We demonstrate precise control of the coupling of each of two trapped ions to the mode of an optical resonator. When both ions are coupled with near-maximum strength, we generate ion--ion entanglement heralded by the detection of two orthogonally pol
arized cavity photons. The entanglement fidelity with respect to the Bell state $Psi^+$ reaches $F geq (91.9pm2.5)%$. This result represents an important step toward distributed quantum computing with cavities linking remote atom-based registers.
A quantum network requires information transfer between distant quantum computers, which would enable distributed quantum information processing and quantum communication. One model for such a network is based on the probabilistic measurement of two
photons, each entangled with a distant atom or atomic ensemble, where the atoms represent quantum computing nodes. A second, deterministic model transfers information directly from a first atom onto a cavity photon, which carries it over an optical channel to a second atom; a prototype with neutral atoms has recently been demonstrated. In both cases, the central challenge is to find an efficient transfer process that preserves the coherence of the quantum state. Here, following the second scheme, we map the quantum state of a single ion onto a single photon within an optical cavity. Using an ion allows us to prepare the initial quantum state in a deterministic way, while the cavity enables high-efficiency photon generation. The mapping process is time-independent, allowing us to characterize the interplay between efficiency and fidelity. As the techniques for coherent manipulation and storage of multiple ions at a single quantum node are well established, this process offers a promising route toward networks between ion-based quantum computers.
Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states. 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 ions, neutral atoms, atomic ensembles, and nitrogen-vacancy spins. 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, these paths transition parameters determine the phase and amplitude of the final entangled state, unless the memory is initially prepared in a superposition state, a step that requires coherent control. Here we report the fully tunable entanglement of 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 presents itself as a promising method for distributing information within quantum networks.
We demonstrate several building blocks for an ion-photon interface based on a trapped Ca ion in an optical cavity. We identify a favorable experimental configuration and measure system parameters, including relative motion of the trapped ion and the
resonator mode. A complete spectrum of cavity-assisted Raman transitions between the $4^{2}S_{1/2}$ and $3^{2}D_{5/2}$ manifolds is obtained. On two of these transitions, we generate orthogonally polarized cavity photons, and we demonstrate coherent manipulation of the corresponding pair of atomic states. Possible implementations of atom-photon entanglement and state mapping within the ion-cavity system are discussed.
