Imaging with quantum states of light

The production of pairs of entangled photons simply by focusing a laser beam onto a crystal with a nonlinear optical response was used to test quantum mechanics and to open new approaches in imaging. The development of the latter was enabled by the emergence of single-photon-sensitive cameras that are able to characterize spatial correlations and high-dimensional entanglement. Thereby, new techniques emerged, such as ghost imaging of objects — in which the quantum correlations between photons reveal the image from photons that have never interacted with the object — or imaging with undetected photons by using nonlinear interferometers. In addition, quantum approaches in imaging can also lead to an improvement in the performance of conventional imaging systems. These improvements can be obtained by means of image contrast, resolution enhancement that exceeds the classical limit and acquisition of sub-shot-noise phase or amplitude images. In this Review, we discuss the application of quantum states of light for advanced imaging techniques.Using quantum states of light for imaging both reveals quantum phenomena and enables new protocols that result in images that surpass classical limitations. Such systems require both quantum light sources and often the ingenious use of detector technologies.Key pointsImprovements in available camera technologies have enabled the efficient detection and characterization of quantum behaviours in continuous spatial variables.The use of cameras in the context of quantum optics allows the detection and use of high-dimensional quantum states.Quantum states of light can be harnessed to implement quantum imaging protocols that allow improved imaging over classical techniques; such protocols can lead to improved estimation of the transmission, reflectance and phase of an imaged object, in addition to offering improved resolution images of the object.Quantum imaging techniques allow new types of imaging, such as ghost imaging, quantum imaging with undetected photons or the implementation of interaction-free measurements in the context of imaging.Sources of pairs of photons with different wavelengths allow the lack of high-fidelity detectors at exotic wavelengths to be overcome through ghost imaging techniques and quantum nonlinear interferometric imaging techniques.