All our former experience with application of quantum theory seems to say:
{\it what is predicted by quantum formalism must occur in laboratory} But the
essence of quantum formalism - entanglement, recognized by Einstein, Podolsky,
Rosen and Schr\"odinger - waited over 70 years to enter to laboratories as a
new resource as real as energy This holistic property of compound quantum systems, which involves
nonclassical correlations between subsystems, is a potential for many quantum
processes, including ``canonical'' ones: quantum cryptography, quantum
teleportation and dense coding However, it appeared that this new resource is
very complex and difficult to detect Being usually fragile to environment, it
is robust against conceptual and mathematical tools, the task of which is to
decipher its rich structure This article reviews basic aspects of entanglement including its
characterization, detection, distillation and quantifying In particular, the
authors discuss various manifestations of entanglement via Bell inequalities,
entropic inequalities, entanglement witnesses, quantum cryptography and point
out some interrelations They also discuss a basic role of entanglement in
quantum communication within distant labs paradigm and stress some
peculiarities such as irreversibility of entanglement manipulations including
its extremal form - bound entanglement phenomenon A basic role of entanglement
witnesses in detection of entanglement is emphasized

Quantum entanglement

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 computers promise to increase greatly the efficiency of solving problems such as factoring large integers, combinatorial optimization and quantum physics simulation. One of the greatest challenges now is to implement the basic quantum-computational elements in a physical system and to demonstrate that they can be reliably and scalably controlled. One of the earliest proposals for quantum computation is based on implementing a quantum bit with two optical modes containing one photon. The proposal is appealing because of the ease with which photon interference can be observed. Until now, it suffered from the requirement for non-linear couplings between optical modes containing few photons. Here we show that efficient quantum computation is possible using only beam splitters, phase shifters, single photon sources and photo-detectors. Our methods exploit feedback from photo-detectors and are robust against errors from photon loss and detector inefficiency. The basic elements are accessible to experimental investigation with current technology.

A scheme for efficient quantum computation with linear optics.

Quantum teleportation — the transmission and reconstruction over arbitrary distances of the state of a quantum system — is demonstrated experimentally. During teleportation, an initial photon which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the initial one. Quantum teleportation will be a critical ingredient for quantum computation networks.

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Experimental quantum teleportation

Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).

Autonomic healing of polymer composites

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 two-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.

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Violation of Bell Inequalities by Photons More Than 10 km Apart

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

Pulsed Energy-Time Entangled Twin-Photon Source for Quantum Communication

We present a setup for quantum cryptography based on photon pairs in energy-time Bell states and show its feasibility in a laboratory experiment. Our scheme combines the advantages of using photon pairs instead of faint laser pulses and the possibility to preserve energy-time entanglement over long distances. Moreover, using four-dimensional energy-time states, no fast random change of bases is required in our setup: Nature itself decides whether to measure in the energy or in the time base, thus rendering eavesdropper attacks based on “photon number splitting” less efficient. PACS numbers: 03.67.Dd, 03.67.Hk Quantum communication is probably one of the most rapidly growing and most exciting fields of physics within the last years [1]. Its most mature application is quantum cryptography [also called quantum key distribution (QKD)], ensuring the distribution of a secret key between two parties. This key can be used afterwards to encrypt and decrypt secret messages using the one time pad [2]. In opposition to the mostly used “public key” systems [2], the security of quantum cryptography is not based on mathematical complexity, but on an inherent property of single quanta. Roughly speaking, since it is not possible to measure an unknown quantum system without modifying it, an eavesdropper manifests herself by introducing errors in the transmitted data. During the past years, several prototypes based on faint laser pulses have been developed, demonstrating that quantum cryptography not only works inside the laboratory, but in the “real world” as well [1,3,4]. Besides, it has been shown that two-photon entanglement can be preserved over large distances [5], especially when being entangled in energy and time [6]. As pointed out by Ekert in 1991 [7], the nonlocal correlations engendered by such states can also be used to establish sequences of correlated bits at distant places. Besides improvements in the domain of QKD, recent experimental progress in generating, manipulating, and measuring the so-called Bell states [8], has lead to fascinating applications like quantum teleportation [9], dense coding [10], and entanglement swapping [11]. In a recent paper, we proposed and tested a novel source for quantum communication generating a new kind of Bell states based on energy-time entanglement [12]. In this paper, we present a first application, exploiting this new source for quantum cryptography. Our scheme follows Ekert’s initial idea concerning the use of photon-pair correlations. However, in opposition, it implements Bell states and can thus be seen in the broader context of quantum communication. Moreover, the fact that energy-time entanglement can be preserved over long distances renders our source particulary interesting for long-distance applications. To understand the principle of our idea, we look at Fig. 1. A short light pulse emitted at time t0 enters an interferometer having a path length difference which is large compared to the duration of the pulse. The pulse is thus split into two pulses of smaller amplitudes, following each other with a fixed phase relation. The light is then focused into a nonlinear crystal where some of the pump photons are down-converted into photon pairs. Working with pump energies low enough to ensure that generation of two photon pairs can be neglected, a created photon pair is described by jc 1 p 2 jsPjsP 1 e if jlPjlP . (1)

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Quantum cryptography using entangled photons in energy-time bell states

Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a ${\mathrm{KNbO}}_{3}$ crystal and are sent into all-fiber interferometers using a telecommunications fiber network. The two interferometers of this Franson-type test of the Bell inequality are located 10.9 km apart from one another. Two-photon fringe visibilities of up to 81.6% are obtained. These strong nonlocal correlations support the nonlocal predictions of quantum mechanics and provide evidence that entanglement between photons can be maintained over long distances.

Experimental demonstration of quantum correlations over more than 10 km

We report on a new kind of experimental investigations of the tension between quantum nonlocally and relativity. Entangled photons are sent via an optical fiber network to two villages near Geneva, separated by more than 10 km where they are analyzed by interferometers. The photon pair source is set as precisely as possible in the center so that the two photons arrive at the detectors within a time interval of less than 5 ps (corresponding to a path length difference of less than 1 mm). One detector is set in motion so that both detectors, each in its own inertial reference frame, are first to do the measurement! The data always reproduces the quantum correlations, making it thus more difficult to consider the projection postulate as a compact description of real collapses of the wave-function

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J. Brendel

Papers

Violation of Bell Inequalities by Photons More Than 10 km Apart

Pulsed Energy-Time Entangled Twin-Photon Source for Quantum Communication

Quantum cryptography using entangled photons in energy-time bell states

Experimental demonstration of quantum correlations over more than 10 km

Experimental test of nonlocal quantum correlation in relativistic configurations