Laser Cool instructions


Table of Contents

About the game
Playing the game
How to decelerate atoms

About the game

Laser Cool is a kind of billiard game with some unusal features. Instead of sticks one uses laser beams to push atoms (replacing the balls) into a detector (replacing the holes). The shorter the time you need to "detect" all atoms, the better is your place in the hiscore.

There are some subtleties compared to ordinary billiard: the atoms can only be detected if they are slow enough. And unlike sticks the laser beams become "soft" when the atoms do not have the right velocity.

All these features follow the basic physical laws for cooling and trapping of real atoms. In 1997 the Nobel price in Physics was awarded for clever extensions of this basic scheme. But to play Laser Cool you absolutely need nothing to know about Physics. Just have fun and  try to beat the best time.


Playing the game

When you start the game with the button over the field with the atoms and lasers the atoms begin to move and the timer starts to count. Initially there are 10 atoms and you have to get them all into the detector to score. But be careful: only atoms which are slow enough are "visible" for the detector. This is the case if the atom turns green. You can push an atom by pointing a laser beam on it. The position and orientation of the laser beam can be manipulated by clicking and dragging with the mouse on the field.

When you click and drag the lasers are replaced by squares which indicate their width and direction. Clicking and dragging only affects the "selected" laser. You can select a laser by left-clicking the circle in the laser box. The selected laser has a black border around this circle. By right-clicking on this circle you can switch the laser beam on and off.

The circle in each laser's box can change the color between red, white, and blue. This indicates the detuning of the laser which is important for slowing down the atoms (see "How to decelerate atoms").

Below the field with the atoms and lasers you will find two control fields (one for each laser) which are painted in the same color as the corresponding laser itself. They both contain two scrollbars. The first scrollbar allows to vary the width of the laser beam. Of course a wider laser beam makes it easier to hit the atoms, but there is a drawback: like an ordinary flashlight that is defocused the laser beam becomes less intense as it gets broader. Hence, the atoms are not pushed as hard as they are with a narrower laser beam.

The second scrollbar allows you to adjust the detuning of the laser. This is very much like tuning a guitar string. If the dot displaying the detuning (see "The Laser Cool window") is white, then the laser is not detuned. This is the perfect choice to push resting atoms. If you want to push atoms moving towards (away from) the laser you should use a red (blue) detuning, respectively. The reason for this is explained in "How to decelerate atoms".

Now it's perhaps the best time to play a little bit with the game. If you have problems getting the atoms slow enough to be detected come back and read the following section.


How to decelerate atoms

Since the detector can only "see" slow atoms it is crucial to reduce their speed. This can be done by varying the detuning of the lasers to exploit the so-called Doppler effect. You know the Doppler effect from everyday life: if a police car is moving towards you its siren sounds higher, and if the car moves away from you it sounds lower.

The same happens with the atoms and the laser beams. For the laser beam each atom has a siren attached. If the atom moves towards the laser this siren sounds higher, if the atoms moves away the siren sounds lower for the laser beam.

Now imagine the laser beam to be a billiard stick with a microphone attached. This stick can only push atoms if it "hears" a siren at the correct frequency. If the atoms are moving the frequency of the siren is changed and the stick stops pushing the atoms.

This is where the detuning comes into play. Detuning the laser is nothing but to tell the "stick with the microphone" at which frequency it can push the atoms:

This is all you need to know to get the basic idea of laser cooling: If you have many atoms moving at a high velocity point your laser beam whith a red detuning towards the atoms. Those moving away from the laser are not affected, but those moving towards your laser are decelerated until their siren doesn't have the correct frequency anymore. Thus the mean velocity of the atoms is reduced.

In a real laboratory one uses two red-detuned laser beams with opposite direction. Then the atoms will always move towards one of the lasers so that they are always decelerated. This is the so-called Doppler-cooling of atoms. However, since the atoms of Laser Cool are sitting in a box, this might not be the optimal strategy to beat the best time.


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