Long-distance quantum communication in a decoherence-free subspace

英敏 熊谷

Hidetoshi Kumagai

Jian-Wei Pan, Yu-Ao Chen, Qiang Zhang

We present several experimental progresses for advanced quantum communication based on multi-photon entanglement. Quantum secret sharing (QSS) and third-man quantum cryptograph (TQC) shows the advancement in multi-party quantum communication. Quantum error rejection (QER) holds the promise for error-free transfer in long-distance quantum communication. Fault-tolerant quantum cryptography in a decoherence-free subspace (DFS) and quantum-relay-assisted key distribution respectively overcome the difficulties caused by decoherence and photon loss, the two main crucial problems in long-distance quantum communication.

Hans J. Briegel

U. Dorner, Alexander Klein, Dieter Jaksch

We study a quantum repeater which is based on decoherence free quantum gates recently proposed by Klein {\it et al.} [Phys. Rev. A, {\bf 73}, 012332 (2006)]. A number of operations on the decoherence free subspace in this scheme makes use of an ancilla qubit, which undergoes dephasing and thus introduces decoherence to the system. We examine how this decoherence affects entanglement swapping and purification as well as the performance of a quantum repeater. We compare the decoherence free quantum repeater with a quantum repeater based on qubits that are subject to decoherence and show that it outperforms the latter when decoherence due to long waiting times of conventional qubits becomes significant. Thus, a quantum repeater based on decoherence free subspaces is a possibility to greatly improve quantum communication over long or even intercontinental distances.

Michael J. Zwerger, B. P. Lanyon, Tracy E. Northup +3 more

Abstract Quantum repeaters provide an efficient solution to distribute Bell pairs over arbitrarily long distances. While scalable architectures are demanding regarding the number of qubits that need to be controlled, here we present a quantum repeater scheme aiming to extend the range of present day quantum communications that could be implemented in the near future with trapped ions in cavities. We focus on an architecture where ion-photon entangled states are created locally and subsequently processed with linear optics to create elementary links of ion-ion entangled states. These links are then used to distribute entangled pairs over long distances using successive entanglement swapping operations performed using deterministic ion-ion gates. We show how this architecture can be implemented while encoding the qubits in a decoherence-free subspace to protect them against collective dephasing. This results in a protocol that can be used to violate a Bell inequality over distances of about 800 km assuming state-of-the-art parameters. We discuss how this could be improved to several thousand kilometres in future setups.