Memories of light

Today, computers use electrons travelling in metal wires to process and store information. In the optical computers of the future, these tasks will be performed by photons travelling in optical fibres or thin films. However, one of the toughest challenges yet to be faced is building high-speed optical memory. Now Japanese researchers have discovered a possible solution.

A powder sample of samarium-doped ZnS nanocrystals (3 nm in diameter) was irradiated with laser light and the resulting excitation spectrum revealed a hole, which matched the wavelength of the irradiating laser. This "hole burning" effect has been observed before.

The sample was then rotated with respect to the incoming radiation. When the sample was moved through only a few tenths of a degree, the original hole in the excitation spectrum vanished. By making holes at various wavelengths and differing angles, it was demonstrated that the "data" (wavelength and incoming angle) were being stored with high resolution.

The scientists suggest that what is recorded are the interference patterns of multiple-scattered light in the sample, and this could eventually be used for effective three-dimensional data storage in optical systems.

Buckyballs generate waves

The interference pattern formed by particles passing through a grating is a distinctive signature of the duality of quantum particle and wave behaviour. Electrons, neutrons and even small molecules produce such patterns, and now scientists in Vienna have moved up an order of magnitude with the observation of the first interference patterns for carbon-60 molecules (buckyballs).

The team directed a collimated beam of carbon-60 molecules through a silicon nitride grating with 50 nm slits and a period of 100 nm. With a velocity of 220 ms-1, the de Broglie wavelength for the molecules was 2.5 pm ­ which is 400 times less than their diameter. Allowing for van der Waals effects between the grating and the carbon-60 molecules, the researchers saw a pattern with a central maximum and first-order diffraction peaks that matched predictions for a carbon-60 molecule interfering as a wave.

Now the team plans to repeat the experiment, using larger molecules or even viruses, to investigate at what scale classical physics takes over from the quantum duality of wave and particle.

SETs raise standards in accuracy

Almost 40 years ago, when the metre was redefined in terms of the wavelength of radiation emitted from a transition in the krypton atom, quantum physics entered the International System of Units. More recently, standards for voltage and resistance have been set by quantum effects (known as the Josephson and quantum Hall effects respectively). Now the US National Institute of Standards and Technology in Colorado has also placed the measurement of capacitance on a quantum basis by exploiting the new technology of single electron transistors.

In a piece of apparatus that is cooled to 40 mK, single quanta of electric charge are able to tunnel through a system of single electron transistor (SET) junctions. These packages of electrical charge are collected on a capacitor. By measuring the resulting voltage change across the capacitor, its capacitance can be determined to an accuracy of 1 ppm without the frequency dependence that has dogged previous standards. The National Institute of Standards and Technology ultimately hopes for an accuracy of 0.01 ppm. AIP

Undulating microstructures

Harvard scientists have found a novel way to make micrometre-sized structures.

When a film of elastic polydimethylsiloxane (PDMS) on a glass slide is heated to 250 ºC and exposed to oxygen, a silicate crust is formed. As the sample cools down, the silicate layer buckles to form waves across the surface. The wave structures can be made with wavelengths as short as 0.5 µm and can be ordered on larger scales by introducing a pattern on the polymer layer to act as a template. Diffraction gratings, and printing and lithographic devices are likely applications of this new technique. The microstructures could also be used as surfaces on which cells could be grown and oriented. AIP

Core calculation

To build an accurate model of the Earth's interior, a thorough knowledge of the high-pressure thermodynamic properties of iron is required. (Iron is the principal component of the Earth's solid inner core and its liquid outer core.) Physicists working at University College London have devised a way of determining the core temperature as a function of pressure. The calculation is based on density functional theory.

Simulation on a Cray T3E supercomputer predicted temperatures for core pressures of between 50 and 350 GPa. The results matched the existing data from shock experiments well. For the expected pressure of 330 GPa, at the boundary between the inner and outer core, the new calculation gives a temperature of 6670 ± 600 K, which is a useful constraint on models of the core, and is also important in our understanding of the Earth's magnetic field, earthquakes and volcanic activity.