Abstract
AbstractQuantum 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-and-resend 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.
Highlights
Quantum 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[3,4,5] or point-to-multipoint links[6]
Efficient BB84 protocol,[27] in the limit of infinitely many signals shared by the users, the fraction R of secure bits that can be extracted from each detected photon is[28]
By replacing the quantum bit error rates QZ and QX in the Z and X basis by those measured of 4.6% and 15.4% in the {H, V} and {D, A} bases, respectively, we find that 0.111 secure key bits can be distilled from our Relay per detected photon
Summary
Quantum 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[3,4,5] 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 where photons are sent by two users to be projected into a Bell state by an intermediate quantum node.[10,11,12] 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. 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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