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Theoretical physics

Why trapped atoms are attractive

Naturevolume 406pages2930 (2000) | Download Citation


Bose-Einstein condensates - ultracold atoms that share the same quantum state - were first created in 1995. Since then, techniques have improved to the extent that theorists now dream of using condensates to model physical systems such as black holes and galaxy clusters.

Before Newton postulated the inverse square law for gravitational force, scientists were already familiar with the inverse square law for light intensity as a function of distance from the source. Today, physics lecturers often introduce students to the gravitational inverse square by comparing it to that of light. A paper from O'Dell et al. in Physical Review Letters 1 suggests that we can now reverse this old analogy, and induce an inverse square attractive force between particles using light itself. This will allow experiments that simulate the gravitational interactions of large numbers of particles. And it illustrates a current trend in the phys-ics of atoms confined in traps — experimental control has become sufficiently strong that one can make clouds of cold, trapped atoms that emulate a surprisingly wide range of physical systems, so we can investigate the physics behind these phenomena under tightly controlled conditions (Fig. 1).

Figure 1: Trapped atoms as a Rosetta stone.
Figure 1


Theorists can now dream of using ultracold matter (left, from ref. 11) to model physical systems ranging from black holes to stellar clusters (right). O'Dell et al. suggest that, with the help of a few lasers, a cloud of ultracold atoms can even be made to simulate gravity.

Experiments with ultracold atoms have accelerated and diversified greatly since the creation of the first Bose–Einstein condensate in 1995. Condensates are samples of ultra-cold atoms that share the same quantum state and therefore have remarkable properties. Although the 1995 experiments were impressive breakthroughs in technology, from the viewpoint of fundamental theory, Bose–Einstein condensation itself was 70-year-old news. Today, however, condensates can be made to do things Bose and Einstein never imagined.

This versatility is what makes condensates so exciting, and which allows us to do things that were previously impossible, such as tuning the interactions between particles. Atomic properties can be changed in many ways, by applying laser beams or magnetic fields, but previous techniques were limited to strengthening, weakening or possibly reversing the natural short-range forces between atoms.

O'Dell et al.1 now propose bathing a cloud of trapped atoms in laser light to induce a long-range attractive force between each and every other atom. A static electric field will induce a force between atoms that decays with the inverse fourth power of their separation. But in the oscillating field of a light beam, there is an additional force that falls away as the inverse square. The sum of these forces may be either attractive or repulsive for a given pair of atoms, depending on the angles between the atoms and the electric field. But by shining a whole battery of lasers on the sample from many directions, one can effectively average over all angles, to give a net attraction that is purely inverse square. (The need for so many lasers is a practical obstacle to carrying out such an experiment, but it will hardly be a permanent barrier.)

To prevent the laser fields from simply destroying the cloud of trapped atoms, the laser wavelengths must be much longer than the size of the sample, and their frequencies must be far from any resonant frequencies of the atoms. Under these conditions one can safely use intense laser fields to create a strong artificial ‘gravity’ between the particles that is many orders of magnitude greater than their genuine gravitational attraction.

The authors show that a Bose–Einstein condensate subjected to this artificial gravity-like interaction will exhibit interesting phase transitions at low temperature, and may even remain bound together under its own ‘gravity’, with the external trapping field turned off. (In this case, ‘normal’ gravity limits the duration of the experiment, as the atomic cloud falls out of the laser field. But the US space agency NASA is already considering proposals to run trapped-atom experiments in orbit, to avoid such problems.) This self-bound state will allow researchers to simulate aspects of stellar physics that are difficult to examine in the laboratory. And one can expect further possibilities in this direction, such as simulations of galaxies and star clusters.

In the past year, there have been several other proposals to use light fields to persuade cold, trapped atoms to model the physics of quite different systems. Bose–Einstein condensate analogues have been suggested for cosmic strings (structural defects) formed in the early Universe2, superconductor– insulator phase transitions3 and Josephson oscillations4 in solid-state systems, and general relativistic black holes5. The reason such schemes are now thinkable is that the necessary experimental techniques have become available. Experimenters can choose the way their atoms interact6, project them in coherent beams7, load them into optical lattices8, and efficiently ‘engineer’ them into exotic collective states9,10. Decades of technical work are now yielding a profound control over matter and light. Ingenious simulations, such as that proposed by O'Dell et al., will be useful benchmarks as this technology advances further, as well as being one of its most promising applications.

There is no way that millions of trapped atoms — interacting quantum mechanically and nonlinearly — will merely duplicate a theoretician's model, so the proposed models are genuine experiments. But they are extremely controlled experiments in which we can examine the principles involved in complex phenomena in a greatly simplified medium. Such highly controlled analogues can serve as Rosetta stones, in which (as it were) the message of an undeciphered script appears duplicated in a familiar language, so that we can begin translating. One hopes that this basic strategy will be extended to more and more scientific questions as techniques improve.

There will, of course, be vital aspects of many natural phenomena that will simply be lost in this kind of simplification. One will not find the keys dropped in a dark alley under the lamppost just because the light is stronger there. But it is worth trying to shine the light a bit further, as it seems we now can, because we can see so much more in strong light. We can even see gravity.


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  1. Institute for Theoretical Atomic and Molecular Physics, 60 Garden Street, MS 14, Cambridge, 02138, Massachusetts, USA

    • James Anglin


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Correspondence to James Anglin.

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