlying fundamental physics.
Ghost imaging is an imaging technique in which the image of an object is revealed only in the correlation measurement between two beams of light, whereas the individual measurements contain no imaging information. Normally, the resolution of the image, which even exceeds the Rayleigh limit, is shown to be related to the transverse coherent length (lc) of the speckle pattern. In this Letter, we demonstrate experimentally that the speckle size can be compressed by a coherent population trapping (CPT) process in atom vapor, and the resolution of GI can be greatly enhanced by the CPT process. The technique we exploit is quite efficient and robust, and it may be useful in the field of quantum and classical two-photon imaging, all-optical image processing, and quantum communication.