A. Stute, B. Casabone, P. Schindler, T. Monz, P. O. Schmidt, B. Brandstätter, T. E. Northup & R. Blatt
Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states1, 2. A quantum interface can be realized by entangling the state of a single photon with the state of an atomic or solid-state quantum memory, as demonstrated in recent experiments with trapped ions3, 4, neutral atoms5, 6, atomic ensembles7, 8 and nitrogen-vacancy spins9. The entangling interaction couples an initial quantum memory state to two possible light–matter states, and the atomic level structure of the memory determines the available coupling paths. In previous work, the transition parameters of these paths determined the phase and amplitude of the final entangled state, unless the memory was initially prepared in a superposition state4 (a step that requires coherent control). Here we report fully tunable entanglement between a single 40Ca+ ion and the polarization state of a single photon within an optical resonator. Our method, based on a bichromatic, cavity-mediated Raman transition, allows us to select two coupling paths and adjust their relative phase and amplitude. The cavity setting enables intrinsically deterministic, high-fidelity generation of any two-qubit entangled state. This approach is applicable to a broad range of candidate systems and thus is a promising method for distributing information within quantum networks.
Reference: Nature 485, 482-485 (24 May 2012)