Quantum imaging can beat classical resolution limits, imposed by the diffraction of light. In particular, it is known that one can reduce the image blurring and increase the achievable resolution by illuminating an object by entangled light and measuring coincidences of photons. If an n-photon entangled state is used and the nth-order correlation function is measured, the point-spread function (PSF) effectively becomes \(\sqrt{n}\) times narrower relatively to classical coherent imaging. Quite surprisingly, measuring n-photon correlations is not the best choice if an n-photon entangled state is available. We show that for measuring (n − 1)-photon coincidences (thus, ignoring one of the available photons), PSF can be made even narrower. This observation paves a way for a strong conditional resolution enhancement by registering one of the photons outside the imaging area. We analyze the conditions necessary for the resolution increase and propose a practical scheme, suitable for observation and exploitation of the effect.

量子成像可以超越由光的衍射强加的经典分辨率限制。特别是，众所周知，可以通过用纠缠光照射物体并测量光子的重合来减少图像模糊并提高可实现的分辨率。如果使用n光子纠缠态并测量n阶相关函数，则点扩展函数 (PSF) 有效地变得比经典相干成像窄\(\sqrt{n}\)倍。相当令人惊讶的是，如果n光子纠缠态可用，则测量n光子相关性并不是最佳选择。我们证明对于测量 ( n − 1)-光子巧合（因此，忽略可用光子之一），PSF 可以做得更窄。这一观察结果为通过记录成像区域外的一个光子来增强条件分辨率铺平了道路。我们分析了分辨率提高的必要条件，并提出了一个实用的方案，适合观察和开发效果。