Observation of Entanglement Between a Single Trapped Atom and a Single Photon.

An outstanding goal in quantum information science is the faithful mapping of quantum information between a stable quantum memory and a reliable quantum communication channel 1 . This would allow, for example, quantum communication over remote distances 2 , quantum teleportation 3 of matter and distributed quantum computing over a ‘quantum internet’. Because quantum states cannot in general be copied, quantum information can only be distributed in these and other applications by entangling the quantum memory with the communication channel. Here we report quantum entanglement between an ideal quantum memory—represented by a single trapped 111 Cd + ion—and an ideal quantum communication channel, provided by a single photon that is emitted spontaneously from the ion. Appropriate coincidence measurements between the quantum states of the photon polarization and the trapped ion memory are used to verify their entanglement directly. Our direct observation of entanglement between stationary and ‘flying’ qubits 4 is accomplished without using cavity quantum electrodynamic techniques 5 , 6 , 7 or prepared non-classical light sources 3 . We envision that this source of entanglement may be used for a variety of quantum communication protocols 2 , 8 and for seeding large-scale entangled states of trapped ion qubits for scalable quantum computing 9 .