# QUANTUM NETWORK Résumé de rapport

Project ID:
509487

Financé au titre de:
FP6-MOBILITY

Pays:
Germany

## Final Activity Report Summary - QUANTUM NETWORK (Theoretical studies on linear optics quantum network with atoms and photons and its experimental realization)

The project aimed at the objectives of theoretical studies on the feasible realisation and the potential applications of quantum network with linear optics and atomic ensembles. Particularly, the project achieves the following results:

1. We proposed an efficient, fault-tolerant long-distance quantum communication architecture with atomic ensembles and linear optics (as well as an alternative scheme). The architecture is based on robust two-photon interference which is about 10^8 times more stable than single-photon interference used in the scheme proposed by L.-M. Duan et al. [Nature (London) 414, 413 (2001)]. Incorporating several significant recent advances on atomic-ensemble-based techniques our scheme faithfully implements quantum repeater and thus enables a realistic avenue for relevant experiments with many photons.

2. The above result enables us to further show that atomic-ensemble-based quantum network is feasible by using the atomic-ensemble quantum memory of photonic polarization qubits and linear optics. Especially, one can create any graph states (e.g. two-dimensional cluster states) with many atomic ensembles and linear optics to enable one-way quantum computing.

3. We have made other relevant progresses on quantum information processing with atoms and linear optics:

1) We proposed a novel scheme for creating atomic entanglement with integrated atom optics.

2) We proposed the first deterministic and efficient quantum-key-distribution protocol with high-dimensional entanglement and linear optics.

3) We reported the first experimental test of a two-photon all-versus-nothing violation of local realism by using the so-called hyper-entanglement and linear optics.

To summarise, we believe the execution of the project is successful. The main message of our project is that one can exploit the advantages of both linear optics manipulating photons with very high accuracy and atoms / atomic ensembles as quantum memory. At the interface of atoms and photons, we can integrate the existing linear-optical quantum protocols and atomic-ensemble-based schemes on photon storage and high-efficiency photon counting into a single unit. This would allow us to realize all ingredients required for long-distance quantum communication.

The robust entanglement creation mechanism we proposed during the project can be used to entangle a complex multi-party quantum network for efficient one-way quantum computing with cluster states. Such an atomic-ensemble-based quantum network provides an exciting perspective for manipulating a large number of photons for applications ranging from multiparty quantum information protocols to fundamental quantum experiments.

1. We proposed an efficient, fault-tolerant long-distance quantum communication architecture with atomic ensembles and linear optics (as well as an alternative scheme). The architecture is based on robust two-photon interference which is about 10^8 times more stable than single-photon interference used in the scheme proposed by L.-M. Duan et al. [Nature (London) 414, 413 (2001)]. Incorporating several significant recent advances on atomic-ensemble-based techniques our scheme faithfully implements quantum repeater and thus enables a realistic avenue for relevant experiments with many photons.

2. The above result enables us to further show that atomic-ensemble-based quantum network is feasible by using the atomic-ensemble quantum memory of photonic polarization qubits and linear optics. Especially, one can create any graph states (e.g. two-dimensional cluster states) with many atomic ensembles and linear optics to enable one-way quantum computing.

3. We have made other relevant progresses on quantum information processing with atoms and linear optics:

1) We proposed a novel scheme for creating atomic entanglement with integrated atom optics.

2) We proposed the first deterministic and efficient quantum-key-distribution protocol with high-dimensional entanglement and linear optics.

3) We reported the first experimental test of a two-photon all-versus-nothing violation of local realism by using the so-called hyper-entanglement and linear optics.

To summarise, we believe the execution of the project is successful. The main message of our project is that one can exploit the advantages of both linear optics manipulating photons with very high accuracy and atoms / atomic ensembles as quantum memory. At the interface of atoms and photons, we can integrate the existing linear-optical quantum protocols and atomic-ensemble-based schemes on photon storage and high-efficiency photon counting into a single unit. This would allow us to realize all ingredients required for long-distance quantum communication.

The robust entanglement creation mechanism we proposed during the project can be used to entangle a complex multi-party quantum network for efficient one-way quantum computing with cluster states. Such an atomic-ensemble-based quantum network provides an exciting perspective for manipulating a large number of photons for applications ranging from multiparty quantum information protocols to fundamental quantum experiments.