What is the actual way you cool atoms to a low enough temperature that you can observe their quantum behavior at a macroscopic level, like in the Bose-Einstein Condensate?


After reading that, and seeing videos about Bose-Einstein Condensation, I can understand it at a high level, but I want to know how they actually do the experiment in detail. How do you isolate the atoms (like in that experiment above, "two thousand rubidium-87 atoms to below 170 nK"), and know that they are cooled to that specific temperature? What are they using to measure that temperature?

I can see how theoretically this could work out, but I want to know practically how they actually did the experiment (in words that a non-PhD physicist can understand). Because as a non-experimental physicist, I have a hard time imagining that the machines/materials they are using to lower the temperature and do all the measurements aren't somehow coming into the equation and influencing things, so to me it seems almost impossible haha.

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    $\begingroup$ One technique is laser cooling en.wikipedia.org/wiki/Laser_cooling, where light is used to carry thermal energy away from an electromagnetically suspended cloud of these atoms. $\endgroup$ – CuriousOne Sep 12 '14 at 4:35
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    $\begingroup$ And after laser cooling you do evaporative cooling - you loosen trap and "hot" atoms just fly away. $\endgroup$ – Jarosław Komar Sep 12 '14 at 4:50
  • $\begingroup$ How do porcupines mate? Very carefully! How do experimental physicists create ultra-low temperatures? The same way porcupines mate! In other words, there is an art to carrying out these experiments without having parts of the apparatus interact with these atoms in unintended ways. $\endgroup$ – CuriousOne Sep 12 '14 at 6:13

There are several ways to create Bose-Einstein condensates or systems that behave that way, there are ultracold atomic gases, solid state quasiparticles, and even photon condensates.

Since you are obviously interested in ultracold atomic gases, I am going to cite Experimental methods of ultracold atomic physics by Kurn and Thywissen:

The material must be constructed prior to each measurement, starting from a new, hot atomic vapor. The typical experimental sequence is

(1) using laser cooling and trapping to gather atoms from the vapor into a magneto-optical trap, with temperatures on the order of $100 \mu K$,

(2) trapping the atoms in a conservative potential, e.g. a magnetic (Sec. 2.2) or optical-dipole (Sec. 2.3) trap,

(3) evaporatively cooling the atoms by gradually lowering the depth of the conservative trap and letting atypically high energy atoms escape the trap, reaching temperatures in the quantum-degenerate regime (usually sub-$\mu K$) and

(4) putting the final touches on the material by turning on the system Hamiltonian (interaction strength, lattice type, spin admixture, etc.) that one wants to examine.

Once finally produced, the lifetime of the ultracold material is short, and so the entire experimental sequence for probing the material is quick, typically much shorter than the time it took to prepare the material in the first place. At the end of each measurement, the sample is discarded. This cycle is repeated at the cycle time of a few seconds to a few minutes, depending on the speed of the accumulation and cooling stages. In examining a graph of data from a cold atom experiment (For example, Figs. 3 or 4), one should value the fact that each point on the graph represents one or several repetitions of a make-probe-discard experimental run. In light of this protocol, it is somewhat stunning to hear of cold-atom experiments that require hundreds or thousands of "shots", each reproducing a gas under almost identical conditions, to obtain the high precision required to reveal new phenomena or test recent theories.


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