Ultra-cold atoms trapped by light, with their inherent quantum coherence and controllability, provide an attractive system for quantum information and for the simulation of complex problems in condensed matter physics. Quantum information processing requires that individual qubits be manipulated and that they be deterministically entangled with one another, a process usually accomplished by controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress toward this goal by demonstrating entanglement among an ensemble of atoms confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between an isolated pair of atoms. We have used an optical lattice of double-well potentials to isolate and manipulate pairs of atoms, inducing controlled entangling interactions within each pair. Our experiment is the first realization of proposals to use controlled exchange coupling in a system of neutral atoms. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, wavefunction exchange symmetry required for these identical bosonic particles leads to distinct state-dependent dynamics. We observe repeated exchange of spin between atoms occupying different vibrational levels with a coherence time exceeding several milliseconds. This observation represents the first demonstration of the essential component of a quantum SWAP gate in neutral atoms, whose partial implementation, the sqrt(SWAP), is a universal gate for quantum computation.
Pub Type: Journals
cold atoms, optical lattice, quantum computing