Abstract
Fibre transport of multi-dimensional photonic quantum states promises high information capacity per photon without space restriction. This work experimentally demonstrates transmission of spatial ququarts through multi-core optical fibres and measurement of the entanglement between two fibres with quantum state analyzers, each composed of a spatial light modulator and a single-mode fibre. Quantum state tomography reconstructs the four-dimension entangled state that verifies the nonlocality through concurrences in two-dimensional subspaces, a lower bound of four-dimensional concurrence and a Bell-type CGLMP inequality.
Highlights
Multi-core fibres (MCFs) have been developed for high-power fibre laser amplifiers based on phase-locked beam combinations[19] and for space-division-multiplexing optical communications[20, 21]
Because the distances between the core images are greater than the transverse shift of photons inside the PPLN crystal, only four combinations between the identical-index cores i = 1, 2, 3, 4 of both MCF1 and MCF2 yield non-zero overlap integrals that lead to photon pair generation[26]
The post-selected quantum state propagating through the MCFs can be expressed as a four-dimensional entangled state: Ψ = a 1 1 1 2 + b 2 1 2 2 + c 3 1 3 2 + d 4 1 4 2, (1)
Summary
Multi-core fibres (MCFs) have been developed for high-power fibre laser amplifiers based on phase-locked beam combinations[19] and for space-division-multiplexing optical communications[20, 21]. Quantum communication through an MCF line based on a faint laser source has recently been reported[22, 23]. After transmission through the MCFs, each ququart (d = 4) is analyzed through a spatial light modulator (SLM) and spatial-mode filtering by a single-mode fibre (SMF). The two-ququart entanglement is verified by two-qubit concurrences in six pairwise two-mode subspaces, a lower bound of multi-dimensional bipartite concurrence[24] and a generalized Bell inequality for high dimensions[25], calculated from the result of quantum state tomography composed of 256 projection measurements
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