Bose-Einstein Condensate

On March 1^st and 2^nd I attended two lectures by the physics Nobel laurite (2001) Wolfgang Ketterle on Bose-Einstein condensates and his current research. Most of this post comes from the notes I took at his lectures, textbooks, common sense and various online articles.

Most people know at least three states of matter: solids, liquids and gasses. From popular science and TV some even know a state of matter like plasma. To make plasma, normally a gas is superheated (among other things). Now, if instead we cooled the gas to extremely cold temperatures, we would get something else. That something else (for bosonic atoms) is known as a Bose-Einstein condensate.

Bose-Einstein Condensates

Before the quantum revolution in physics, atoms were thought of as particles and nothing more. Atoms were thought to follow the rules of classical mechanics and the field of physics was regarded as dead. However, scientists like Bohr, Einstein, Heisenberg and Schrödinger brought life back to physics by seeing the holes that classical mechanics left on the atomic level, and proposing the theory of quantum mechanics. Among the countless new principles that came from quantum mechanics, came the realization that atoms (and all other matter, for that matter) display wave like properties. Normally wave like properties are not observed in atoms, unless they are standing still, or close to it. To achieve that, the free energy of atoms has to be eliminated by cooling.

Cooling is precisely what Wolfgang Ketterle at MIT and his colleagues from other universities accomplished in 1995 when they formed the first BEC. Sodium gas was cooled until it reached the nanokelvin range and at that temperature the waves of individual sodium atoms overlapped so much that they formed one large matter-wave known as a Bose-Einstein Condensate. The men responsible for displaying the condensate Einstein and Bose predicted in 1924/25 received their Nobel Prizes in 2001 and currently continue their work on BECs, superfluids and fermonic condensates.

To easily picture a BEC it is best to think about lasers and light bulbs. A light bulb emits various wavelengths in various directions. A laser does its best to emit one continuous wave. Standard gasses are like a light bulb, they are randomly bouncing around in random directions. A BEC is like a laser, it is one continuous matter wave.

Temperature, Cooling and Measurement

We all have a basic understand of temperature from experiencing it first hand. Winter is cold, summer is hot. But what does temperature really mean? Under physical rules temperature is just another measure of energy; a measure of free energy in a system to be more precise. In a gaseous system, almost all of the free energy is expressed as kinetic energy, so when a gas is cooled it looses kinetic energy. At room temperature air molecules move at around 300m/s. At the temperatures achieved in the MIT-Harvard Center for Ultracold Atoms these same molecules would move at around 0.001m/s. In other words: it is so cold you might not even make it to the mall.

The record temperature achieved by MIT is 450 picokelvin (4.5*10^-10 Kelvin). Comparatively the temperature of outer space is around 3 Kelvin and room temperature is around 300 Kelvin (273.15 Kelvin = 0 Celsius). Basically, the temperature inside the experimental chamber is about a trillion times colder than your room.

Cooling atoms to such a low temperature poses many technical challenges. Ketterle’s team overcomes these challenges by laser and evaporative cooling the system. The sample is held inside an evacuated vacuum chamber by a magnetic field while lasers, microwaves and other magnetic fields are used to cool the system.

Laser cooling is the first step. Intuitively we perceive that shinning a light on something makes it hotter. That phenomenon is based on absorption of light. In laser cooling the light is bounced off the atoms in such a way that it has more energy leaving than it did entering. That seems impossible at first, but due to the Doppler Effect (also known as blue-shift) it is possible. The light is tuned to a frequency slightly below one that the atoms can absorb, and thus atoms can only absorb light when they are flying towards the light (when the light is blue-shifted to the right frequency by their movement). The atoms release the photon right after at the proper frequency, and thus energy is lost. The process looks very exciting. Sadly laser cooling can not take the atoms to the nanokelvin temperatures, so evaporative cooling has to be used.

For evaporative cooling, Ketterle states a very good metaphor: a coffee cup. Everyone knows what happens to a cup of coffee if it is left out on the table for a long time: it cools down. One of the reasons the coffee cools down, is because all the atoms are bouncing around inside of it, and atoms with high-kinetic energy are more likely to get enough energy to fly out of the cup. Thus, over time more and more atoms with higher than average kinetic energy are lost and the system cools.

If you are in a hurry, and you need to cool the coffee, then you blow on it. Blowing agitates the system more and helps speed the process of high-energy particles escaping. Evaporative cooling at MIT follows a similar principle. The sample is put in a “cup” and then the machine “blows” on it.

The atoms are suspended in a bowl shaped magnetic trap. The particles of various energies bounce around, and higher energy particles are more likely to reach the corners of the trap. At the corners, other magnetic fields or microwaves move the escaping particles away from the main system and thus the sample is quickly cooled to nanokelvin temperatures.

Once everything is cooled down, the scientists need to actually have a way to measure the temperature. Since it is impossible to just stick a thermometer into the system, another way needs to be used. Ketterle’s team simply uses gases’ property of expansion to measure an exact kinetic energy of the particles and thus the exact temperature. The magnetic field is turned off, and the sample starts to drop towards the ground and expand in all directions. The team takes a shadow picture of the sample as it drops and then records the temperature based on expansion of the system. This method is extremely precise and accurate, but sadly the sample is destroyed,

How to Know a BEC When You See One

The way a BEC is detected by the researchers is based on the density of the sample when it is dropped, and how it disassociates. When a standard gas sample is dropped it expands more or less evenly and has a density distribution that looks like a three dimensional bell curve. When the gas has transformed into a BEC it assumes the shape of the container (even though it does not expand to fill the whole container) and it is much denser. The whole process is recorded and analyzed with black and white shadow photography. If a BEC is present, then a sharp peek in density is observed.