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ARE BOSE-EINSTEIN CONDENSATES SUPERFLUID? Previously physicists have demonstrated that Bose Einstein condensates (BEC is created when trapped atoms are chilled so low that they begin to overlap) constitute a single macroscopic quantum state, which implies superfluidity. However, physicists would like to see frictionless flow more directly. Two new experiments pave the way toward this goal. A NIST/Colorado group has observed quantized vortices in a condensate of rubidium atoms, while an MIT group has observed that excitations can move through a condensate of sodium atoms and lose little or no energy if the velocity is below a certain critical value. In the Colorado/NIST work the BEC state consists of atoms residing in two separate spin states (referred to as 1 and 2). Using microwaves and a separate probe laser beam working at the fringe of the condensate, the spins of 1-state atoms are flipped, turning them into 2-state atoms in one sector of the condensate after another. This sets a vortex of 2-state atoms into motion around the outer part of the condensate while 1-state atoms remain at rest at the core of the vortex. Thus the vortex is like a smoke-ring of 2-state atoms (with a filling of 1-state atoms) rotating about every 3 seconds. Furthermore, it has exactly one unit of angular momentum. Meanwhile the MIT group uses a focused laser beam to punch a hole in the BEC blob (the light repels atoms from its focus) and then scans the hole along at various speeds. The moving hole is equivalent to a moving object. Below a scan velocity of about 2 mm/sec, no energy dissipation was observed.
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The existence of such a critical velocity for frictionless motion is an attribute of superfluidity. One reason for this kind of BEC research, other than for studying fundamental aspects of a novel form of atomic matter, is that it might afford a new way of learning about superfluidity and superconductivity.
SEPARATING CHEMICAL ISOTOPES WITH A TABLE-TOP TERAWATT LASER has been demonstrated by researchers at the University of Michigan, providing a more compact alternative to the bulky techniques for extracting isotopes, and introducing a new method for making ultrapure thin films, which can be used, in electronic devices. Using a technique known as chirped pulse amplification, University of Michigan researchers produced laser pulses that deliver between 10 trillion and 1 quadrillion watts (10 1000 terawatts) of power per square centimeter for an extremely short instant between 150 and 200 quadrillionths (10 15) of a second. Aimed at a target inside a vacuum chamber containing the isotopes of interest, the pulse vaporized some of the isotopes, which escaped in the form of ions (charged atoms). Intense magnetic fields associated with the pulses exerted forces on the ions, which deposited them at different locations on a nearby silicon disk depending on the isotopes weight. With their technique, the researchers separated boron-10 from boron-11 and gallium-69 from gallium-71. Its an open question if their technique will be feasible on the large scales required for separating isotopes at nuclear facilities, but the researchers are initially setting their sights on other applications, such as depositing pure thin films of isotopes directly onto microelectronic devices.
CHILLING MIRRORS WITH LIGHT. In astronomy the effect of atmospheric turbulence on the quality of images acquired by ground-based telescopes can be greatly reduced by adaptive optics, a corrective process in which parts of the telescope mirror are flexed mechanically by piezoelectric motors in an amount typically equal to a fraction of the wavelength of the incoming light.
In interferometric measurements adjustments in mirrors are also desirable, not because of turbulence in the intervening medium but because of thermal noise in the mirror itself. The LIGO and VIRGO interferometers, searching for gravity waves, need very still mirrors, the better to observe the flexing of spacetime on a scale far smaller than the size of an atom. A new technique might help in this regard. Physicists at the Ecole Normale Superieure and Universit-P.et M. Curie in Paris, can measure the thermal agitation of mirrors and reduce this unwanted noise by a factor of 20, with pressure from laser light. This corresponds to a spatial sensitivity of the mirror at a level of a billionth of an angstrom.
COUNTING UP TO 100 MILLION. The science of measurement, metrology, has been moving away from standards based on artifacts such as a meter stick and toward the use of quantum phenomena to provide reliable, accurate and, if possible, portable calibrations that can be used by researchers in the field. Examples are resistance defined inn terms of the quantum Hall effect and voltage in terms of the Josephson effect. Consider capacitance, the measure of how well a tiny reservoir can store electrical charge. NIST already has the best capacitance standard, accurate to 0.02 parts per million (ppm). But this device is cumbersome and, more importantly, its accuracy is frequency dependent. For rendering the value of capacitance in circuits operating outside a certain frequency range, the standard is no better than 2 ppm. A promising new approach to capacitance (pioneered at NIST) uses a single-electron transistor (SET), which contains at its heart a tiny refuge for electrons where the arriving charges can be counted one at a time, all the way up to 100 million or more. When combined with an accurate voltage measurement this becomes an accurate capacitance standard (C=Q/V). The SET approach has now achieved a measurement accuracy of about 2 ppm, and the NIST researchers hope soon to reach 0.1 ppm. The setup is relatively portable and its output is largely independent of frequency.
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QUANTUM COOL. Physicists at Simon Fraser University in Vancouver are trying to get electrical circuits to cool themselves electrostatically. To do this they employ both quantum and classical physics. First, the classical: a gas can cool down by pushing against a piston; some of the gass thermal energy is converted into mechanical energy. Second, the quantum: electrons flowing from one GaAs layer into another via another a thin layer of AlGaAs will move with optimum efficiency if the electron energy matches a preferred resonant energy in the AlGaAs layer. This three-layer setup, called a quantum well, is at the heart of grocery-store laser scanners and CD players. As circuitry shrinks, disposing of waste heat from even tiny electric currents becomes an ever greater problem. The Simon Fraser researchers are proposing that the electrons in a quantum well cool themselves by moving against not a piston but against an opposing electric field, a field in addition to the one moving the electrons through their circuit. This way of combining the quantum (the electrons as waves tunneling through a thin layer) and the classical (the electrons as a working fluid in a sort of Carnot heat engine) might lead to a completely new category of microelectronic quantum device.
B
