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Physics World

Bouncing atoms promise new quantum playground

Christopher R. Ekstrom

Published Feb 1, 1994• 0 citations
Publisher

Abstract

Almost everyone has spent hours as a child bouncing rubber balls on hard surfaces. Later, we learn that the ball is made up of tightly bound atoms. The same game has been played by physicists at the Ecole Normale Superieure (ENS) in Paris who have bounced a ball of trapped, cold caesium atoms more than eight times on a curved mirror created with laser light (C G Aminoff et al. Phys. Rev. Lett. (1993) 71 3083). This work could lead to experiments in which quantum collective effects of atoms play a major role.

DOI
10.1088/2058-7058/7/2/28

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Almost everyone has spent hours as a child bouncing rubber balls on hard surfaces. Later, we learn that the ball is made up of tightly bound atoms. The same game has been played by physicists at the Ecole Normale Superieure (ENS) in Paris who have bounced a ball of trapped, cold caesium atoms more than eight times on a curved mirror created with laser light (C G Aminoff et al. Phys. Rev. Lett. (1993) 71 3083). This work could lead to experiments in which quantum collective effects of atoms play a major role.
Quantum mechanics is the physics of the very small and the very cold. When particles are small and cold they take on wave properties and thus act differently to anything you can imagine in the world you see around you. Throwing tennis balls through brick walls, walking through two adjacent doors at the same time, even having a cat that is both dead and alive at the same time might seem weird to you, but in quantum mechanics this is quite normal. It is this strange playground of physics that has attracted people to quantum mechanics, and the advent of cold atom technologies allows us to, not only theoretically but physically, study these weird systems. In recent years, cold atoms have provided an excellent testbed for investigating these quantum effects. As the system is cold, it is incredibly clean and noise-free due to the lack of thermal vibrations and collisions ...
The ability to juggle atoms may lead to a new breed of atomic clocks with unrivalled accuracy. By launching ultra-cold balls of caesium atoms in rapid succession, it should be possible to measure the frequency of atomic transitions with exceptional precision. Atomic transitions in caesium are currently used to define the fundamental unit of time and form the basis of atomic clocks, but existing caesium clocks use a beam of fast atoms emitted from an oven. While the clocks have an accuracy equivalent to one second in every 3 million years, atomic-clock experts believe that the precision has reached its limit.
Physicists often like talking about atoms colliding together like billiard balls. In reality, things are very different. Collisions that occur between atoms at temperatures near absolute zero, for example, have some highly unusual properties. These collisions are no longer governed by classical physics, but by quantum mechanics. Studying such collisions is important since cold atoms are beginning to be used in a range of experiments, from precision measurements of physical constants to atom optics and atom interferometry.
New techniques of laser cooling have produced atoms with extremely low temperatures; Christophe Salomon and colleagues at the Ecole Normale Supérieure (ENS), Paris, have just measured caesium atoms with temperatures of only 2.5 ± 0.6 μK, the lowest kinetic temperature yet achieved (1990 Europhys. Lett. 12 683). This impressive experimental achievement is a good example of physics in action since it is the elegant and clear theoretical explanation of the underlying physics of the laser cooling processes, developed at the ENS, which has enabled them to push further than anyone else.
For many years atomic-collision experiments basically involved measuring the deflection of fast projectiles that had passed through gaseous targets, or measuring the amount of light given off by atoms and molecules as they were bombarded with different projectiles. In recent years, however, the atomic-physics community has aspired to much more: we want to prepare a target of non-interacting atoms or molecules in a particular quantum state, strike it with projectiles of perfectly known speed, direction and internal state, and then record the time, frequency, speed, energy, direction, spin and internal state of every fragment that emerges from the collisions.