Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers, including quantum computation, communication and metrology. The realization of quantum networks composed of many nodes and channels requires new scientific capabilities for generating and characterizing quantum coherence and entanglement. Fundamental to this endeavour are quantum interconnects, which convert quantum states from one physical system to those of another in a reversible manner. Such quantum connectivity in networks can be achieved by the optical interactions of single photons and atoms, allowing the distribution of entanglement across the network and the teleportation of quantum states between nodes.

The quantum internet

The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here the theoretical and experimental status quo of this very active field are reviewed. The potentials of different approaches are compared quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.

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Quantum repeaters based on atomic ensembles and linear optics

The internet-a vast network that enables simultaneous long-range classical communication-has had a revolutionary impact on our world. The vision of a quantum internet is to fundamentally enhance internet technology by enabling quantum communication between any two points on Earth. Such a quantum internet may operate in parallel to the internet that we have today and connect quantum processors in order to achieve capabilities that are provably impossible by using only classical means. Here, we propose stages of development toward a full-blown quantum internet and highlight experimental and theoretical progress needed to attain them.

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Quantum internet: A vision for the road ahead

Quantum memory is essential for the development of many devices in quantum information processing, including a synchronization tool that matches various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a mechanism to convert heralded photons to on-demand photons. In addition to quantum computing, quantum memory will be instrumental for implementing long-distance quantum communication using quantum repeaters. The importance of this basic quantum gate is exemplified by the multitude of optical quantum memory mechanisms being studied, such as optical delay lines, cavities and electromagnetically induced transparency, as well as schemes that rely on photon echoes and the off-resonant Faraday interaction. Here, we report on state-of-the-art developments in the field of optical quantum memory, establish criteria for successful quantum memory and detail current performance levels.

Optical quantum memory

During the past decade the interaction of light with multiatom ensembles has attracted much attention as a basic building block for quantum information processing and quantum state engineering. The field started with the realization that optically thick free space ensembles can be efficiently interfaced with quantum optical fields. By now the atomic ensemble-light interfaces have become a powerful alternative to the cavity-enhanced interaction of light with single atoms. Various mechanisms used for the quantum interface are discussed, including quantum nondemolition or Faraday interaction, quantum measurement and feedback, Raman interaction, photon echo, and electromagnetically induced transparency. This review provides a common theoretical frame for these processes, describes basic experimental techniques and media used for quantum interfaces, and reviews several key experiments on quantum memory for light, quantum entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. The two types of quantum measurements which are most important for the interface are discussed: homodyne detection and photon counting. This review concludes with an outlook on the future of atomic ensembles as an enabling technology in quantum information processing.

Quantum interface between light and atomic ensembles

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of $\ensuremath{\chi}=0.84(11)$ and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.

Interfacing Collective Atomic Excitations and Single Photons

Generation of non-classical correlations (or entanglement) between atoms1,2,3,4,5,6,7, photons8 or combinations thereof 9,10,11 is at the heart of quantum information science. Of particular interest are material systems serving as quantum memories that can be interconnected optically 3,6,7,9,10,11. An ensemble of atoms can store a quantum state in the form of a magnon—which is a quantized collective spin excitation—that can be mapped onto a photon12,13,14,15,16,17,18 with high efficiency19. Here, we report the phase-coherent transfer of a single magnon from one atomic ensemble to another via an optical resonator serving as a quantum bus that in the ideal case is only virtually populated. Partial transfer deterministically creates an entangled state with one excitation jointly stored in the two ensembles. The entanglement is verified by mapping the magnons onto photons, whose correlations can be directly measured. These results should enable deterministic multipartite entanglement between atomic ensembles.

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Single-photon bus connecting spin-wave quantum memories

We demonstrate a heralded quantum memory where a photon announces the mapping of a light polarization state onto a single collective-spin excitation (magnon) shared between two atomic ensembles. The magnon can be converted at a later time into a single polarized photon with polarization fidelity over 90(2)% for all fiducial input states, well above the classical limit of $\frac{2}{3}$. The process can be viewed as a nondestructive quantum probe where a photon is detected, stored, and regenerated without touching its---potentially undetermined---polarization.

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.103.043601

Heralded single-magnon quantum memory for photon polarization States.

http://absimage.aps.org/image/MAR07/MWS_MAR07-2006-007573.pdf

Haruka Tanji

Papers

Interfacing Collective Atomic Excitations and Single Photons

Single-photon bus connecting spin-wave quantum memories

Heralded single-magnon quantum memory for photon polarization States.

Interfacing Collective Atomic Excitations and Single Photons

Heralded Single-Magnon Quantum Memory for Photon Polarization States