Two-qubit entangling gates between distant atomic qubits in a lattice

Abstract: Arrays of qubits encoded in the ground-state manifold of neutral atoms trapped in optical (or magnetic) lattices appear to be a promising platform for the realization of a scalable quantum computer. Two-qubit conditional gates between nearest-neighbor qubits in the array can be implemented by exploiting the Rydberg blockade mechanism, as was shown by D. Jaksch et al. [Phys. Rev. Lett. 85, 2208 (2000)]. However, the energy shift due to dipole-dipole interactions causing the blockade falls off rapidly with the interatomic distance and protocols based on direct Rydberg blockade typically fail to operate between atoms separated by more than one lattice site. In this work, we propose an extension of the protocol of Jaksch \emph{et al.}\ for controlled-Z and controlled-NOT gates which works in the general case where the qubits are not nearest-neighbor in the array. Our proposal relies on the Rydberg excitation hopping along a chain of ancilla non-coding atoms connecting the qubits on which the gate is to be applied. The dependence of the gate fidelity on the number of ancilla atoms, the blockade strength and the decay rates of the Rydberg states is investigated. A comparison between our implementation of distant controlled-NOT gate and one based on a sequence of nearest-neighbor two-qubit gates is also provided.