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Physics News Update
Number 613, November 13, 2002 by Phil Schewe, James Riordon, and Ben Stein

A Bi-Photon de Broglie Wavelength

A bi-photon de Broglie wavelength has been directly measured in an interference experiment for the first time. In the early days of quantum mechanics, Louis de Broglie argued that if waves could act like particles (photoelectric effect) then why couldn't particles act like waves?

They could, as was borne out in numerous experiments (the double-slit experiment for electrons was voted the "most beautiful" experiment in a recent poll—see Physics World, Sept 2002).

In fact, intact atoms in motion and even molecules can be thought of as "de Broglie waves." Molecules as large as buckyballs (carbon-60) have been sent through an interferometer, creating a characteristic interference pattern (see Update 579).

The measured wavelength for a composite object like C-60 will in part depend on the internal bonds of the molecule. What then if the corporate object is a pair of entangled photons?

One of the more fascinating predictions made regarding quantum entanglement (Jacobson et al., Physical Review Letters, 12 Jun 1995) was the suggestion that the de Broglie wavelength for an ensemble consisting of N entangled photons (each with a wavelength of L) would be L/N.

This proposition has been verified now by physicists at Osaka University (Keiichi Edamatsu, 81-6-6850-6507, eda@mp.es.osaka-u.ac.jp) for the case of two entangled photons. The daughter photons were created by the process of parametric down-conversion, in which an incident photon entering a special crystal will split into two correlated photons. These photons are then sent through an interferometer (see figure).

The resultant interference pattern shows that the photons behave as if they acted as a single entity with a wavelength half that for either photon alone, a feature which might improve the sharpness of future quantum lithography (the narrowness of lines on a circuit board being no better than the wavelength of light used in the fabrication process).

But since the parent photon already had this shorter wavelength, what will have been gained by splitting the photon in half? The advantage will come when, at some point in the future it will be possible to generate entangled photons from non-entangled photons of the same wavelength, a process called hyper-parametric scattering. (Edamatsu et al., Physical Review Letters, 18 November 2002.)

Icicle Instability

No two snowflakes are alike, according to common wisdom. Icicles, on the other hand, are all alike--that is, the ripples that embellish the surfaces of most icicles are similar regardless of variations in air temperature, humidity, icicle thickness, or growth rate.

An icicle grows when thin sheets of water flow down the icicle shaft. A portion of the flowing water freezes and the rest drips from the icicle tip. But the ice that's left behind doesn't build up uniformly; instead, it is selectively deposited at certain locations.

As a result, icicles are covered in ring-like ripples extending along their lengths, which always measure about 1 cm from peak to peak.

Researchers at Hokkaido University's Institute of Low Temperature Sciences in Japan (Naohisa Ogawa and Yoshinori Furukawa: ogawa@particle.sci.hokudai.ac.jp, frkw@lowtem.hokudai.ac.jp) have developed a theoretical model that explains the surprisingly universal structure of icicles.

According to the new model, two effects are important as an icicle grows. The first effect is the Laplace instability, which is related to the latent heat released from an icicle's surface and dispersed into the air through the thin water layer. The instability arises because heat is more rapidly lost from the convex surfaces than that from the concave surfaces, which makes ice build up faster on an icicle's convex protrusions than on the concave indentations, thus amplifying ripples.

The second factor is the fluid effect. Flow in the thin water layer decreases the temperature distribution along the layer, making it uniform and thus inhibiting the Laplace instability.

As it happens, these two competing effects ensure that all icicle ripples have the same wavelength, although the ripple height can vary from one icicle to another. The theory also predicts that the ripples should migrate down an icicle at about half the speed that the icicle grows--a prediction the researchers hope will soon be verified experimentally.

In addition the researchers expect that their model should be helpful in explaining the structures of mineral stalagmites commonly found in limestone caves. (N. Ogawa and Y. Furukawa, Physical Review E, October 2002)

Powerful T-Lux Spotted in Virginia

Terahertz radiation, far-infrared light with frequencies around 1012 Hz, is difficult to make in useful amounts with electronic devices. It is, however, potentially valuable for a number of important applications, such as performing spectroscopy on proteins and buried structures in semiconductors.

A new experiment conducted at the Jefferson Lab free electron laser (FEL) has now produced a broadband batch of coherent THz light with an average beam power of 20 watts, some 100,000 times better than previous sources.

The T-light is produced in 500-femtosecond spurts when comparably timed bunches of electrons pass through a tiny region of magnetic field. (Carr et al., Nature, 14 November 2002.)