Hotter condensates

Hotter condensates　2007-09-07 11:11

To create Bose–Einstein condensates, physicists usually cool atomic gases to within a few billionths of a degree of absolute zero. Ryan Balili and David Snoke explain how these macroscopic quantum systems can in fact be explored at relatively high temperatures using polaritons

According to quantum mechanics, an object’s wave nature allows it to pass through a barrier that would be utterly impenetrable classically. And yet any normal person would hesitate in trying to run through a solid wall. If they did, the best they could hope for would be to bounce off the wall unharmed.

So why do we not observe tunnelling and other quantum phenomena in everyday life? The reason is that these phenomena only take place at the scale of the wavelength of the atoms that make up a macroscopic object, and these wavelengths are far too small for the effect to be visible. Equal to Planck’s constant (h) divided by momentum, the de Broglie wavelength of a typical atom at room temperature is about 10–22 m.

To observe a particle’s wavelike behaviour we must lower its momentum. If the momentum of a group of particles is so low that the wavelength of the particles becomes comparable to the distance between them, the wave functions of the individual particles start to superpose constructively, or “add up”. The highly ordered state that results is known as Bose–Einstein condensation, in which all the particles behave as a single wave. This effect can only take place among particles known as “bosons”, which have integer values of angular momentum, or spin.

Since the creation of the first Bose–Einstein condensate (BEC) in an atomic gas 12 years ago, physicists have tended to achieve these extremely small momenta by cooling the particles (and thereby lowering their velocities). But the temperatures needed are extremely low – generally just a few billionths of a degree, which requires very sophisticated refrigeration techniques involving laser cooling. An alternative, which is now being pursued by several labs around the world, is to create a particular kind of very light particle.

In the September issue of Physics World , Ryan Balili and David Snoke see how this particle, called a “polariton”, is helping physicists to understand the quantum nature of individual particles and to invent new technologies, such as low-power lasers.

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