# A Review of Super-Resolution Imaging through Optical High-Order Interference [Invited]
## 5 Super-Resolution via Quantum Antibunching Effect  
Single-photon emitters have also been employed to realize the SQL super-resolution imaging. In 2012, [PERSON] and [PERSON] proposed that the nonclassical emission of fluorescent markers can be employed for realizing super-resolution imaging [36]. For the fluorophores used in bioimaging, photons are emitted one by one, leading to the antibunching effect [55; 56]. By taking advantage of the antibunching effect, measurement of the nth-order antibunching signal gives rise to the enhanced resolution scaling as \(\sqrt{n}\). Physically, the SQL super-resolution imaging through antibunching is resulted from the lack of the in-position n-photon constructive interference signal, in contrast to the case through classical bunching effect that is resulted from the excess of the in-position n-photon constructive interference signal. Similar to the bunching induced SQL imaging, the antibunching induced SQL imaging generates an incoherent image of the object.  
The antibunching induced super-resolution imaging has been realized in a standard wide-field epi-fluorescence microscope [37], as shown in Figure 5I. The non-Poisson emitter is CdSe/CdS/ZnS colloidal quantum dot excited by a pulse laser at 532 nm, and emits light photon with a wavelength at 617 nm. An EMCCD has been used for measuring the high-order correlation function [57] with a repetition rate at 1 kHz. By introducing two-photon and three-photon coincidence measurement, the fluorescence imaging resolution is increased by a factor of 1.3 and 1.5 as shown in Figure 5 II-b and 5 II-c, respectively.  
With antibunching emitters, [PERSON] et al. have demonstrated super-resolution fluorescence imaging in a confocal microscope [38]. The sample to be imaged is a diamond with clusters of NV centers, which are excited by a focused laser beam at 532 nm and emit fluorescent light within  
Figure 5: (I): Schematic of the experimental setup for realizing super-resolution image based on antibunching effect of quantum single-photon emitters. The sample to be imaged consists of many independent single-photon emitters. Multi-photon coincidence counts are measured with an EMCCD camera. (II): Super-resolution images measured with second-order (b) and third-order (c) antibunching effect, which clearly show the resolution enhancement in comparison with the conventional intensity image (a). Scale bars in (a–c): 1 \(\upmu\)m. (d–f) are zoom-in images of the zones marked by red box in (a–c), while (g) shows the one-dimensional plot of the cross-sections marked by the solid line in (d–f). Scale bars in (d–f): 400 nm. The figure is reproduced with permissions from Ref. [37] of ACS.  
640-800 nm. n-photon coincident measurement is taken with several single-photon detectors operating in Geiger mode [58], as shown in Figure 6I. By scanning the sample, SQL super-resolution imaging is obtained with two-photon and three-photon coincidence measurement as shown in Figure 6 II. Moreover, with the prior knowledge of the number of the quantum emitters \(k_{0}\), better resolution can be reached by further calculating the higher-order cumulant function with the hitherto known Glauber higher-order (kth-order) correlation function \(g^{(k)}=0\) when \(k>k_{0}\).