Physics News Update, Number 492

Number 492, July 6, 2000 by Phillip F. Schewe and Ben Stein

DISCOVERY OF SOFT DIPOLE RESONANCE IN HELIUM-6. In some neutron-rich nuclei extra neutrons can constitute a "halo" or satellite nucleus. Furthermore, when these compound nuclei are excited the parts oscillate back and forth. For example, in the case of lithium-6, the three protons can oscillate together opposite the three neutrons (an arrangement called "giant dipole resonance" or GDR). I

n the case of helium-6, one expects that an oscillating resonance should occur in which one unit, consisting of two protons and two neutrons (an alpha particle, in other words), moves in opposition to a unit made of two neutrons (see figure at Physics News Graphics).

However, vigorous experimental work had up to now failed to find the oscillation, and physicists had begun to doubt its existence. This long-looked-for "soft dipole resonance" has now been observed for the first time by a Tokushima-Konan-RCNP-Osaka collaboration of physicists in Japan (Shintaro Nakayama, nakayama@ias.tokushima-u.ac.jp, 011-81-88-656-7236). They created the He-6 resonance when a Li-7 beam struck a target of Li-6 atoms. (Nakayama et al., Physical Review Letters, 10 July; Select Articles.)

A SUPERCONDUCTING "SCHRODINGER'S CAT" has been demonstrated in the lab by a group at Stony Brook. Quantum phenomena can be big things; examples include supercurrents, consisting of billions of electron pairs, moving around a macroscopically sized superconductor, or ensembles of billions of photons making up a pulse of laser light, all residing in a single quantum state.

By contrast, quantum superposition, in which the system exists in two states (such as having two different values of angular momentum or being in two different places) at the same time, has mostly been a small thing, or a thing of few parts. Examples: a single ion simultaneously in two places (several nm apart) within an atom trap (David Wineland, NIST); or wavelike manifestations of C-60 molecules split and sent along separate paths of an atom interferometer (Anton Zeilinger, Univ Vienna).

In the Stony Brook experiment (Jonathan Friedman, 631-632-8079, jonathan.friedman@sunysb.edu) the superposition of quantum states is both big in size and in the number of parts. The quantum system in question is a supercurrent (containing billions of electron pairs) flowing around a 140-micron-sized superconducting quantum interference device (SQUID) circuit. As for the superposition of states in this case, it consists of the fact (improbably enough) that the supercurrent can flow in both directions at the same time (note: the current is in a superposition of clockwise or anticlockwise; it is never zero).

Normally the two supercurrent quantum states (clockwise and counterclockwise flow) sit in two separate potential wells (in the abstract space of quantum states). But the Stony Brook researchers (James Lukens heads the team) apply a gentle blast of microwaves that nudges the quantum current states part of the way out of their valleys, high enough to make quantum tunneling between the states possible (facilitating currents flowing in both directions at the same time) but not so high as to break up the electron pairs which are the heart of the superconducting condition. One hope is that this type of large coherent quantum state, well isolated from the outside (nonquantum) environment, could be put to service in quantum computing. (Friedman et al., Nature, 6 July 2000.)

ADAPTIVE OPTICS HAS NOW BEEN APPLIED TO HUMAN VISION. Adaptive optics is a technique developed by astronomers to correct for telescope image blurriness caused by atomospheric turbulence. A laser samples the air overhead, computes the nature of the instantaneous turbulence, and then prompts a deformable mirror which can quickly alter the incoming light wave front to restore a sharp image of the desired object in the sky.

David Williams of the University of Rochester adapts this approach to the case of human vision. He sends a tiny light beam into the eye for sensing minute imperfections in the retina. The reflected light is analyzed to describe the wavefront of light coming from the retina. With this approach Williams (david@cvs.rochester.edu, 716-275-8672) has made the most accurate map of the retina (see Physics Today, January 2000). The whole process can then be operated in reverse so that light coming toward the eye can be corrected using a deformable mirror actuated by a previously formulated map of the imperfections of the eye's optics. The result: sharper vision, perhaps even a sort of "super vision."

Scientists from the University of Rochester and the company of Bausch & Lomb are jointly working on the problem of applying the new technology to human vision. No one would actually wear a version of the full apparatus, but the process will at least be valuable in providing better contact lenses and in diagnosing retinal diseases. (Talk presented at the American Astronomical Society meeting in Rochester, NY in June; also see Nature, 11 Feb 1999.)