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

The author's experimental activities in ocean optics that are related to the inherent optical properties of natural waters are discussed. The specific subjects discussed are (1) measurements of Mueller matrices for several ocean water regions; (2) measurements of the spectral absorption of pure water throughout the visible spectrum; (3) the development of an in situ absorption meter that provides higher accuracy than is available for any other inherent optical property; (4) a relatively simple expression for the refractive index of water as a function of temperature, salinity, and wavelength; and (5) development of the first instrument to directly determine the backscattering coefficient b(b).

Studies on some of the inherent optical properties of natural waters [Invited]

Abstract An experimental realization of Bohm's spin-1/2 EPR gedanken-experiment is possible using the spin-1/2 nuclei of the isotope 199 Hg. The dissociation of dimers of the 199 Hg 2 isotopomer, using a spectroscopically selective stimulated Raman process, leads to the generation of an entangled state between the two 199 Hg atoms. The measurement of nuclear spin correlations between the two atoms in this entangled state is achieved by detection of the atoms using a spin state selective two photon excitation-ionization scheme. The experiment will not only close the detector efficiency loophole, but in addition will permit enforcement of the locality condition. Effects of imperfect analyzers will also be discussed.

On Some Aspects of an Hg Based EPR Experiment

Absorption of light at various wavelengths (i.e. absorption spectroscopy) is a powerful tool for identifying the presence of chemical compounds or specific substances in a sample. Cavity ring down spectroscopy (CRDS) is a well-known technique for very high sensitivity absorption spectroscopy. Another technique, integrating cavity spectroscopy has the additional unique feature of providing accurate absorption data even in the presence of severe scattering. This paper describes a combination of these two techniques that has led to an extremely powerful and useful new technology—integrating CRDS.

https://iopscience.iop.org/article/10.1088/0031-8949/91/4/043004/pdf

Integrating cavity ring-down spectroscopy (ICRDS) and the direct measurement of absorption coefficients

The time dependence of the frequency shift (δω) due to colliding-pulse phase modulation (CPPM) is computed, for collision of femtosecond optical pulses, both in nonresonant and in saturable media. In particular, thin material slabs (up to several pulse widths τ) are considered, and although the results are more generally applicable, special attention is given to a typical saturable-absorber jet in a colliding-pulse mode-locked laser. Results show that CPPM in nonresonant media causes noticeable effects on the chirp, both for thin slabs with L ≈ laser wavelength, where the coherence of the pulses leads to interference effects, and for thicker slabs with L ≈ cτ. Complications to CPPM by saturable media are discussed, and it is shown that the consequences of dispersion by the ground-state population, and finite phase-relaxation time, may be important for short, low-energy pulses. The results show, for parameters typical of contemporary colliding-pulse mode-locked lasers, what conditions are favorable for short, stable pulse formation in solitonlike pulse shaping. It is also shown that there are conditions for which the pulses produced would be spread by any sign of group-velocity dispersion.

Colliding-pulse phase modulation and chirping of ultrashort optical pulses in thin slabs of nonresonant and of saturable media

A new method has been developed for determining the absolute calibration of a light-scattering meter. The method is applicable when the light source is a well-collimated beam of small diameter, such as from a laser. No special reflectance or transmittance standards are required and the method is simple to implement. The measured calibration factor includes refractive effects at water/glass/air interfaces as well as the changes in scattering volume with scattering angle.

Edward S. Fry

Papers

Studies on some of the inherent optical properties of natural waters [Invited]

On Some Aspects of an Hg Based EPR Experiment

Integrating cavity ring-down spectroscopy (ICRDS) and the direct measurement of absorption coefficients

Colliding-pulse phase modulation and chirping of ultrashort optical pulses in thin slabs of nonresonant and of saturable media

Absolute Calibration of a Scatterance Meter