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Quantum Communication and Quantum Networking

Matter-Matter Entanglement for Quantum Communication

Julien Laurat

Published Oct 11, 2012• 0 citations
OpenAlex

Abstract

The distribution of entanglement between different parties enables quantum communication protocols, such as quantum key distribution or teleportation. Furthermore, control of entanglement between material systems is an essential capability for scalable architectures. This talk will focus on the experimental gen-eration of such entanglement in different setups, in the regime of single photons or in the regime of continuous variables. The first part will review experiments done recently in the single excitation regime. First, by following the seminal paper of Duan, Lukin, Cirac and Zoller (DLCZ) [1], entanglement between single collective excitations stored in two re-mote atomic ensembles can be generated. In the DLCZ protocol, entanglement is created in a probabilistic but heralded way from quantum interference in the measurement process. The detection of a photon from one or the other atomic ensemble in an indistinguishable fashion results in an entangled state with one collective spin excitation shared coherently [2]. This entanglement has been used for the initial implementation of functional quantum nodes for polarization en-

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The distribution of entanglement between different parties enables quantum communication protocols, such as quantum key distribution or teleportation. Furthermore, control of entanglement between material systems is an essential capability for scalable architectures. This talk will focus on the experimental gen-eration of such entanglement in different setups, in the regime of single photons or in the regime of continuous variables. The first part will review experiments done recently in the single excitation regime. First, by following the seminal paper of Duan, Lukin, Cirac and Zoller (DLCZ) [1], entanglement between single collective excitations stored in two re-mote atomic ensembles can be generated. In the DLCZ protocol, entanglement is created in a probabilistic but heralded way from quantum interference in the measurement process. The detection of a photon from one or the other atomic ensemble in an indistinguishable fashion results in an entangled state with one collective spin excitation shared coherently [2]. This entanglement has been used for the initial implementation of functional quantum nodes for polarization en-
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.
Quantum information has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.
The creation of single collective excitations in atomic ensembles via spontaneous Raman emission plays a key role in several quantum communication protocols, starting with the seminal Duan-Lukin-Cirac-Zoller protocol [L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)]. This process is usually analyzed theoretically under the assumptions that the write laser pulse inducing the Raman transition is far off resonance and that the atomic ensemble is only homogeneously broadened. Here we study the impact of near-resonance excitation for inhomogeneously broadened ensembles on the collective character of the created atomic excitation. Our results are particularly relevant for experiments with hot atomic gases and for potential future solid-state implementations.
Quantum entanglement has emerged as one of the most intriguing and revolutionary phenomena in quantum physics, offering unparalleled opportunities in secure communication, quantum computing, and advanced information processing. By enabling correlations between particles that persist regardless of spatial separation, entanglement challenges classical intuitions and forms the backbone of many quantum technologies. This paper investigates the theoretical framework of quantum entanglement, evaluates its experimental realizations across photonic and matter-based platforms, and explores its transformative role in next-generation quantum communication systems. Through mathematical modeling, simulation, and comprehensive literature analysis, the study examines entanglement-based quantum key distribution (QKD), quantum teleportation protocols, and the pivotal role of decoherence in limiting scalability. Furthermore, the potential of entanglement swapping and multi-node quantum repeaters is assessed in the context of developing a global quantum internet. The findings highlight both the immense promise and the persistent technical challenges that must be overcome to transition from laboratory demonstrations to robust, real-world quantum networks.