Physics News Update, Number 465

Number 465, January 7, 2000 by Phillip F. Schewe and Ben Stein

AMPLIFYING AN ATOM WAVE while maintaining its original phase has been demonstrated for the first time, bringing about an atom laser that is the closest equivalent yet to an optical laser. The first atom lasers (Update 305) were passive devices: researchers simply prepared a Bose-Einstein condensate of atoms (Update 233), and then extracted some of the BEC atoms to form a beam.

In the latest round of demonstrations, two research groups (one at MIT and one at the University of Tokyo) have independently demonstrated an atom laser that amplifies its initial beam, in a way that's remarkably similar to how optical lasers augment an initial light wave. Unlike light, however, atoms cannot be created from the vacuum, so researchers must rely on a pre-existing supply of atoms to serve as the initial beam to be amplified.

In the MIT demonstration, researchers shine a pair of laser pulses on a sodium BEC. First, some of the BEC atoms absorb a photon from a high-frequency beam and emit a photon towards a lower-frequency beam. These atoms recoil in the same direction, forming a weak atom wave. Then the lower-frequency beam is shut off, and some of the other BEC atoms absorb light from an intensified pulse coming from the high-frequency laser. The presence of the initial atom wave stimulates these atoms to emit a photon in the direction of the lower-frequency beam. This resulted in a phase-coherent amplified beam about 4 times as strong as the initial atom wave.

The Tokyo group demonstrated similar results with a rubidium-87 BEC. In both demonstrations, the amplification is limited by the size of the BEC, which is depleted in the process. However, an atom-wave amplifier promises improvements in such applications as atom-wave gyroscopes and lithography. (Inouye et al., Nature, 9 December 1999; Kozuma et al., Science, 17 December.)

HIGH PROTON POLARIZATION, up to 32%, has been achieved at liquid-nitrogen temperatures (77 K) and with modest 0.3-Tesla magnetic fields in an experiment at Kyoto University in Japan. Among a proton's attributes is the orientation of its intrinsic spin; this directionality can come into play when the proton interacts with the spins of other particles or with a radio frequency field. For comparison, proton polarization levels in MRI medical imaging is a paltry .0003% (still good enough for spotting tumors) using room temperature and magnetic fields typically of 1 Tesla (10,000 gauss). Targets for particle physics using accelerators can achieve polarizations of up to about 70% but even higher fields (2 or 5 T) are needed as well as low liquid-helium temperatures (typically 0.3 K).

In the Kyoto experiment, the electrons in pentacene (an aromatic organic molecule chain) are polarized optically with a laser beam. Next, microwaves force the polarization to be transferred to protons in the molecules. The researchers (M. Iinuma, 011-81-824-24-7373, iinuma@photon.hepl.hiroshima-u.ac.jp) suspect that their approach will find applications in particle physics (where targets polarized in smaller fields and warmer temperatures would permit the detection of slower charged particles amid high intensity beams) and in chemistry/biology (where the new method provides higher sensitivity than the existing NMR).

Polarized protons would be portable in a small box for more than 3 hours at almost zero magnetic field. The new polarization method should also benefit MRI imaging (where high polarization can improve spatial resolution of pictures ), the task of transfering spin to normally-hard-to-detect C-13 atoms, and NMR-based quantum computing (wherein information storage and processing are vested in spins). The Kyoto physicists, through various improvements, hope to extend their method to room temperatures. (Iinuma et al., Physical Review Letters, 3 January 2000; Select Article; figure at www.aip.org/png)

COSMIC RAYS OBSERVED BY GRAVITY-WAVE DETECTOR at the Frascati Laboratory in Italy consists of a 2300-kg aluminum cylinder cooled to a temperature of 0.1 K. The plan is that a passing gravitational wave (broadcast, say, by the collision of two neutron stars) would excite a noticeable vibration in the cylinder. NAUTILUS has not yet recorded any gravitational waves, but scientists have now witnessed the cylinder vibrated by energetic particle showers initiated when cosmic rays strike the atmosphere. The signal generated by the rays is believable because conventional cosmic-ray detectors surrounding the bar also lit up when they were struck by the particles. In effect the detector is able to discern a mechanical vibration as small as 10^-18 meters, corresponding to an energy deposit as small as 10^-6 eV. (Astone et al., Physical Review Letters, 3 January 2000; Select Article. Contact Giuseppina Modestino, modestino@lnf.infn.it, 011-39-694-032-756.)