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.

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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...

We propose a quantum repeater protocol which builds on the well-known DLCZ protocol [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature 414, 413 (2001)], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multi-mode memories based on the principle of photon echo, using solids doped with rare-earth ions. The use of multimode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.The distribution of entanglement over long distances is an important challenge in quantum information. It would extend the range for tests of Bell's inequalities, quantum key distribution and quantum networks. [5,6]. A basic element of all protocols is the creation of entanglement between neighboring nodes A and B, typically conditional on the outcome of a measurement, e.g. the detection of one or more photons at a station between two nodes. In order to profit from a nested repeater protocol [1], the entanglement connection operations creating entanglement between non-neighboring nodes can only be performed once one knows the relevant measurement outcomes. This requires a communication time of order L 0 /c, where L 0 is the distance between A and B. Conventional repeater protocols are limited to a single entanglement generation attempt per elementary link per time interval L 0 /c. Here we propose to overcome this limitation using a scheme that combines photon pair sources and memories that can store a large number of distinguishable temporal modes. We also show that such memories could be realized based on the principle of photon echo, using solids doped with rare-earth ions.Our scheme is inspired by the DLCZ protocol [2], which uses Raman transitions in atomic ensembles that lead to nonclassical correlations between atomic excitations and emitted photons [7]. The basic procedure for entanglement creation between two remote locations A and B in our protocol requires one memory and one source of photon pairs at each location, denoted M A(B) and S A(B) respectively. The two sources are coherently excited such that each has a small probability p/2 of creating a pair, corresponding to a stateHere a and a ′ (b and b ′ ) are the two modes corresponding to S A (S B ), φ A (φ B ) is the phase of the pump laser at location A (B), and |0 is the vacuum state. The O(p) term introduces errors in the protocol, leading to the requirement that p has to be kept small, cf. below. The photons in modes a and b are stored in the local memories M A and M B . The modes a ′ and b ′ are coupled into fibers and combined on a beam splitter at a station between A and B. The modes after the beam splitter, where χ A,B are the phases acquired by the photons on their way to the central station. Detection of a single photon inã, for example, creates a state, where a and b are now stored in the memories. This can be rewritten as an entangled state of the two m...

A Franson-type test of Bell inequalities by photons 10.9 km apart is presented. Energy-time entangled photon-pairs are measured using two-channel analyzers, leading to a violation of the inequalities by 16 standard deviations without subtracting accidental coincidences. Subtracting them, a 2-photon interference visibility of 95.5% is observed, demonstrating that distances up to 10 km have no significant effect on entanglement. This sets quantum cryptography with photon pairs as a practical competitor to the schemes based on weak pulses. PACS. 03.65Bz, 03.67.D, Quantum theory is nonlocal. Indeed, quantum theory predicts correlations among distant measurement outcomes that cannot be explained by any theory which involves only local variables. This was anticipated by Einstein, Podolski and Rosen [1] and by Schrödinger [2], among others, and first demonstrated by John Bell in 1964 with his now famous inequality [3]. However, the nonlocal feature cannot be exploited for superluminal communication [4]. Hence, there is no contradiction with relativity, though there is clearly a tension. Physicists disagree about the significance and importance of this tension. This led Abner Shimony to name this situation "peaceful coexistence between quantum mechanics and relativity" [5].Why should one still bother about quantum nonlocality despite that all experiments so far are in agreement with quantum theory [6,7]? The traditional motivations are based on fundamental questions on the meaning and compatibility of our basic theories, quantum mechanics and relativity: to date, no experiment to test Bell's inequality has been loophole free [8][9][10] and no experiment so far has directly probed the tension between quantum non locality and relativity. Recently, additional motivations to investigate quantum non-locality arose based on the potential applications of the fascinating field of quantum information processing: all of quantum computation and communication is based on the assumption that quantum systems can be entangled and that the entanglement can be maintained over long times and distances [11].In 1997 we have demonstrated that two-photon correlations remain strong enough over 10 km so that a violation of Bell inequalities could be expected [12]. In this letter we report on a new experiment using two-channel analyzers in which all 4 coincidence rates have been measured simultaneously, thus allowing to obtain directly the correlation coefficient that defines the Bell inequalities. Our experiment demonstrates a violation of Bell's inequalities with photons more than 10 km apart [13], even without subtracting the accidental coincidences. In addition, an experiment with three interferometers, two on one end and the third at the other end (10 km away) is presented. The two nearby interferometers analyse the incoming photons randomly, the choice being made by a passive beam splitter. This setup enables to test directly the CHSH form of Bell-inequalities [14]. Our experiment establishes also the feasibility of quantum cryptography wi...