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

Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.

/pdf/electromagnetically-induced-transparency-optics-in-coherent-2dhges92td.pdf

Electromagnetically induced transparency : Optics in coherent media

The first quantum technology that harnesses quantum mechanical effects for its core operation has arrived in the form of commercially available quantum key distribution systems. This technology achieves enhanced security by encoding information in photons such that an eavesdropper in the system can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, which promise exponentially greater computational power for particular tasks. Photonics is destined to have a central role in such technologies owing to the high-speed transmission and outstanding low-noise properties of photons. These technologies may use single photons, quantum states of bright laser beams or both, and will undoubtedly apply and drive state-of-the-art developments in photonics.

Photonic quantum technologies

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.

/pdf/quantum-repeaters-based-on-atomic-ensembles-and-linear-303iyntnod.pdf

Quantum repeaters based on atomic ensembles and linear optics

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

Nonlinear Optics at Low-Light Levels

We describe experiments and theory showing the generation of counterpropagating paired photons with coherence times of about 50 ns and waveforms that are controllable at a rudimentary level. Using cw lasers, electromagnetically induced transparency and cold 87Rb atoms we generate paired photons into opposing single-mode optical fibers at a rate of approximately 12 000 pairs per second.

http://web.stanford.edu/~seharris/PublicationsLinksPDFs/2000/210.pdf

Generation of paired photons with controllable waveforms.

Electromagnetically induced transparency in an optically thick, cold medium creates a unique system where pulse-propagation velocities may be orders of magnitude less than c and optical nonlinearities become exceedingly large. As a result, nonlinear processes may be efficient at low-light levels. Using an atomic system with three, independent channels, we demonstrate a quantum interference switch where a laser pulse with an energy per area of $\ensuremath{\sim}23$ photons per ${\ensuremath{\lambda}}^{2}/(2\ensuremath{\pi})$ causes a $1/e$ absorption of a second pulse.

/pdf/low-light-level-nonlinear-optics-with-slow-light-58jzcg3j76.pdf

Low-light-level nonlinear optics with slow light

We report the first experimental demonstration of four-wave mixing using electromagnetically induced transparency in cold atoms Backward-wave, phase-matched difference-frequency conversion is achieved at optical powers of a few nanowatts and at energies of less than a picojoule

https://web.stanford.edu/~seharris/PublicationsLinksPDFs/2000/206.pdf

Frequency mixing using electromagnetically induced transparency in cold atoms.

This study reports the experimental demonstration of a high-brightness source of paired photons based on an double-/spl Lambda/ atomic level configuration in cold /sup 87/Rb atoms. The source is capable of producing pairs of correlated photons with bandwidths as small as 5 MHz at into a single spatial mode. Strong counter-propagating coupling and pump lasers are applied and the system spontaneously emits pairs of photons at Stokes and anti-Stokes frequencies. The backward-wave geometry ensures that a spontaneously emitted photons will be oppositely directed and that they can be detected without additional filtering.

https://ieeexplore.ieee.org/iel5/10482/33238/01567243.pdf

Vlatko Balić

Papers

Nonlinear Optics at Low-Light Levels

Generation of paired photons with controllable waveforms.

Low-light-level nonlinear optics with slow light

Frequency mixing using electromagnetically induced transparency in cold atoms.

Narrow-bandwidth paired photons with controllable waveform