The non-local Hong-Ou-Mandel effect

TL;DR

This paper demonstrates a non-local version of the Hong-Ou-Mandel effect using a four-path interferometer, revealing new insights into multiphoton interference and entanglement with potential applications in quantum optics.

Contribution

It introduces a novel non-local interference scheme that links multiphoton interference with entanglement, expanding the understanding of linear optics in quantum information.

Findings

Non-local HOM effect observed in spatially separated photons.

Local phase shifts reveal non-classical correlations.

Establishes a fundamental relation between interference and entanglement.

Abstract

Two-photon interference effects arise because photons are indistinguishable particles. In the wellknown Hong-Ou-Mandel (HOM) effect, the transmission of two photons at a beam splitter interferes destructively with the reflection of both photons, requiring both photons to "bunch up" by leaving the beam splitter on the same side. Here, we show that the interference between locally propagating photons and photons exchanged by a mode swap can be implemented by post-selecting spatially separated photon outputs of a four-path interferometer. Even though the photons detected at spatially separated locations must have travelled along paths that never met up at the same beam splitter, the Hong-Ou-Mandel effect can be observed in correlations between the output ports that originate from the association of detection events with non-local output modes defined by the two single photon inputs. Local phase shifts can be used to map out non-classical correlations between the photons detected at different output locations, clarifying the role of linear optics in generating entanglement between spatially separated photons. Our work thus establishes a fundamental relation between multiphoton interference and entanglement, opening the door to new possibilities in optical quantum technologies.