Introduction to BEC




 

Dallin Durfee (Ph.D. 1999) wrote this introduction into our research around 1997:

For those of you who aren't familiar with our work, perhaps a few questions need to be answered...



What is the purpose of your atom trap?

We use our atom trap to trap and cool sodium atoms to extremely low temperatures. By cooling the atoms to just 0.000001 degrees centigrade above absolute zero (-273 degrees C), we can study quantum mechanical effects which are completely obscured at higher temperatures. One special phenomenon that we study is called Bose-Einstein condensation.


What does your experiment look like?

It looks like a bunch of vacuum flanges bolted together and then covered with miles and miles of electrical cables and water tubing. Click below for a view of the apparatus.

THE EXPERIMENT

ANOTHER VIEW OF THE EXPERIMENT

Click the icon below to see a close-up of one of the most important parts of the experiment.

CLOSE-UP OF THE EXPERIMENT

The vacuum chamber sits on top of an optics table, and several racks of electronics surround the table. Two computers control the experiment. Next door to the atom trap is a climate controlled room where our lasers and most of the optics for the experiment can be found. Light from argon-ion R6G dye lasers are available from this room. Light from these lasers is split into several beams, conditioned and shuttered, and sent through a tube which connects the laser room with the room that the experiment sits in.


What is quantum mechanics?

Quantum mechanics is a way to describe the laws of the universe by thinking of everything as waves. Quantum mechanics tells us that you, your dog, and the computer that you are reading this on are all waves, just like waves on water or radio waves. If you drop a rock into a pond and look at the wave that it creates, it is difficult to say where the wave begins or ends because it doesn't have sharp edges. In fact, as the wave spreads, it becomes increasingly difficult to say where the wave ends and the undisturbed part of the pond begins. And if we drop two rocks into a pond side by side, when the waves created by both rocks begin to overlap interesting patterns will appear. Some points in the pattern will be motionless as the waves pass through it. At those points whenever the peak of one wave passes over it, the valley of the other wave passes at the same time. The two waves cancel each other out and leave an undisturbed spot on the pond. At other points wave crests from both waves arrive at the same time. These spots fluctuate even more than they would if we had dropped a single rock into the pond. Quantum mechanics tells us that all matter, including me, you and your cat, exhibits these same wavelike properties.

The wave-like nature of matter is very apparent in the structure of atoms. In larger objects, like the ones we deal with in our every day world, the wave-like nature is undetectable. In other words, it is easy to see that an electron orbiting an atom is a wave. But the fact that your cat is a wave is obscured by its mass and thermal energy. The laws of physics that Isaac Newton and Galileo discovered are really only approximations to the more fundamental laws of quantum mechanics.

In our experiment we take a gas of many atoms (about 5 to 10 million) and by lowering the temperature we make the atoms wavelengths very long. This makes our atoms behave in ways that would have startled Sir Isaac.

For a more comprehensive introduction to quantum mechanics there are several good web pages you can read. One is maintained by the University of Exeter. For a simpler and more entertaining introduction to modern physics (including quantum mechanics), head to your local library and check out "Mr. Tompkins in Wonderland," or "Mr. Tompkins in Paperback," by George Gamow (Cambridge University Press 1993 sbn 0 521 44771 2).


How are the atoms trapped?

The atoms start out in an oven which is held at 350 degrees centigrade. These hot atoms are allowed to escape through a hole in the oven and shoot out in a beam traveling at about 800 meters per second (1800 miles per hour). We aim a laser beam in the opposite direction of the atomic beam. The laser beam hits the atoms and slows them down to about 20 meters per second (45 miles per hour) at the center of our vacuum chamber, where a magneto-optical trap (MOT) captures them. The MOT traps the atoms with six laser beams coming in from all directions. These beams push the atoms into the center of the chamber. After collecting a large number of atoms in the MOT, we turn off the lasers and turn on a large magnetic field which confines the atoms magnetically. In the magnetic trap we cool the atoms down to very low temperatures and study them.


How are the atoms cooled?

The technique which we use to get extremely cold atoms is called rf-induced evaporation. It is very similar to the way evaporation works in a cup of hot coffee. A cup of coffee is made up of many molecules flying around and bumping into each other. The temperature of the coffee is just a measure of the average energy that these quickly moving molecules have. From time to time two molecules will collide in such a way that one of the two ends up with most of the energy, sometimes even gaining enough energy to fly out of the cup. Since these molecules are going relatively fast compared to the rest of the molecules, they take with them more than their fair share of energy, and the molecules which are left behind have less energy on average than they did before the fast molecules shot out. For every molecule that is kicked out, the temperature of the coffee decreases a tiny amount.

In our trap we actively induce evaporation using radio waves. Ground state sodium atoms can have several different spin orientations. Atoms in one of the orientations are attracted to weak magnetic fields. These are the ones which are trapped in our experiment. A different spin orientation is attracted to high magnetic fields. Since our magnetic trap has a magnetic field minimum at the center, these "strong-field seeking" atoms are pushed out of the trap. To induce evaporation, we simply use radio waves to flip the spins of the most energetic atoms in the trap. With their spins flipped, they fly out of the trap. Since we only eject the most energetic atoms, they take away more than their fair share of energy. When the rest of the atoms re-thermalize (by bouncing off of each other several times), the net energy per atom has dropped, and the atom cloud is cooler.


What is a Bose-Einstein condensate?

In the early part of this century, as quantum mechanics was just being discovered, it was found that all particles can be divided into two classes. Fermions, named after Enrico Fermi, obey the "Pauli exclusion principle," that no two identical fermions can be in the same quantum state at the same time. This means that fermionic systems will have many energetic particles flying around even as the temperature goes down to absolute zero, since only one particle can be in the lowest energy state.

The other type of particles are called Bosons, named after Satyendra Nath Bose. Bose, an Indian physicist, worked out the statistics for photons (the particles which make up light). Albert Einstein then adapted the work by Bose to apply it to other Bosonic particles and atoms. While doing this, Einstein found that not only is it possible for two Bosons to share a quantum state, but that they actually prefer being in the same state. He predicted that at a finite temperature, almost all of the particles in a Bosonic system would congregate in the ground state. When this happens, the quantum wave functions of each particle start to overlap, the atoms get locked into phase with each other, and loose their individual identity. This phenomena was named "Bose-Einstein condensation." Using this effect it is possible to put a large group of atoms in a single quantum state and study the wave-like nature of matter.

Click on the icon below for a graphical representation of what Bose-Einstein condensation really is.

WHAT IS BEC?

For a time, Einstein's prediction was considered to be a mathematical artifact or even a mistake. Then in the 1930's, while Fritz London was investigating superfluid liquid helium, he realized that the phase transition in liquid helium could be understood in terms of Bose-Einstein condensation. The analysis of liquid helium was muddied, however, by the fact that helium atoms in a liquid interact strongly with one another. For many years now scientists have been working towards the creation of a Bose condensate in a less complicated system. It turned out that laser cooling combined with rf evaporation of alkali atoms was the key to make this possible. In the summer of 1995, BEC was reported by scientists at JILA, followed by similar reports from RICE and from our lab here at MIT. Since that time, all three groups have been busy studying the properties of Bose-Einstein condensates.

Click on the link below for a graphical timeline of the race for BEC.

RACE TO BEC