  We demonstrate a probabilistic entangling quantum gate between two distant
trapped ytterbium ions. The gate is implemented between the hyperfine "clock"
state atomic qubits and mediated by the interference of two emitted photons
carrying frequency encoded qubits. Heralded by the coincidence detection of
these two photons, the gate has an average fidelity of 90+-2%. This entangling
gate together with single qubit operations is sufficient to generate large
entangled cluster states for scalable quantum computing.
