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The Atom Laser

by John G. Cramer

Alternate View Column AV-84
Keywords: atom laser Bose Einstein condensate quantum interference
Published in the July-1997 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 02/09/97 and is copyrighted ©1997 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

    The hot physics news at the beginning of 1997 was a breakthrough in experimental physics, the demonstration of an atom laser. A team of atomic physicists led by Prof. Wolfgang Ketterle at MIT published papers in the journal Physical Review Letters describing their method and in Science magazine describing their results. The work is an outgrowth of the 1995 success of the Boulder group of Cornell and Wieman in producing a "Bose-Einstein condensate," an ultra-cold aggregation of atoms that are all in exactly the same quantum state. [See my column "Bose-Einstein Condensation, a New Form of Matter," in the March-1996 issue of Analog.]

    The words "atom laser" appearing in a science fiction magazine conjure up visions of death rays and space battles. The real device is rather different in its implications and ultimately is much less likely to be a weapon than a tool for probing the innermost secrets of Nature. Therefore, let's begin by reviewing some basic facts about quantum statistics and the ground rules for the operation of lasers.

    All atomic-scale particles have the property of "spin" or intrinsic angular momentum, like tiny tops that must always spin and never stop. It is reasonable to think that if an object like an atom were rotated by 360o it would return to its original state. In quantum mechanics that assumption leads to a rule which requires spin to be "quantized", in that it must always have a value that is an integer multiple of h/2pi, where h is called Planck's constant (h = 6.626 x 10-34 Joule seconds and pi = 3.14159...). Photons of light, for example, obey this rule by having one h/2pi unit of spin that points either along or against their direction of motion.

    It came as a real surprise when in the 1920's experimental physicists discovered that the electron breaks this quantization rule by having a spin that is 1/2 of an h/2pi unit. This means that electrons (along with quarks, neutrinos, protons, neutrons, and many atoms) are not the same after a 360o rotation and must be rotated through 720o (two full turns) to return to their original states. The distinction between particle with integer spins (like photons) and half-integer spins (like electrons) is very important in atomic physics because the two types of particles have distinctively different statistical properties. The particles with half- integer spin have the statistical behavior first described by Enrico Fermi and Paul Dirac and are called "fermions", while integer spin particles are called "bosons" and show a statistical behavior first described by S. N. Bose and Albert Einstein.

    Fermions behave as highly territorial individualists as described by the Pauli exclusion principle, with only one particle allowed in each quantum state. If one fermion is occupies a particular quantum "box" described by spin orientation, momentum, and position, then all other fermions are Pauli-excluded from that box.

    Bosons, on the other hand, tend to congregate together like gregarious groupies. If one boson is in a particular quantum box, all other bosons are "invited in" to share the same box. The more bosons that pile into the box, the stronger becomes the tendency for others to join them. In such a state, a very large number of particle will have a single quantum wave function. This is a Bose-Einstein condensate, as previously described here. It is a state of matter which had been predicted since the 1920s but which was observed in atomic systems only since 1995.

    The optical laser is a good example of the uses of the grouping-tendencies of bosons. "LASER" was originally an acronym standing for "Light Amplification from the Stimulated Emission of Electromagnetic Radiation." However, the term has now become a part of the language, a noun that refers to any device that produces coherent radiation. Therefore, even if the device emits atoms instead of light, as in the present case, it is still a laser.

    The key phrase in the laser acronym is "stimulated emission", a central property of Bose-Einstein systems. A conventional optical laser operates as follows. By pumping in energy from outside, confined atoms or molecules are all placed in the same long-lived "excited state", so that each is rather like a tiny cocked mousetrap on the atomic scale. Each excited atom or molecule can "fire", producing a new photon and changing back to its lowest state of energy. However, because the excited state is chosen to be long-lived this does not immediately happen. This ensemble of excited atoms or molecules is called a "population inversion".

    The boson "groupie" tendencies of the photons of light means that if an excited atom or molecule is presented with a photon identical to the one it is about to emit, then producing the new photons becomes easier. Thus the photon emission is "stimulated" by Bose-Einstein statistics. The more photons that are present, the larger the stimulation. The stimulation factor S rises as the factorial of the number of photons involved. [S=2!=2 for two photons, S=3!=6 for three photons, S=4!=24 for four photons, S=5!=120 for five photons, S=6!=720 for six photons, etc.].

