Number 859, March 25, 2008 by Phillip F. Schewe and Jason S. Bardi
Short Light.
The world’s shortest single photon has been produced by physicists at Oxford University. Light can be thought of as a series of waves or, in the dualistic view of reality prescribed by quantum science, as a collection of quanta, particle-like parcels of light energy referred to as photons. At any place along a light beam there may be many photons present or in special cases just one. Creating single photons is not easy to do. It is possible to make photons in pairs by sending laser light through special crystals. Even a pure-color laser beam will consist of many photons; but occasionally one of these photons will be “down converted,” that is, will turn into two photons each with half the energy of the original photon. When a pair has been created, the detection of one of these half-energy photons heralds the presence of its twin.
Furthermore, these photons are entangled, meaning that the properties of one photon are
inextricably linked to those of its partner and detecting one can ruin the quantum state of the other. By minimizing these quantum correlations, the researchers obtained heralded photons with exceptionally high quality and short duration.
In the Oxford experiment the pairs of photons made had a central wavelength of about 830 nm, at the border between visible and near-infrared light. Each of these photons was (in units of time) about 65 femtoseconds (65 x 10^-15 sec) long. In units of space, they were about 20 microns long.
The shortest previously produced single photon was about 1picosecond (10^-12 sec) long. Even shorter pulses of light-stretching only hundreds of attoseconds-have been made, but these pulses consist of many photons. One of the Oxford researchers, Peter Mosley (p.mosely1@physics.ox.ac.uk, 44-01865-282640), says that this new experiment represents the first time that textbook photons-identical, localized wavepackets containing a single quantum of energy-have been produced in a lab. (Mosley et al., Physical Review Letters upcoming article)
Atom Cloaking
A new study shows how a region of space could be rendered invisible to matter waves. In recent years the possibility of optical cloaking has become a hot topic (e.g., Science, 8 Sept 2006). Even cloaking with sound waves has been proposed (http://www.aip.org/pnu/2008/split/853-2.html). Now physicists in Professor Xiang Zhang’s group at the University of California, Berkeley, are trying to extend the cloaking idea to atom waves (chilled atoms whose quantum wavelike properties are more important than their particle-like properties) moving through a medium.
The “medium” in question here is a concentric optical lattice, generated by standing electromagnetic waves with spatially controlled amplitudes and phases. Cloaking of an object bathed in light works by modulating the effective mass and potential of atom waves traversing the shell surrounding the object. The shell is analogous to the metamaterials (tailored materials often consisting of arrays of tiny rods and ring-shaped metal structures) used in the optical case.
One of the Berkeley researchers, Shuang Zhang (zhangs@berkeley.edu), says that the atom-wave equivalent of an index of refraction would be the modulation of the effective atomic mass inside the optical lattice (see figure at www.aip.org/png). Zhang says that apart from cloaking, the creation of a metamaterial for atom waves might also help in focusing atom waves into tiny spot (super-lensing) or for steering particle beams at will. (Zhang et al., Physical Review Letters, 28 march 2008)
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