Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ''droplet-epitaxy'' of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ∼900 nm, and metal-organic vapor-phase epitaxy on InP emitting at ∼1550 nm, with mean FSS 4× smaller than for Stranski-Krastanow QDs.

Quantum cryptography allows confidential information to be communicated between two parties, with secrecy guaranteed by the laws of nature alone. However, upholding guaranteed secrecy over networks poses a further challenge, as classical receive-andresend routing nodes can only be used conditional of trust by the communicating parties, which arguably diminishes the value of the underlying quantum cryptography. Quantum relays offer a potential solution by teleporting qubits from a sender to a receiver, without demanding additional trust from end users. Here we demonstrate the operation of a quantum relay over 1 km of optical fibre, which teleports a sequence of photonic quantum bits to a receiver by utilising entangled photons emitted by a semiconductor light-emitting diode. The average relay fidelity of the link is 0.90 ± 0.03, exceeding the classical bound of 0.75 for the set of states used, and sufficiently high to allow error correction. The fundamentally low multiphoton emission statistics and the integration potential of the source present an appealing platform for future quantum networks. INTRODUCTIONQuantum key distribution 1,2 systems based on weak-coherent optical pulses have been reported that allow unique cryptographic keys to be shared between directly connected users on point-to-point 3-5 or point-to-multipoint links 6 . To establish fully quantum multipartite networks, that avoid trusting intermediate parties, 7 it is necessary to route quantum signals through a backbone of quantum nodes. 8 This can be achieved by leveraging quantum entanglement to set-up non-local correlations between measurements by end users. Examples of such schemes are distribution of entangled photon pairs to end users, where local measurements are performed, 9 or conversely, where photons are sent by two users to be projected into a Bell state by an intermediate quantum node. 10-12 Photonic quantum repeaters 13 and relays 8 use both of these effects to teleport entangled or single qubits, respectively, in a manner that can be chained to create a fully quantum network for which theoretically proven quantum security can be preserved.Here we report operation of a quantum relay over 1 km of optical fibre using entangled photons generated by a lightemitting diode to teleport photonic qubits encoded on weak coherent pulses emitted by a laser. Compared with previously reported quantum relays 14 and photonic teleportation over significant distances, 15,16 our system is directly electrically driven using a simple semiconductor device, offering a route to largescale network deployments. Teleporting weak coherent states offers potential enhancements to state-of-the art quantum key distribution systems, as it creates output photons with sub-Poissonian statistics immune to the photon number-splitting attack, 17,18 and protects against intrusions. 19

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