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.

Few-photon routing via chiral light-matter couplings

Quantum router is one of the essential elements in the quantum network. Conventional routers only direct a single photon from one quantum channel into another. Here, we proposed a few-photon router. The active element of the router is a single qubit chirally coupled to two independent waveguides simultaneously, where each waveguide mode provides a quantum channel. By introducing the operators of the scatter-free space and the controllable space, the output state of the one-photon and two-photon scattering are derived analytically. It is found that the qubit can direct one and two photons from one port of the incident waveguide to an arbitrarily selected port of the other waveguide with unity, respectively. However, two photons cannot be simultaneously routed to the same port due to the anti-bunch effect.

Unconditionally Secure Relativistic Quantum Qubit Commitment

Quantum qubit commitment is a stronger version of the quantum bit commitment. It is impossible to realize unconditionally secure quantum qubit commitment in nonrelativistic domain. In this paper, we propose an unconditionally secure relativistic quantum qubit commitment protocol for the first time, which will have some unique applications in the upcoming era of quantum network.

Nonlocality of a type of multi-star-shaped quantum networks

The correlations in quantum networks have attracted strong interest due to the fact that linear Bell inequalities derived from one source are useless for characterizing multipartite correlations of general quantum networks. In this paper, { a type of multi-star-shaped quantum networks are introduced and discussed. Such a network consists of three-grade nodes: the first grade is named party (node) $A$, the second one consists of $m$ nodes marked $B^1,B^2,\ldots,B^m$, which are stars of $A$ and the third one consists of $m^2$ nodes $C^j_k (j,k=1,2,\ldots,m)$, where $C^j_k (k=1,2,\ldots,m)$ are stars of $B^j$. We call such a network a $3$-grade $m$-star quantum network and denoted by $SQN(3,m)$, being as a natural extension of bilocal networks and star-shaped networks.} We introduce and discussed the locality and strong locality of a $SQN(3,m)$ and derive the related nonlinear Bell inequalities, called $(3,m)$-locality inequalities and $(3,m)$-strong locality inequalities. To compare with the bipartite locality of quantum states, we define the separability of $SQN(3,m)$ that imply the locality and then locality of $SQN(3,m)$. When all of the shared states of the network are pure ones, we prove that $SQN(3,m)$ is nonlocal if and only if it is entangled.

Photon routing based on non-chiral interaction between atoms and waveguides

The photon router plays an essential role in the optical quantum network. However, conventional routers generally couple photons chirally into waveguides to achieve complete transmission from the input port to the required port. Here, we use non-chiral photon-atom interactions for targeted routing. The system consists of two V-type three-level atoms and two parallel waveguides. In addition, the two atoms are driven by external coherent fields, respectively. With a real-space Hamiltonian, the probability of photon transmitted to four ports can be obtained. The study shows that a single photon input from the left port of the waveguide-a can be deterministically transferred to any of the four ports of the two waveguides by adjusting the detuning of the atom and the driving field on the atom, as well as the distance between the two atoms.