Abstract

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 interferometry 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.

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