Quantum Imaging

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Imaging is an important technological tool in many disciplines, such as biomedical research, nanotechnology, and basic physics research. However, due to the wave nature of light there are limits in resolution and contrast that can be achieved in classical imaging techniques. On the other hand, quantum entanglement may offer an improvement over these classical limits, leading to the subject of quantum imaging. The best known quantum imaging protocols are two-photon microscopy and spectroscopy, quantum holography, quantum lithography, and quantum illumination. There are two reasons why it is important to understand the precise distinction between classical and quantum imaging. First, it will help identify new methods for improved imaging, potentially leading to new technology. Second, it will reveal a fundamental aspect of physics that has hitherto remained elusive, namely what makes quantum optics more powerful in imaging than classical optics. The obvious answer to this question, i.e., quantum entanglement, has already been proved false to some extent. While entanglement is probably necessary, it is certainly not sufficient. This is reminiscent of quantum computing, where it was shown that entanglement is necessary but not sufficient for obtaining the promised exponential speed-up over classical computing.The first problem we encounter when we try to understand the difference between classical and quantum imaging is the lack of an operational quantitative measure of the imaging quality. We therefore need a quantitative measure for practical imaging protocols that provides a well-defined threshold between the classical and the quantum regime. Secondly, what exactly is the role of entanglement in quantum imaging? Furthermore, is it possible to derive fundamental bounds on quantum imaging with respect to the imaging quality measure? Thirdly, once a fundamental limit on the objective imaging measure has been found, an obvious question is what procedures saturate this bound.