**Significance Statement**

Second-order interference phenomenon analyzed by Hanbury Brown and Twiss results in advanced understanding of coherence and interference. Interestingly, the in-depth debate raised by Hanbury Brown and Twiss interferometry resulted to the inception of quantum optics in view of its promising applications, intriguing analysis on multi-photon interference, quantum information processing, and metrology.

In the argument, second-order interference can be observed when light from a single chaotic source is detected on two sensors and correlation measurements done while changing time delay or relative position between the two separate detectors, i.e. temporal and spatial second-order interferences, respectively. However, the two detectors are observed not to retrieve any first order interference. Notwithstanding, interference is observed at second-order only if the spatial separation and time delay remain within the coherence area and coherence time of the light source, respectively.

Researchers led by Professor Vincenzo Tamma demonstrated that pure second-order interference between pairs of disjoint optical paths emitted from a single chaotic source, first demonstrated in the temporal domain (V. Tamma and J. Seiler, New J. Phys. 18, 032002 (2016)), could be seen in the spatial domain. This unique spatial interference exhibits potential application for remote objects sensing. Their work is published in __Optics Express____.__

The authors considered an optical interferometer in which light from a single chaotic source, after being split in a beam splitter, propagated through two double-pinhole masks positioned in output channels of the beam splitter. The gap between the pinholes in each mask is such that the detectors positioned behind each mask could detect no first-order interference.

They considered the correlation between the photon number fluctuations at selected transverse positions of the detectors. This way, they were able to predict a spatial second-order interference, even with the pinholes in each mask separated by a gap larger than the transverse coherence length of the source. Interference occurs between two pairs of disjoint optical paths that are characterized by the two pairs of remote pinholes.

The information regarding the spatial structure as well as the relative position of the masks is encoded within the phase between the two pairs of interfering paths, irrespective of the distance between the source and the two masks. The authors demonstrated that this information could be retrieved in appropriate experimental scenarios. The precision of the measurement could be enhanced by altering a few experimental parameters instead of raising light frequency. This work has recently triggered the first experimental realization of this sensing technique for the characterization of two remote double pinholes (M. D’Angelo, A. Mazzilli, F.V. Pepe, A. Garuccio, and V. Tamma, Sci. Rep. 7, 2247 (2017)).

The team indicated how the proposed interference phenomenon can be also implemented to simulate quantum logic operations, even the Controlled-NOT gate, by following the proposal in V. Tamma and J. Seiler, New J. Phys. 18, 032002 (2016). These results have been recently verified experimentally (T. Peng, V. Tamma, and Y.H. Shih, Sci. Rep. 6, 30152 (2016)).

The contributions from any possible pair of paths of the thermal light field components to the two detectors lead effectively to the interference between two pairs of disjoint but correlated paths passing through a single pinhole per mask. In particular, the transverse gap between the pinholes in each pair is smaller as compared to the transverse coherence distance of the light source. However, this is not the case with the other possible path pairs, which cannot therefore contribute to the interference. This phenomenon is quite different from other second-order interference occurrence from multiple chaotic sources. Therefore, their work provided an insight of the physics of multi-path interference as well as spatial coherence.

The results of their study also indicated this spatial interference effect has potential applications for remote objects sensing without first-order coherence.

**Reference**

Michele Cassano, Milena D’Angelo, Augusto Garuccio, Tao Peng, Yanhua Shih, and Vincenzo Tamma.** Spatial interference between pairs of disjoint optical paths with a single chaotic source. **Vol. 25, No. 6 | 20 Mar 2017 | OPTICS EXPRESS 6590.