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arXiv:1303.5343 · Physical Review Letters

Entanglement’s Benefit Survives an Entanglement-Breaking Channel

Zheshen Zhang, Maria Tengner, Tian Zhong, Franco N. C. Wong, Jeffrey H. Shapiro

Published Jul 1, 2013• 147 citations
arXivHTMLPDF

Abstract

Entanglement is essential to many quantum information applications, but it is easily destroyed by quantum decoherence arising from interaction with the environment. We report the first experimental demonstration of an entanglement-based protocol that is resilient to loss and noise which destroy entanglement. Specifically, despite channel noise 8.3 dB beyond the threshold for entanglement breaking, eavesdropping-immune communication is achieved between Alice and Bob when an entangled source is used, but no such immunity is obtainable when their source is classical. The results prove that entanglement can be utilized beneficially in lossy and noisy situations, i.e., in practical scenarios.

DOI
10.1103/physrevlett.111.010501

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Abstract: We report the first experimental demonstration of an entanglement-based secure communication protocol that is resilient to entanglement-breaking loss and noise on the communication channel. Passive eavesdropping immunity is demonstrated in bit-error rate measurements and Alice’s information advantage over Eve is estimated. Entanglement is a key ingredient to many quantum information applications, but quantum decoherence arising from loss and noise easily destroys entanglement. Quantum illumination (QI) [1]–[3] is a novel concept of utilizing entanglement in an entanglement-breaking lossy and noisy environment to achieve a substantial performance benefit. First proposed for enhancing the signalto-noise ratio for detecting a weakly-reflecting target in the presence of strong background noise [1], quantum illumination was later shown, theoretically, to enable high data-rate classical communication that is immune to passive eavesdropping [3]. In this work we report the first experimental demonstration of the QI communication protocol [4], showing a strong performance benefit of using bosonic entanglement over an entanglement-breaking channel and QI’s passive-eavesdropping immunity. Quantum illumination achieves a substantial benefit from its initial entanglement, despite the roundtrip lossy, noisy propagation channel’s having left the retained and returned light
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Quantum illumination is a paradigm for using entanglement to gain a performance advantage—in comparison with classical‐state systems of the same optical power—over lossy, noisy channels that destroy entanglement. Previous work has shown how it can be used to defeat passive eavesdropping on a two‐way Alice‐to‐Bob‐to‐Alice communication protocol, in which the eavesdropper, Eve, merely listens to Alice and Bob’s transmissions. This paper extends that analysis to consider how Alice and Bob can minimize their vulnerability to Eve’s doing active eavesdropping, i.e., when she injects her own light into the channel.
A quantum communication network can be constructed by distributing a multipartite entangled state to space-separated nodes. Entangled optical beams with highest flying speed and measurable brightness can be used as carriers to convey information in quantum communication networks. Losses and noises existing in real communication channels will reduce or even totally destroy entanglement. The phenomenon of disentanglement will result in the complete failure of quantum communication. Here, we present the experimental demonstrations on the disentanglement and the entanglement revival of tripartite entangled optical beams used in a quantum network. We experimentally demonstrate that symmetric tripartite entangled optical beams are robust in pure lossy but noiseless channels. In a noisy channel, the excess noise will lead to the disentanglement and the destroyed entanglement can be revived by the use of a correlated noisy channel (non-Markovian environment). The presented results provide useful technical references for establishing quantum networks.
The inevitable interaction between quantum systems and environment induces effects of decoherence which may be so strong as to destroy any initial entanglement between the systems, a phenomenon known as "entanglement breaking". Here we show the simplest examples where the combination of two entanglement-breaking channels into a joint correlated-noise environment reactivates the distribution of entanglement, with classes of entangled states which are perfectly transmitted from a middle station (Charlie) to two remote stations (Alice and Bob). Surprisingly, this reactivation is induced by the presence of purely-classical correlations in the joint environment, whose state is separable with zero discord. This paradoxical effect is proven for quantum systems with Hilbert spaces of any dimension, both finite and infinite.
Entanglement underpins a variety of quantum-enhanced communication, sensing, and computing capabilities. Entanglement-assisted communication (EACOMM) leverages entanglement preshared by communicating parties to boost the rate of classical information transmission. Pioneering theory works showed that EACOMM can enable a communication rate well beyond the ultimate classical capacity of optical communications, but an experimental demonstration of any EACOMM advantage remains elusive. In this Letter we report the implementation of EACOMM surpassing the classical capacity over lossy and noisy bosonic channels. We construct a high-efficiency entanglement source and a phase-conjugate quantum receiver to reap the benefit of preshared entanglement, despite entanglement being broken by channel loss and noise. We show that EACOMM beats the Holevo-Schumacher-Westmoreland capacity of classical communication by up to 16.3%, when both protocols are subject to the same power constraint at the transmitter. As a practical performance benchmark, we implement a classical communication protocol with the identical characteristics for the encoded signal, showing that EACOMM can reduce the bit-error rate by up to 69% over the same bosonic channel. Our work opens a route to provable quantum advantages in a wide range of quantum information processing tasks.