Sensing with undetected photons enables the measurement of absorption and phase shifts at wavelengths different from those detected. Here, we experimentally map the balance and loss parameter space in a non-degenerate nonlinear interferometer, showing the recovery of sensitivity despite internal losses at the detection wavelength. We further explore an interaction-free operation mode with a detector-to-sample incident optical power ratio of >200. This allows changes in attowatt levels of power at 3.4 $\mu$m wavelength to be detected at 1550 nm, immune to the level of thermal black-body background. This reveals an ultra-sensitive infrared imaging methodology capable of probing samples effectively `in the dark'.
Categories: quant-ph physics.optics