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Robust long-distance quantum communication with atomic ensembles and linear optics

Bo Zhao, Zeng‐Bing Chen, Yu-Ao Chen, Jörg Schmiedmayer, Jian-Wei Pan

Published Sep 20, 2006• 6 citations
OpenAlex

Abstract

Quantum communication deals with absolutely secure transfer of classical messages by means of quantum cryptography or faithful transfer of unknown quantum states by means of quantum teleportation between distant sites. The essential element for quantum communication is to create remote entangled pairs of high fidelity. The main difficulty to create such a distant entangled pair is that both photon loss and decoherence grow exponentially with distance. However, with the help of quantum repeater, it is demonstrated that both the entanglement sources and the time needed to create a single distant entangled pair of high fidelity could scale polynomially with the communication length. Here we propose a robust quantum repeater by extending the original Duan-Lukin-Cirac-Zoller (DLCZ) scheme based on atomic ensembles and linear optics. Our protocol entails the advantage of two photon interference, which is more robust than single photon interference used in the DLCZ scheme. In comparison with our previous proposal, the entangled memory qubits are manipulated across long distance, and local entanglement pairs are avoided. Our proposal provides an exciting possibility for realistic long distance quantum communication.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 23, 2005•1 citation
Quantum communication deals with absolutely secure transfer of classical messages by means of quantum cryptography or faithful transfer of unknown quantum states by means of quantum teleportation between distant sites. The essential element for quantum communication is to create remote entangled pairs of high fidelity. The main difficulty to create such a distant entangled pair is that both photon loss and decoherence grow exponentially with distance. However, with the help of quantum repeater, it is demonstrated that both the entanglement sources and the time needed to create a single distant entangled pair of high fidelity could scale polynomially with the communication length. Here we propose a robust quantum repeater by extending the original Duan-Lukin-Cirac-Zoller (DLCZ) scheme based on atomic ensembles and linear optics. Our protocol entails the advantage of two photon interference, which is more robust than single photon interference used in the DLCZ scheme. In comparison with our previous proposal, the entangled memory qubits are manipulated across long distance, and local entanglement pairs are avoided. Our proposal provides an exciting possibility for realistic long distance quantum communication.
In this Letter we propose a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)10.1038/35106500]. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance stability requirements by about 7 orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.
We present a detailed analysis of a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L.M. Duan et al. Nature (London) 414, 413 (2001)]. The architecture is based on two-photon Hong-Ou-Mandel-type interference which relaxes the long-distance interferometric stability requirements by about seven orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons, thereby removing the weakest point in the DLCZ scheme. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.
The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for long-distance quantum communication with atomic ensembles is analyzed. Its fidelity and throughput in entanglement distribution, entanglement swapping, and quantum teleportation is derived within a framework that accounts for multiple excitations in the ensembles as well as loss and asymmetries in the channel. The DLCZ performance metrics that are obtained are compared to the corresponding results for the trapped-atom quantum communication architecture that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT and NU). Both systems are found to be capable of high-fidelity entanglement distribution. However, the DLCZ scheme only provides conditional teleportation and repeater operation, whereas the MIT-NU architecture affords full Bell-state measurements on its trapped atoms. Moreover, it is shown that achieving unity conditional fidelity in DLCZ teleportation and repeater operation requires ideal photon-number resolving detectors. The maximum conditional fidelities for DLCZ teleportation and repeater operation that can be realized with nonresolving detectors are 1/2 and 2/3, respectively.
Quantum repeaters create long-distance entanglement between quantum systems while overcoming difficulties such as the attenuation of single photons in a fiber. Recently, an implementation of a repeater protocol based on single qubits in atomic ensembles and linear optics has been proposed [Duan et al., Nature (London) 414, 413 (2001)]. Motivated by rapid experimental progress towards implementing that protocol, here we develop a more efficient scheme compatible with active purification of arbitrary errors. Using similar resources as the earlier protocol, our approach intrinsically purifies leakage out of the logical subspace and all errors within the logical subspace, leading to greatly improved performance in the presence of experimental inefficiencies. Our analysis indicates that our scheme could generate approximately one pair per 3min3\phantom{\rule{0.3em}{0ex}}\mathrm{min}3min over 1280km1280\phantom{\rule{0.3em}{0ex}}\mathrm{km}1280km distance with fidelity $(F\ensuremath{\geqslant}78%)$ sufficient to violate Bell's inequality.
Quantum repeaters promise to deliver long-distance entanglement overcoming loss in realistic quantum channels. A promising class of repeaters, based on atomic ensemble quantum memories and linear optics, follows the proposal by L.-M. Duan et al., Nature (London) 414, 413 (2001). Here we analyze this protocol in terms of a very general model for the quantum memories employed. We derive analytical expressions for scaling of entanglement with memory imperfections, dark counts, loss, and distance, and we apply our results to two specific quantum memory protocols. Our methods apply to any quantum memory with an interaction Hamiltonian at most quadratic in the mode operators and are in principle extendible to more recent modifications of the original proposal of Duan, Lukin, Cirac, and Zoller.
The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for entangling distant atomic ensembles is addressed and analyzed. Its performance, in terms of fidelity and throughput, is compared to that of the quantum communication architecture using trapped rubidium-atom quantum memories that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT/NU). It is shown that the DLCZ protocol for entanglement distribution achieves a better throughput versus distance behavior than does the MIT/NU architecture, with both being capable of high entanglement fidelities. The DLCZ scheme also admits to a conditional teleportation scheme based on its entangled atomic ensembles, whereas the MIT/NU architecture affords unconditional teleportation based on its trapped-atom quantum memories. It is shown that achieving unity fidelity in DLCZ teleportation requires photon-number resolving detectors; the maximum teleportation fidelity that can be realized with non-resolving detectors is 1/2.