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Number 615, November 27, 2002 by Phil Schewe, James Riordon, and Ben Stein

Refraction at the Atomic Level

Light propagation in a cavity can now be controlled through interactions with a collection of fewer than 10 atoms, a new experiment shows.

In general, the speed of light can be lowered from the vacuum value by passing it through a dense medium. Light speed can also be altered if the light pulse consists of a superposition of light waves at different frequencies and if the medium is dispersive (if its index of refraction varies for different frequencies). Using this dispersive approach, light was slowed to a halt in a Bose-Einstein condensate containing a million atoms (Update 521).

Now researchers from the University of Tokyo (Japan) and NIST (US) have managed the feat of altering a light pulse's speed in a microcavity with a medium whose density scarcely differs from vacuum—namely a handful of rubidium atoms.

The secret to the control is a long dwell time. The 70-micron-long cavity is so reflective (its "Q" value is high) that the pulse reflects many times before leaking out. This allows the light to interact with the handful of atoms repeatedly, as if there were many more atoms present.

According to the researchers (Yukiko Shimizu, shimizu-yukiko@aist.go.jp) this radical departure may be useful in quantum computing schemes. The pulses used in the experiment were themselves quite ephemeral, amounting to only four tenths of a photon (on average) in the cavity at any one time. The next goal is entangle a single photon with a single atom. (Shimizu et al., Physical Review Letters, 2 December 2002.)

Cool Ferric Wheels

A new form of magnetic cooling has been demonstrated on tiny ring-shaped molecules. One obvious form of cooling is for one sample of particles to give excess energy to another, surrounding, ensemble of particles. Another way of chilling atoms (used to produce Bose-Einstein condensates) is simply to allow hotter atoms to escape.

To see how "magnetic cooling" works in an ensemble of molecules consider first only the electrons spins in the molecule. The spins constitute a system all by themselves and can be "cooled" adiabatically (that is, without heat flowing in or out) by decreasing the strength of an applied magnetic field. Then some of the heat of molecular motion can be transferred to the spins; a lower molecular temperature is achieved.

This "adiabatic demagnetization" was routinely used to achieve the low temperatures (milli-kelvin) needed for studying helium-3. The principle can even be extended to the spins of nuclei, and in this way the lowest cryogenic temperature ever was reached, 50 nK in copper.

Now physicists at Erlangen-Nurnberg University in Germany (contact Oliver Waldmann, now at Ohio State, waldmann@mps.ohio-state.edu, 614-292-3705) have demonstrated, for the first time, the inverse effect: cooling molecules by increasing the strength of the applied field. This adiabatic magnetization was achieved with "ferric wheels," ring-shaped molecules featuring six iron atoms plus a few ligand hangers-on (see figure). Research like this, involving the reactions between spins and molecules, and the coherence of states over time might be beneficial to a future quantum computing scheme. (Waldmann et al., Physical Review Letters, 9 December 2002.)

Gentle Lithography

Lithography is the key process in microchip fabrication whereby circuit elements are built up or "written" onto a backing in a series of steps that can include chemical action, heating, and irradiation. Many attempts are underway both to devise simpler forms of lithography and to produce smaller circuit elements. The use of scanning tunneling microscope (STM) probes to fashion small structures by moving individual atoms or molecules is one way to do this, albeit at a very slow rate.

One new step in this direction is provided by Peter Kruse and Robert Wolkow (National Research Council, Ottawa), who report a "gentle lithography," one requiring no heating, etching, or exposure to photons, in which a silicon surface is covered by a monolayer of benzene molecules. Thereafter the benzene can be selectively removed in long strips (as if a combine were harvesting grain), with an STM probe, to produce deliberate patterns with spatial resolutions as small as 2 nm. Then another species of molecule, such as ethylene, can be laid down in the cleared areas. According to the researchers, patterned ethylene (after it's been heat treated) could lead to the creation of silicon carbide structures. (Kruse and Wolkow, Applied Physics Letters, 2 December 2002.)