We distributed four-dimensional time-bin entangled photons over 100-km optical fiber. We reconstructed the quantum density operator by utilizing cascaded Mach-Zehnder interferometers, and obtained an average fidelity of 0.935 ± 0.015.
High-dimensional quantum entanglement can enrich the functionality of quantum information processing. For example, it can enhance the channel capacity for linear optic superdense coding and decrease the error rate threshold of quantum key distribution. Long-distance distribution of a high-dimensional entanglement is essential for such advanced quantum communications over a communications network. Here, we show a long-distance distribution of a four-dimensional entanglement. We employ time-bin entanglement, which is suitable for a fibre transmission, and implement scalable measurements for the high-dimensional entanglement using cascaded Mach-Zehnder interferometers. We observe that a pair of time-bin entangled photons has more than 1 bit of secure information capacity over 100 km. Our work constitutes an important step towards secure and dense quantum communications in a large Hilbert space.
We describe an experimental implementation to generate and measure high-dimensional time-bin-entangled qudits. Two-photon time-bin entanglement is generated via spontaneous four-wave mixing in single-mode fiber. Unbalanced Mach-Zehnder interferometers transform selected time bins to polarization entanglement, allowing standard polarization-projective measurements to be used for complete quantum state tomographic reconstruction. Here we generate maximally entangled qubits (d=2), qutrits (d=3), and ququarts (d=4), as well as other phase-modulated nonmaximally entangled qubits and qutrits. We reconstruct and verify all generated states using maximum-likelihood estimation tomography.
We describe our recent experiment on the distribution of time-bin entangled photon pairs over 300 km of optical fiber, which set a record for long-distance distribution of quantum information.This experiment was achieved by using a high-speed and high signal-to-noise ratio entanglement generation/evaluation setup consisting of periodically poled lithium niobate waveguides and superconducting singlephoton detectors.We observed the violation of Bell's inequality and thus confirmed that the entangled photon pairs preserved the quantum correlation after they were separated by 300 km of fiber.