Deterministic entanglement and tomography of ion–spin qubits

We have implemented a universal quantum logic gate between qubits stored in the spin state of a pair of trapped 40Ca ions. An initial product state was driven to a maximally entangled state deterministically, with 83% fidelity. We present a general approach to quantum state tomography which achieves good robustness to experimental noise and drift, and use it to measure the spin state of the ions. We find the entanglement of formation is 0.54.