Deterministic entanglement swapping with an ion-trap quantum computer

Entang lement—once only a subject of disputes about the foundation of quantum mechanics—has today become an essential issue in the emerging field of quantum information processing, promising a number of applications, including secure communication, teleportation and powerful quantum computation. Therefore, a focus of current experimental work in the field of quantum information is the creation and manipulation of entangled quantum systems. Here, we present our results on entangling two qubits in an ion-trap quantum processor not through a direct interaction of the ion qubits but instead through the action of a protocol known as entanglement swapping 1 . Our ion-trap system enables us to implement all steps of the entanglement swapping protocol in a fully deterministic way. Thus, two ion qubits can be prepared on demand in a welldefined entangled state. This particular feature may facilitate the implementation of quantum repeaters 2 or aid in distributing entangled states in ion-trap quantum computers 3 . Entanglement among quantum systems usually results from a common origin of the systems or a direct local interaction between the systems. In both cases, the entangling operation requires the systems to be at the same point in space at a certain time. In entanglement swapping, however, the entangling operation is achieved as follows: starting from two entangled pairs, A1A2 and B1B2, a joint measurement of A2 and B2 in the Bell basis projects pair A1B1 in an entangled state even though A1 and B1 might be far apart. This can be used for applications such as quantum communication or teleportation, which require that quantum systems at distant locations are entangled. Entanglement swapping facilitates the provision of such separated pairs of entangled systems, as two quantum systems in two distinct locations can be entangled through a chain of intermediary entangled pairs. Furthermore, if entanglement swapping is combined with entanglement purification, as proposed in quantum repeater schemes 2 , the final entangled state can have a higher fidelity than is achievable with a direct transfer of an entangled system. Up to now, entanglement swapping has been implemented with photon pairs 4,5 and with ion‐photon entangled states 6 . These implementations are appealing as the photons can be