    This rapidly increasing stimulation factor leads to an avalanching wave of light photons that pass through the ensemble of atoms or molecules and emerge with a large number of photons, all in the same quantum state, all traveling together with the same wavelength and with their spins pointing in the same direction. This is a laser beam. It is possible because photons from previous atomic transitions "invite" other photons to join their state through stimulated emission as they move past, triggering an avalanche of atomic transitions and forming a beam of photons that are all in exactly the same quantum state. Such a beam of photons is called "coherent" because the waves of all the photons are exactly in phase, rising and falling together as they move through space. Coherence gives special characteristics to the beam, including coherent interference, enhanced interactions, and "speckle" formation.

    OK, if that's a normal optical laser is, then what's an atom laser? And in particular, since excited atoms don't emit other atoms, how could we arrange for the population inversion and the stimulated emission necessary for laser action?

    The answers to these questions are provided by the magnetic trap used to produce the Bose-Einstein condensate of atoms. This magnetic trap, made with a pair of loop coils above and below the clustered atoms with oppositely directed currents, produces a 4-leaf clover "quadrupole" field that interacts with the aligned spins of the atoms to hold them in place. In the trap several million sodium atoms are cooled to temperatures well below 2 microkelvins. There the Bose-Einstein condensate of sodium atoms forms, and all atoms go into the same quantum state with identical quantum wave functions.

    The trick that Ketterle's group uses is to perturb a trapped condensate of sodium atoms with radio waves that can be absorbed by the trapped atoms, propelling them to an untrapped state. This then is the population inversion: the trap itself behaves like an excited atom of an optical laser, ready to emit sodium atoms and de-excite, while the untrapped sodium atoms are like the photons emitted from the optical laser. Each new untrapped atom enhances the probability that more atoms in the same quantum state will be released, and a clump of sodium atoms in the same quantum state emerges from the trap. The strength of the radio waves is adjusted so that clumps containing about 100,000 sodium atoms emerge from the trap in a coherent pulse.

    The photons leaving an optical laser emerge traveling at the speed of light, but the untrapped-state atoms that emerge from the Bose-Einstein condensate are at rest and separate from the trapped atoms only because of the pull of gravity. The atom clumps literally drip slowly downwards out of the condensate. The clump of emerging atoms is coherent like the light pulse from an optical laser, but its speed, for the same downward distance of travel, is no greater than a dropped rock would have.

    Having created the conditions for an atom laser that produces a coherent clump of atoms, it was necessary for Ketterle's group to demonstrate that the emerging atoms were truly in a coherent state, that the device was truly a laser, not just a leaky atom-scale trap. They accomplished this by simultaneously producing two Bose-Einstein condensates and perturbing them both with the same radio waves, so that both produced atom-clump drips that fell downward, expanding as they fell. After they had traveled a certain distance downward the clumps began to overlap, and a zebra-striped pattern was observed to appear in the overlap region.

    The zebra-striped pattern is a direct result of quantum interference between the two clumps of atoms, each clump behaving like a single wave. In places where the two wave functions are in phase, rising and falling together, they reinforce and the probability of detecting an atom there is twice as great as if there had been no definite phase relation between the atom's wave functions. In places where the two wave functions are opposite in phase, one rising as the other falls, they cancel and probability of detecting an atom there is zero. The result is a set of bright and dark bands that are an unambiguous signal that two atom lasers have indeed been created and arranged so that their beams can coherently interfere.

    Finally, lets briefly consider the SF implications of this discovery. At least in its initial form, the atom laser with its dripping beams does not remotely resemble the ray gun of space operas. The number of atoms in Ketterle's atom laser pulse, about 10^5 atoms, is very small compared to perhaps 10^24 atoms in a rifle bullet, and the rifle bullet moves at a much higher speed. While the clump of atoms could in principle be speeded up by transport through some acceleration device, the low intensity would limit its potential use as a weapon.

    The new and interesting thing about the atom laser is the coherence of the atoms in its beam. In the next few years we can expect to witness a series of clever experiments exploiting coherence to test and probe various aspects of Nature, particularly in the areas of gravitational and atomic physics.

    And we can speculate a bit on the SF side. One of the most interesting outcomes of the development of optical lasers was the production of holograms, interference patterns that can be used to re-create a three dimensional images of the laser-illuminated object that produced the original interference pattern. Can this be done with atoms? Could one re-create, even mass produce, three-dimensional objects by illuminating recorded interference patterns with coherent beams from atom lasers that crossed and deposited their atoms in the desired pattern? That certainly suggests possibilities for science fiction and perhaps even for real physics.


First Observation of a Bose-Einstein Condensate:

    M. H. Anderson et al., Science 269 198 (1995).

First Operation of an Atom Laser:

    M.-O. Mewes et al., Physical Review Letters 78, 582 (1997); M. R. Andrews et al., Science 275 637 (1997);

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