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

If a photon of definite polarization encounters an excited atom, there is typically some nonvanishing probability that the atom will emit a second photon by stimulated emission. Such a photon is guaranteed to have the same polarization as the original photon. But is it possible by this or any other process to amplify a quantum state, that is, to produce several copies of a quantum system (the polarized photon in the present case) each having the same state as the original? If it were, the amplifying process could be used to ascertain the exact state of a quantum system: in the case of a photon, one could determine its polarization by first producing a beam of identically polarized copies and then measuring the Stokes parameters1. We show here that the linearity of quantum mechanics forbids such replication and that this conclusion holds for all quantum systems.

A single quantum cannot be cloned

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

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.

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Electromagnetically induced transparency : Optics in coherent media

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

Summary form only given.Laser sources with energies in the microjoule to millijoule range are used in a multitude of applications. Many applications require narrow linewidth and tunability. The traditional approach employed is to use a low power, narrow linewidth master laser to injection seed a high power slave oscillator. In order for the oscillator to operate at the seeder frequency the cavity of the former has to be resonant with the seeder mode. Two alternative approaches are currently employed to achieve this goal: (1) the minimization of the build-up time or (2) the ramp-fire technique, in which the Q-switch is fired, when a resonance is detected while one cavity mirror is ramped rapidly. An alternative approach is presented. The beam of a low power seed source is gated with an electro-optic gate and then consecutively amplified first by a high gain fiber amplifier (up to 60 dB gain) followed by four-passing through a bulk amplifier. In contrast to a Q-switched fiber laser, our configuration does not form a closed cavity and no resonance condition must be fulfilled.

Generation of narrow linewidth pulses via amplification in a chain of fiber/bulk amplifiers

All previous experimental tests of Bell inequalities have required additional assumptions. The strong Bell inequalities (i.e. those requiring no additional assumptions) have never been tested. An experiment has been designed that can, for the first time, provide a definitive test of the strong Bell inequalities. Not only will the detector efficiency loophole be closed; but the locality condition will also be rigorously enforced. The experiment involves producing two Hg-199 atoms by a resonant Raman dissociation of a mercury dimer ((199)Hg2) that is in an electronic and nuclear spin singlet state. Bell inequalities can be tested by measuring angular momentum correlations between the spin one-half nuclei of the two Hg-199 atoms. The method used to make these latter measurements will be described.

https://ntrs.nasa.gov/api/citations/19950007553/downloads/19950007553.pdf

The strong Bell inequalities: A proposed experimental test

We consider the use of solid rare gas matrices to separately store two atoms whose spin one-half nuclei are in an entangled state. We believe this is the first feasible example of the possibility of storing an entangled state. The two separated, entangled atoms are prepared by photodissociation of a diatomic molecule that is in a nuclear spin singlet state.

Independent storage of the two components of an entangled state

We observe Ramsey fringes in the absorption line shape for a weak optical field probing the transition between an unperturbed quantum state and one that is dynamically Stark shifted by a strong pump field. {copyright} {ital 1996 The American Physical Society.}

/pdf/observation-of-interference-fringes-in-autler-townes-line-2c7p4ufanh.pdf

Edward S. Fry

Papers

Generation of narrow linewidth pulses via amplification in a chain of fiber/bulk amplifiers

The strong Bell inequalities: A proposed experimental test

Independent storage of the two components of an entangled state

Observation of interference fringes in Autler-Townes line shapes

Do EPR-Bell correlations require a non-local interpretation of quantum mechanics? I: Wigner approach