Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.

Quantum Cryptography

Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on an eavesdropper's power. This article provides a concise up-to-date review of QKD, biased toward the practical side. Essential theoretical tools that have been developed to assess the security of the main experimental platforms are presented (discrete-variable, continuous-variable, and distributed-phase-reference protocols).

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The security of practical quantum key distribution

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

/pdf/invited-review-article-single-photon-sources-and-detectors-5bsn8auo37.pdf

Invited review article: Single-photon sources and detectors

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 we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.

Quantum repeaters based on atomic ensembles and linear optics

This work presents an overview of progress and developments in the field of fiber optic sensor technology, highlighting the major issues underpinning recent research and illustrating a number of important applications and key areas of effective fiber optic sensor development.

http://snake.persiangig.com/document/sensor1.pdf

Fiber optic sensor technology: an overview

We report on a new kind of correlated photon-pair source based on a waveguide integrated on a Periodically Poled Lithium Niobate substrate. Using a pump laser of a few micro-Watts at 657 nm, we generate degenerate photon-pairs at 1314 nm. Detecting about 1500 coincidences per second, we can infer a conversion rate of 10-6 pairs per pump photon, which is four orders of magnitude higher than that obtained with previous bulk sources. These results are very promising for the realization of sources for quantum communication and quantum metrology experiments requiring a high signal-to-noise ratio or working with more than one photon-pair at a time.

Highly efficient photon-pair source using a Periodically Poled Lithium Niobate Waveguide

We report the realization of a fiber-coupled polarization entangled photon-pair source at 1310 nm based on a birefringent titanium in-diffused waveguide integrated into periodically poled lithium niobate. By making use of a dedicated and high-performance setup, we characterized the quantum properties of the pairs by measuring two-photon interference in both Hong–Ou–Mandel and standard Bell inequality configurations. For the two sets of measurements we obtained interference net visibilities reaching nearly 100%, which represent important and competitive results compared to those for the similar waveguide-based configurations already reported. These results prove the relevance of our approach as an enabling technology for long-distance quantum communication.

https://iopscience.iop.org/article/10.1088/1367-2630/12/10/103005/pdf

A polarization entangled photon-pair source based on a type-II PPLN waveguide emitting at a telecom wavelength

We report a simple fabrication process for realizing waveguides on periodically poled lithium niobate which preserves both the nonlinearity and the domain inversion. This so-called soft proton exchange has been used to generate highly efficient optical parametric fluorescence in the 1.48–2.01 μm region using a pump around 830 nm. The measured normalized efficiency is 130% W−1 cm−2 for an effective interaction length of 1.3 cm. This experimental figure is very close to the maximum theoretically predicted value of 140% W−1 cm−2.

Soft proton exchange on periodically poled LiNbO3: A simple waveguide fabrication process for highly efficient nonlinear interactions

Laser oscillation in an Nd:MgO:LiNbO3 single-mode channel waveguide has been demonstrated. Under IR dye laser pumping, a threshold of 1.5 mW and a slope efficiency of 14%; were achieved. Laser oscillation has also been observed with diode pumping.

Laser oscillation of single-mode channel waveguide in Nd:MgO:LiNbO3

A report is presented on the fabrication and efficient CW operation at 1.085 mu m of Nd:MgO:LiNbO/sub 3/ proton-exchanged channel waveguide lasers and amplifiers optically pumped at 0.814 mu m. Thresholds as low as 1.5 mW and slope efficiencies up to 34% were achieved in monolithic miniature laser devices. Up to 14 mW of output power could be achieved without observation of photorefractive damage. Diode laser pumping was also demonstrated. For the amplifier a small-signal gain of 7.5 dB was achieved for 22 mW of coupled pump power. Experimental results were analyzed with the use of a mode overlap formalism and a value of 0.93 was found for the pump quantum efficiency. Absorption spectra, fluorescence spectra, and lifetime measurements are also reported for the bulk and waveguide material. >

D.B. Ostrowsky

Papers

Highly efficient photon-pair source using a Periodically Poled Lithium Niobate Waveguide

A polarization entangled photon-pair source based on a type-II PPLN waveguide emitting at a telecom wavelength

Soft proton exchange on periodically poled LiNbO3: A simple waveguide fabrication process for highly efficient nonlinear interactions

Laser oscillation of single-mode channel waveguide in Nd:MgO:LiNbO3

Nd:MgO:LiNbO/sub 3/ channel waveguide laser devices