All-fiber source and sorter for multimode correlated photons

Photons occupying multiple spatial modes hold a great promise for implementing high-dimensional quantum communication. We use spontaneous four-wave mixing to generate multimode photon pairs in a few-mode fiber. We show the photons are correlated in the fiber mode basis using an all-fiber mode sorter. Our demonstration offers an essential building block for realizing high-dimensional quantum protocols based on standard, commercially available fibers, in an all-fiber configuration.

Entangling single atoms over 33 km telecom fibre

Heralded entanglement between distant quantum memories is the key resource for quantum networks. Based on quantum repeater protocols, these networks will facilitate efficient large-scale quantum communication and distributed quantum computing. However, despite vast efforts, long-distance fibre based network links have not been realized yet. Here we present results demonstrating heralded entanglement between two independent, remote single-atom quantum memories generated over fibre links with a total length up to 33 km. To overcome the attenuation losses in the long optical fibres of photons initially emitted by the Rubidium quantum memories, we employ polarization-preserving quantum frequency conversion to the low loss telecom band. The presented work represents a milestone towards the realization of efficient quantum network links.

Twin-field quantum key distribution over free-space optical channels in the presence of atmospheric turbulence

Future quantum communication networks envisaged on a global scale will include various networks interlinked via optical fiber and free space channels. In recent years, quantum key distribution (QKD) protocol based on optical fiber has been extensively studied. Twin-field QKD (TF-QKD) may enable 550 km QKD using standard optical fiber without quantum repeaters. However, the performance of TF-QKD in free-space channel is still unclear. In this paper, a free-space channel model is proposed with specific turbulence characterization discussed. Here, the key rate of TF-QKD under multiple scenarios considering the variation of turbulence and different link configuration is investigated. Simulation results demonstrate that the performance of free-space TF-QKD is related to link configuration and turbulence motion which is determined by surface feature, time and height. Furthermore, TF-QKD protocol is a potential scheme for the free-space quantum communication.

Informationally restricted correlations: a general framework for classical and quantum systems

We introduce new methods and tools to study and characterise classical and quantum correlations emerging from prepare-and-measure experiments with informationally restricted communication. We consider the most general kind of informationally restricted correlations, namely the ones formed when the sender is allowed to prepare statistical mixtures of mixed states, showing that contrary to what happens in Bell nonlocality, mixed states can outperform pure ones. We then leverage these tools to derive device-independent witnesses of the information content of quantum communication, witnesses for different quantum information resources, and demonstrate that these methods can be used to develop a new avenue for semi-device independent random number generators.

Teleportation affected by fluctuating electromagnetic field with a boundary

We study the quantum teleportation under fluctuating electromagnetic field in the presence of a perfectly reflecting boundary. The noisy scheme of quantum teleportation affected by electromagnetic fluctuation is proposed. Then we calculate and investigate the behaviors of entanglement and fidelity, which are closely related to the plane boundary and atomic polarization. After a period of evolution, entanglement and fidelity evolve to zero and nonzero stable value respectively. Fidelity is closely related to the weight parameter and phase parameter of the teleported state. Besides, small two-atom separation makes entanglement and fidelity have better enhancement. Furthermore, the presence of boundary, atomic polarization and two-atom separation offers us more freedom to adjust the performance of the quantum teleportation. The results would give us new insight into quantum communication in an open quantum system since quantum teleportation plays an important role in quantum communication and quantum information.