Rydberg-blockade controlled-not gate and entanglement in a two-dimensional array of neutral-atom qubits

We present experimental results on two-qubit Rydberg-blockade quantum gates and entanglement in a two-dimensional qubit array. Without postselection against atom loss we achieve a Bell state fidelity of $0.73\pm 0.05$. The experiments are performed in an array of single Cs atom qubits with a site to site spacing of $3.8\mu \mathrm{m}$. Using the standard protocol for a Rydberg-blockade $C_Z$ gate together with single qubit operations we create Bell states and measure their fidelity using parity oscillations. We analyze the role of ac Stark shifts that occur when using two-photon Rydberg excitation and show how to tune experimental conditions for optimal gate fidelity.