  To advance quantum information science a constant pursuit is the search for
physical systems that meet the stringent requirements for creating and
preserving quantum entanglement. In atomic physics, robust two-qubit
entanglement is typically achieved by strong, long-range interactions in the
form of Coulomb interactions between ions or dipolar interactions between
Rydberg atoms. While these interactions allow fast gates, atoms subject to
these interactions must overcome the associated coupling to the environment and
cross-talk among qubits. Local interactions, such as those requiring
significant wavefunction overlap, can alleviate these detrimental effects yet
present a new challenge: To distribute entanglement, qubits must be
transported, merged for interaction, and then isolated for storage and
subsequent operations. Here we show how, via a mobile optical tweezer, it is
possible to prepare and locally entangle two ultracold neutral atoms, and then
separate them while preserving their entanglement. While ground-state neutral
atom experiments have measured dynamics consistent with spin entanglement, and
detected entanglement with macroscopic observables, we are now able to
demonstrate position-resolved two-particle coherence via application of a local
gradient and parity measurements; this new entanglement-verification protocol
could be applied to arbitrary spin-entangled states of spatially-separated
atoms. The local entangling operation is achieved via ultracold spin-exchange
interactions, and quantum tunneling is used to combine and separate atoms. Our
toolset provides a framework for dynamically entangling remote qubits via local
operations within a large-scale quantum register.
