  Qubit state detection is an important part of a quantum computation. As
number of qubits in a quantum register increases, it is necessary to maintain
high fidelity detection to accurately measure the multi-qubit state. Here we
present experimental demonstration of high-fidelity detection of a multi-qubit
trapped ion register with average single qubit detection error of 4.2(1.5) ppm
and a 4-qubit state detection error of 17(2) ppm, limited by the decay lifetime
of the qubit, using a novel single-photon-sensitive camera with fast data
collection, excellent temporal and spatial resolution, and low instrumental
crosstalk.
