A single ion and two photons: A programmable three-qubit interface

We demonstrate a three-qubit protocol involving a single ion and two photons, whose functionality can be programmed to serve as a quantum repeater [1] node for sending entangled photons or for receiving photons for entanglement swapping [2] . Moreover, the same protocol serves as complete (read-in – read-out) quantum memory operation. The protocol is based on the sequential absorption of two 854-nm photons resulting in the emission of two 393-nm photons [3] , [4] from a single trapped ion and subsequent state projection of the ion. Depending on the mode of operation of the protocol, a quantum gate is implemented between three qubits, namely, either the ion and the two emitted 393-nm photons, the ion and the two absorbed 854-nm photons or the ion and one absorbed 854-nm and one emitted 393-nm photon.