Section 3: A Sound Design for Slowing Light
Our objective is to slow light from its usual 186,000 miles per second to the speed of a bicycle. Furthermore, we can show that light can be completely stopped, and even extinguished in one part of space and regenerated in a completely different location.
To manipulate light to this extreme, we first need to cool atoms down to very low temperatures, to a few billionths of a degree above absolute zero. As we cool atoms to such low temperatures, their quantum nature becomes apparent: We form Bose-Einstein condensates and can get millions of atoms to move in lock-step—all in the same quantum state as described in Units 5 and 6.
Achieving these low temperatures requires more than the use of a standard household refrigerator: We need to build a special atom refrigerator.
Figure 5: Sketch of experimental setup used to create ultracold atoms.
Source: © Brian Busch and Lene Hau. More info
In the setup we use in our laboratory, most parts have been specially designed and constructed. This includes the first stage of the cooling apparatus: the atom source. We have named it the "candlestick atomic beam source" because it works exactly like a candle. We melt a clump of sodium metal that we have picked from a jar where it is kept in mineral oil. Sodium from the melted puddle is then delivered by wicking (as in a candle) to a localized hot spot that is heated to 650°F and where sodium is vaporized and emitted. To create a well-directed beam of sodium atoms, we send them through a collimation hole. The atoms that are not collimated are wicked back into the reservoir and can be recycled.
Figure 6: The atom source in the experimental setup is specially designed and called the "candlestick atomic beam source."
Source: © Hau Laboratory. More info
This atom source produces a high-flux collimated beam of atoms—there is just one problem: The atoms come out of the source with a velocity of roughly 1500 miles per hour, which is much too fast for us to catch them. Therefore, we hit the atoms head-on with a yellow laser beam and use radiation pressure from that laser beam to slow the atoms. By tuning the frequency of the laser to a characteristic frequency of the atoms, which corresponds to the energy difference between energy levels in the atom (we say the laser is resonant with the atoms), we can get strong interactions between the laser light and the atoms. We use a laser beam with a power of roughly 50 mW (a typical laser pointer has a power of 1 mW), which we focus to a small spot at the source. This generates a deceleration of the atoms of 100,000 g's, in other words 100,000 times more than the acceleration in the Earth's gravitational field. This is enormous; and in a matter of just a millisecond (one-thousandth of a second) —and over a length of one meter—we can slow the atoms to 100 miles per hour.
At this point, we can load the atoms efficiently into an optical molasses created in the middle of a vacuum chamber by three pairs of counter-propagating laser beams. These laser beams are tuned to a frequency slightly lower than the resonance absorption frequency, and we make use of the Doppler effect to cool the atoms. The Doppler effect is familiar from everyday life: A passing ambulance will approach you with the siren sounding at a high pitch; and as the ambulance moves away, the siren is heard at a much lower pitch. Moving atoms bombarded from all sides with laser beams will likewise see the lasers' frequency shifted: If an atom moves toward the source of a laser beam, the frequency of the laser light will be shifted higher and into resonance with the atom; whereas for atoms moving in the same direction as the laser beam, the atoms will see a frequency that is shifted further from resonance. Therefore, atoms will absorb light—or photons—from the counter-propagating beams more than from the co-propagating beams; and since an atom gets a little momentum kick in the direction of the laser beam each time it absorbs a photon, the atoms will slow down. In this way, the laser beams create a very viscous medium—hence the name "optical molasses" in which the atoms will slow and cool to a temperature just a few millionths of a degree above absolute zero. We can never reach absolute zero (at -273°C or -460°F), but we can get infinitely close.
Figure 7: Running the experimental setup requires great focus.
It may be surprising that lasers can cool atoms since they are also being used for welding, in nuclear fusion research, and for other high-temperature applications. However, a laser beam consists of light that is incredibly ordered. It has a very particular wavelength or frequency and propagates in a very particular direction. In laser light, all the photons are in lock-step. By using laser beams, we can transfer heat and disorder from the atoms to the radiation field. In the process, the atoms absorb laser photons and spontaneously re-emit light in random directions: The atoms fluoresce. Figure 7 shows a view of the lab during the laser cooling process. Figure 8 shows the optical table in daylight: As seen, many manhours have gone into building this setup.
Figure 8: A view of the optics table.
Source: © Hau Laboratory. More info