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Number 555, September 6, 2001 by Phil Schewe, James Riordon, and Ben Stein

Evidence for a Re-ionization Era

Evidence for a re-ionization era in the early universe has been glimpsed in the form of a quasar spectrum exhibiting a paucity of radiation at UV and shorter wavelengths.

Let's retrace some cosmological history. In the early years after the big bang conditions were too hot for neutral atoms to form; protons and electrons roamed independently in plasma form. Later, about 300,000 years after the big bang, things were cool enough for electrons and protons to form neutral hydrogen, making the universe transparent to visible light but opaque to higher-energy light which (if there were much of it about, and there wasn't) would be absorbed by these same H atoms. Later still the first stars and quasars started to pump out UV light. This radiation was avidly absorbed by surrounding reservoirs of neutral H, sometimes ionizing the atoms in the process.

As time passed more stars/galaxies/quasars formed, more UV was produced, and more of the neutral H was being turned back into ions. At a certain point, the great majority of H would be re-ionized. Since bare electrons and protons cannot absorb light, UV photons could thereafter proceed largely unhindered through the cosmos.

A new study of quasars (Becker et al., Los Alamos preprint) made with the Sloan Digital Survey telescope looks at this process happening; it shows that quasar spectra out to a redshift of about 6 feature UV emission, but that the furthest-out (earliest after the big bang) quasar yet glimpsed, at a redshift of 6.28, does not, suggesting that this quasar was active in an epoch when neutral H was still plentiful enough to choke off high energy radiation. Thus a re-ionization era would seem to have occurred around Z=6, at a time corresponding roughly to 800 million years after the big bang.

Superconductivity at 117K in a Buckyball Crystal

Superconductivity at 117K in a buckyball crystal has been observed by Bertram Batlogg at Lucent Technologies. A crystal of C₆₀ molecules normally has a lattice spacing of 1.415 nm and becomes superconducting at a temperature of 18 K. But by obliging the crystal to conduct with holes instead of electrons (in a transistor-like setup) and by adding other molecular species to space out the buckyballs a bit, the superconductivity transition temperature can be raised.

Batlogg and his colleagues at Lucent, the University of Konstanz (Germany), and the ETH lab in Zurich, have tried a number of candidates; the best so far is a dopant consisting of tribromo-methane (Br₃CH) molecules, which nudges the C-60 molecules out to a spacing of 1.445 nm and a transition temperature of 117 K, in the same realm as the high-temperature ceramic superconductors (Schön et al., Science, published online, 30 August).

Laser-Like Amplification of Entangled Particles

Laser-like amplification of entangled particles has been achieved by a University of Oxford team. Governed by quantum physics, entangled particles have much stronger correlations, or interrelationships, than anything allowed in classical physics. For example, measuring one entangled particle instantly influences its partner's state, even if the two particles are separated by great distances.

Entangled particles are the bread-and-butter of quantum information schemes such as quantum cryptography, quantum computing, and quantum teleportation. But they are notoriously difficult to create in bulk.

To create entangled photons, for example, researchers can send laser light through a barium borate crystal. Passing through the crystal, a photon sometimes splits into two entangled photons (each with half the energy of the initial photon). However, this only occurs for one in every ten billion incoming photons.

To increase the yield, the Oxford researchers added a step: they put mirrors beyond the crystal so that the laser pulse and entangled pair could reflect, and have the chance to interact. Since the entangled pair and reflected laser pulse behave as waves, quantum mechanics says that they could interfere constructively to generate fourfold more two-photon pairs or interfere destructively to create zero pairs. Following these steps, the researchers increased production of two-photon entangled pairs and also of rarer states such as four-photon entangled quartets.

This achievement could represent a step towards an entangled-photon laser, which would repeatedly amplify entangled particles to create greater yields than previously possible, and also towards the creation of new and more complex kinds of entangled states. (Lamas-Linares et al., Nature, 30 August 2001.)