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Atomic and molecular

Atomic and molecular

Thinking small

01 Mar 1997

As Physics World went to press in mid-February, the DESY laboratory in Hamburg, Germany, was playing down reports that evidence for new physics – rumoured to be supersymmetry – had been found in high-energy scattering events at the HERA collider. Conclusive proof of new physics beyond the Standard Model of particle physics is the dream of physicists working at existing accelerators, or those making plans for new ones. Confirmation that nature is supersymmetric would be an awesome step forward, although the high-energy approach is not the only route to new physics. For example, there is one school of thought which believes that radically new physics will also emerge in condensed-matter systems as more complex materials are synthesized and studied. But what comes after supersymmetry? If this fundamental symmetry between fermions and bosons is not observed at HERA or other accelerators currently operating, it should be seen at the Large Hadron Collider ten years from now. What then? The Planck energy – the energy at which all four fundamental forces are expected to be unified into a “theory of everything” – is so remote from the energies of planned experiments that some observers have taken to calling these theories “ironic” or “post-modern” in the sense that there is no chance of confrontation with experiment. The Planck energy is about 1019 GeV, whereas the optimistic accelerator physicists are currently thinking of energies around 105 GeV. In “Atom interferometry, spacetime and reality” Ian Percival suggests a short cut to the Planck scale based on atom interferometry. There is no guarantee that this approach will work and it will certainly not provide the sort of information that would come from colliding particles at Planck energies. But it could help to distinguish between various quantum theories of gravity, themselves a key ingredient in any theory of everything. These experiments might also help decide between the different extensions to quantum theory that are needed to connect the quantum and classical worlds. Percival’s proposal is to probe physics on the Planck scale – at times around 10-43 seconds – by detecting the effects of fluctuations in spacetime itself at much longer times (about 10-25 seconds). This is essentially the approach adopted by Einstein to establish the reality of atoms based on Perrin’s studies of Brownian motion. The idea might sound off the wall, but so did Casimir’s predictions about forces arising from fluctuations of the vacuum, and these have just been measured. We can also take heart from recent experimental progress in related areas of atomic physics. In “Atomic physics in ion traps” Christopher Monroe and John Bollinger describe experiments in ion traps that are studying the quantum properties of single particles. Trapped ions can be used to test the fundamental symmetries of physics, and this work could also be laying the foundations of quantum computers. The traps are also versatile enough to confine plasmas containing half a million ions. These strongly coupled plasmas have parallels with astrophysical plasmas and condensed matter systems.

In “Bose-Einstein condensation” Christopher Townsend, Wolfgang Ketterle and Sandro Stringari report on recent advances in Bose-Einstein condensation, the process by which large numbers of laser-cooled in a magnetic trap atoms are nudged into a single quantum state. Besides being a model system in many-body physics, this research has also yielded a rudimentary atom laser that, like its optical antecedent, offers numerous opportunities for further fundamental research and applications.

What atom interferometers, ion traps and magnetic traps have in common is that they involve the arrangement of conventional components – lasers, magnets, radio-frequency sources and so forth – on laboratory benches by small groups of researchers. The experimental know-how required is not trivial, and the theory is not easy, but if a laser-like beam of atoms could be directed from a Bose-Einstein condensate into an atom interferometer, the results could be spectacular.

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