Towards fault-tolerant quantum computing with trapped ions

Today, ion traps are among the most promising physical systems for constructing a quantum device harnessing the computing power inherent in the laws of quantum physics(1,2). For the implementation of arbitrary operations, a quantum computer requires a universal set of quantum logic gates. As in classical models of computation, quantum error correction techniques(3,4) enable rectification of small imperfections in gate operations, thus enabling perfect computation in the presence of noise. For fault-tolerant computation(5), it is believed that error thresholds ranging between 10(-4) and 10(-2) will be required-depending on the noisemodel and the computational overhead for realizing the quantum gates(6-8)-but so far all experimental implementations have fallen short of these requirements. Here, we report on a Molmer-Sorensen-type gate operation(9,10) entangling ions with a fidelity of 99.3(1)%. The gate is carried out on a pair of qubits encoded in two trapped calcium ions using an amplitude-modulated laser beam interacting with both ions at the same time. A robust gate operation, mapping separable states onto maximally entangled states is achieved by adiabatically switching the laser-ion coupling on and off. We analyse the performance of a single gate and concatenations of up to 21 gate operations.