Biphoton Quantum State Tomography and Spin-orbit Conversion in C+L Telecom Bands

Abstract

High-dimensional quantum states offer the promise of enhancing information capacity in quantum communications. Towards that goal, we here characterize a fiber-based system with the generation and transmission of biphotons in the telecom C+L bands and capable of converting photons with low-dimensional spin subspace to a high-dimensional orbital angular momentum (OAM) subspace (from -5 to +5 topological charge). We experimentally confirm that Bell's inequality is violated with a CHSH value of 2.43. The reconstructed density matrix through polarization quantum state tomography (QST) indicates a fidelity of 92 %, a purity of 86 % and concurrence of 84 % are achieved. The spin (polarization) to OAM conversion through spatial light modulator (SLM) is shown to yield a conversion efficiency of 41.5 %. This work is a step towards achieving multidimensional entanglement and high-dimensional quantum key distribution at standard fiber telecom wavelengths.