Entanglement and cluster state quantum computing with distant trapped ions
David Moehring
University of Michigan, Ann Arbor, US.
Abstract: Entanglement, which is at the base of all quantum computing algorithms, can now routinely be established through the collective motion of nearby trapped atomic ions. However, entangling remotely-located ions remains a challenge. One possible realization requires the interference of two single photons emitted by the ions, where the quantum information is conveyed through the polarization or frequency of the photons. Towards this end, we have entangled single trapped ion qubits with either type of photonic qubit. The spontaneously-emitted photons show near-perfect anti-bunching, demonstrating that from a single laser pulse at most one photon is scattered by the atom. We have also demonstrated the second order quantum interference of single photons simultaneously emitted from two ions confined in an rf Paul trap. In free space we achieve a visibility of about 60%. Improving the visibility of the two-photon interference (using single-mode fibers) and combining it with the readily available methods of state detection of a trapped ion may allow two ions to be entangled without involving their motion. While this method of remote-ion entanglement is probabilistic, it has been shown that together with local deterministic quantum gates, it is sufficient for scalable quantum computation. Further, with a combination of the quantum repeater and the cluster state approaches, efficient quantum computation is still possible even if all of the entangling quantum gates only succeed with an arbitrarily small probability.