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 memory is important to quantum information processing in many ways: a synchronization device to match various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a tool to convert heralded photons to photons-on-demand. In addition to quantum computing, quantum memory would be instrumental for the implementation of 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: optical delay lines, cavities, electromagnetically-induced transparency, photon-echo, and off-resonant Faraday interaction. Here we report on the state-of-the-art in the field of optical quantum memory, including criteria for successful quantum memory and current performance levels.

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A pulsed source of energy-time entangled photon pairs pumped by a standard laser diode is proposed and demonstrated. The basic states can be distinguished by their time of arrival. This greatly simplifies the realization of 2-photon quantum cryptography, Bell state analyzers, quantum teleportation, dense coding, entanglement swapping, GHZ-states sources, etc. Moreover the entanglement is well protected during photon propagation in telecom optical fibers, opening the door to few-photon applications of quantum communication over long distances.Quantum communication offers fascinating possibilities to the physicists. Some correspond to potential applications, like quantum cryptography, others explore the quantum world of entanglement, like dense coding, entanglement swapping (entangling particules that never interact) or teleportation (transferring the unknown quantum state from one particle to a distant one) [1][2][3][4][5]. In recent years, quantum communication, like all the field of quantum information processing, underwent an impressive flow of theoretical ideas. The experiments, however, were generally far behind. This unbalanced situation still remains, except for the 1-qubit quantum cryptography case (actually, pseudo-1-qubit, since weak coherent light pulses mimic the qubit) [5]. There is thus a clear need for original implementations of the general ideas. In this letter, we propose a compact, robust (and low cost) source producing energy-time entangled pairs of photons (twin-photons) at determined times. The source can be tuned to produce any desired 2-qubit state, in particular the four Bell states. Contrary to other Bell state sources [6], the basic states of our twin-photons are neither based on polarization, nor on momentum, but on time bins. This allows one to separate the basic state easily, without any optical element, and prevents crosstalk during the photon propagation.We first introduce the basic states of our qubit space. Next, we present our source and an experimental demonstration is discussed. Finally, the potential of our source is illustrated by several examples.To understand our source, it is useful to start with the simple device of Figure 1 which can be entirely understood in terms of classical linear optics. First, we analyse it as a preparation device. Let a 1-photon pulse enter the device from the left. Assuming the pulse duration is short compared to the arm length difference long-short of the Mach-Zehnder interferometer, the output consists of two well separated pulses. Let us denote them | short and | long . They form the bases of our qubit space, similar to the usual vertical | V and horizontal | H linear polarization states. Hence the state at the output of our preparation device reads α| short + β| longThe relative norm and phase of the coefficients α and β are determined by the coupling ratio of the beam splitter and the phase shifter, respectively. Hence, any state of the 2-dimensional Hilbert space spanned by the basic states | short and | long can be prepared. The switch of...

The reversible transfer of quantum states of light into and out of matter constitutes an important building block for future applications of quantum communication: it will allow the synchronization of quantum information, and the construction of quantum repeaters and quantum networks. Much effort has been devoted to the development of such quantum memories, the key property of which is the preservation of entanglement during storage. Here we report the reversible transfer of photon-photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100 megahertz to 5 gigahertz. We assess the entanglement-preserving nature of our storage device through Bell inequality violations and by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer. These measurements show, within statistical error, a perfect mapping process. Our broadband quantum memory complements the family of robust, integrated lithium niobate devices. It simplifies frequency-matching of light with matter interfaces in advanced applications of quantum communication, bringing fully quantum-enabled networks a step closer.