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In this article we present a simple repeater scheme based on the negatively-charged nitrogen vacancy centre in diamond (NV-). Each repeater node is built from simple modules comprising an optical cavity containing a single NV-, with one nuclear spin from 15N as quantum memory. The operation in the module only uses deterministic processes and interactions and achieves high fidelity (>99%) operation, and modules are connected by optical fiber. In the repeater node architecture, the processes between modules by photons can be in principle deterministic, however current limitations on optical components lead to the processes to be probabilistic but heralded. The most resource modest repeater architecture contains at least two modules at each node, and the repeater nodes are than connected by telecom wavelength entangled photon pairs. We discuss the performance of quantum repeaters starting from the minimum-resource strategy with several modules (~10) and then incorporating more resource-intense strategies step by step. Our architecture enables large-scale quantum information networks with existing technology.

Dephasing -- phase randomization of a quantum superposition state -- is a major obstacle for the realization of high fidelity quantum logic operations. Here, we implement a two-qubit Controlled-NOT gate using dynamical decoupling (DD), despite the gate time being more than one order of magnitude longer than the intrinsic coherence time of the system. For realizing this universal conditional quantum gate, we have devised a concatenated DD sequence that ensures robustness against imperfections of DD pulses that otherwise may destroy quantum information or interfere with gate dynamics. We compare its performance with three other types of DD sequences. These experiments are carried out using a well-controlled prototype quantum system -- trapped atomic ions coupled by an effective spin-spin interaction. The scheme for protecting conditional quantum gates demonstrated here is applicable to other physical systems, such as nitrogen vacancy centers, solid state nuclear magnetic resonance, and circuit quantum electrodynamics., Comment: 6 pages, 5 figures. Replaced with published version (wording changed in some places, added references)

We report on the experimental investigation of an individual pseudomolecule using trapped ions with adjustable magnetically induced J-type coupling between spin states. Resonances of individual spins are well separated and are addressed with high fidelity. Quantum gates are carried out using microwave radiation in the presence of thermal excitation of the pseudomolecule's vibrations. Demonstrating Controlled-NOT gates between non-nearest neighbors serves as a proof-of-principle of a quantum bus employing a spin chain. Combining advantageous features of nuclear magnetic resonance experiments and trapped ions, respectively, opens up a new avenue towards scalable quantum information processing., Comment: replaced with published version, 6 pages, 4 figures