Atom lasers

Soon after the invention of the laser in the late 1950s many dubbed the discovery as "a solution in search of a problem". Nowadays lasers are used in an enormous range of scientific and technological applications, thanks to their high intensity and their ability to emit light in an extremely narrow range of wavelengths. Indeed, the laser has revolutionized the whole field of optics and now plays a central role in the world's communications networks. The market for lasers, optical amplifiers and other optoelectronic components is worth billions of dollars every year.

Atomic physicists are hoping that the invention of the "atom laser" will spark a similar revolution in the field of atom optics, or matter-wave optics as it is also known. Researchers in this field have relied heavily on the analogy between light and the wave-like nature of atoms. Lenses, mirrors and beam splitters have all been developed to control atomic beams just as their optical counterparts manipulate light. But what has been lacking in atom optics, until recently, is a source capable of producing an intense, highly directional, coherent beam in which all the atoms have the same wavelength just like the photons in a laser beam. This would be an atom laser.

Bose Einstain Condensate

The Bose Einstein Condensate is a group of atoms at the same energy level. The BEC can only be formed at temperatures as low as a few billionths (0.000,000,001) of a degree above Absolute Zero. Atoms will be at different energy levels at normal temperatures, but once the temperature goes down below a certain threshold a large fraction of the atoms will crash down to the lowest energy level. This results on a unique state as an atom in the lowest energy level is spread out a little. As the atom's energy level keeps falling atoms cannot be differentiated from one another and the BEC is formed. All the atoms are in the same place. The atoms are not really spread out, what is really meant is that the possible location of the atom is in that area, the lower the energy level of the atom the bigger the area. The reason for this is due to the uncertainty principle. The Uncertainty Principle states that the more precisely the momentum is determined, the less precisely the position is known in this instance and vice versa. As the atoms get colder their average speed decreases so the momentum is known more precisely therefore the position of the atom becomes more uncertain, leading to the "spreading out" of the atom.

For the Bose Einstein Condensate to form the temperature must be very low. The temperature might be hard to achieve but the difficulty is in the technique and not the equipment that is quite simple and inexpensive compared to many current physics experiments. It cost $50000 to $100000 only.

To first cool the atoms down lasers are used to slow down the atoms, since the slower the atoms are the cooler they are, the laser light can cool down an atom by bouncing off it with more energy than when it hits it. The lasers have to be tuned to a specific frequency for it to affect the atoms or it will not slow the atoms down. The hardest part of laser cooling was to find a way to allow the laser to affect fast moving atoms while not affecting slower ones. The atoms movement and sensitivity to certain frequency was made used of. The laser frequency is adjusted so that the Doppler shift of a fast atom will make the frequency appear just right while a slow atom will not be affected. By using lasers as "Optical Molasses" the atoms are trapped where the lasers crisscross as if they move in another direction they will be hit by more light and move back into the center. There is also a magnetic field which makes the atom feel more push the further away it is from the center, thus the atoms will be contained in the center of the laser beams. The atoms will also be unable to get heated up by other atoms bumping into them as the vacuum pump attached to the apparatus removes all the atoms other than the rubidium atoms that are supposed to get trapped.

However laser cooling cannot cool an atom below a certain temperature as each photon delivers a certain kick to the atom and thus the atom cannot be slowed down anymore that a single photon's kick. Thus another form of cooling was required. It is known as evaporative cooling, basically it is a magnetic trap which makes use of the slight magnetism of each atom to contain the atom. Evaporative cooling works by letting the atoms with the most energy escape from the other atoms thus lowering the average speed of the atoms that lowers the temperature. Evaporative cooling reduces temperature but reduces the amount of atoms too. The BEC is formed when there are enough cold atoms left in the trap. Thus if the trap is not properly controlled and too many atoms are released there will be no BEC. Once evaporative cooling lowers the temperature enough BEC is formed.

The Bose Einstein Condensate is like a laser if atoms are light, it has very special properties which can make it very useful in the future but until it can be produced more easily it will most likely have no useful applications in the near future due to it fragile state. The BEC could be used in very accurate measurements and more interestingly because of its capability to slow light tremendously, the BEC could also be used in optical computing, computers that work on streams of photons instead of electrons.

Atom-laser applications

The possibility of producing a coherent beam of atoms that could be collimated to travel long distances, or brought to a tiny focus like an optical laser, opens up a whole host of applications. Although it would be imprudent to try to predict all of the applications that will arise, there are reasons to believe that the atom laser will be a significant scientific tool in the future. Atom lasers may have a major impact on the fields of atom optics, atom lithography, precision atomic clocks and other measurements of fundamental standards.

One application for which the coherence of an atom laser is critical is atom holography. Just as conventional holography uses the diffraction of a photon beam to reconstruct a 3-D image, atom holography uses the diffraction of atoms. As the de Broglie wavelength of the atoms is much smaller than the wavelength of light, an atom laser could create much higher resolution holographic images. Atom holography might be used to project complex integrated-circuit patterns, just a few nanometres in scale, onto semiconductors.

The first atom holograms were demonstrated in 1996 by Fujio Shimuzu and colleagues at the University of Tokyo, in collaboration with Jun-ichi Fujita at NEC Research, using laser-cooled atoms. However, the matter waves in these experiments were only partially coherent because the atoms were not all in the same quantum state.

In the case of laser-cooled gases, the level of coherence needed to create a hologram is achieved by selecting a small portion of the atoms. Although this increases the spatial coherence of the matter waves, it is at the expense of the flux or number of atoms available for the duration of the experiment (which is often determined by the overstretched patience of the graduate student). The problem would be alleviated by having a source in which most of the atoms are in the same quantum state. An atom laser is such a source and could provide a much more intense and fully coherent beam of atoms.

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Holography is a two-step process. First, a hologram - a sort of diffraction grating containing information about the object - is produced. Then a beam of light (or atoms) is diffracted by the hologram to form the image. In optical holography, the hologram is often made by interfering a laser beam with light that has been reflected from an object. The resulting diffraction pattern is recorded on photographic film. However, the diffraction pattern can also be generated by computer, so that an image can be formed without ever actually using an object.

In the atom-holography experiments, an image has been created by diffracting a coherent beam of atoms through a grating that was manufactured using electron-beam lithography. The image was recorded on a "microchannel plate" - a detector that is sensitive to atoms. So far, atom holography has been able to produce 2-D images, and not the familiar 3-D ones of optical holography.

A related application, which might also benefit from a source of coherent matter waves, is atom interferometry. In an atom interferometer an atomic wave packet is coherently split into two wave packets that follow different paths before recombining. The interference pattern created when the two wave packets recombine tells us something about the phase difference between the two paths. Atom interferometers that are more sensitive than optical interferometers could be used to test quantum theory, and may even be able to detect changes in space-time (see Physics World March 1997 pp43-48). This is because the de Broglie wavelength of the atoms is smaller than the wavelength of light, the atoms have mass, and because the internal structure of the atom can also be exploited.

Until now, all atom-interferometry experiments have used thermal atomic beams, analogous to the lamps that were used in the early days of optics experiments. Filtering is typically used to reduce the energy spread of the beam and achieve the degree of coherence needed to see the interference effects. The coherence length, however, is short, limiting the use of atom beams to interferometers that have "arms" of equal length - otherwise the interference pattern would be washed out.

Atom lasers would allow the use of devices with unequal path lengths, such as Michelson interferometers. Such devices may provide a way to measure lengths over large distances with unprecedented precision.