Quantum communication requires the transfer of quantum states1, or quantum bits of information (qubits), from one place to another. From a fundamental perspective, this allows the distribution of entanglement and the demonstration of quantum non-locality over significant distances2,3,4,5,6. Within the context of applications, quantum cryptography offers a provably secure way to establish a confidential key between distant partners7. Photons represent the natural flying qubit carriers for quantum communication, and the presence of telecommunications optical fibres makes the wavelengths of 1,310 nm and 1,550 nm particularly suitable for distribution over long distances. However, qubits encoded into alkaline atoms that absorb and emit at wavelengths around 800 nm have been considered for the storage and processing of quantum information8,9. Hence, future quantum information networks made of telecommunications channels and alkaline memories will require interfaces that enable qubit transfers between these useful wavelengths, while preserving quantum coherence and entanglement9,10,11. Here we report a demonstration of qubit transfer between photons of wavelength 1,310 nm and 710 nm. The mechanism is a nonlinear up-conversion process, with a success probability of greater than 5 per cent. In the event of a successful qubit transfer, we observe strong two-photon interference between the 710 nm photon and a third photon at 1,550 nm, initially entangled with the 1,310 nm photon, although they never directly interacted. The corresponding fidelity is higher than 98 per cent.
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We thank D.B. Ostrowsky for discussions. Financial support by the Swiss Nation Center for Quantum Photonics and the European IST project RamboQ is acknowledged. S.T. acknowledges financial support from the European Science Foundation programme ‘Quantum Information Theory and Quantum Computation’.
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