How are ultra cold temperatures measured?

[widderjoos]

How are ultra cold temperatures measured? For example, sometimes I see things measured in nanoKelvins. I'm thinking there has to be direct contact since the vacuum is already hotter than this, but how is it actually done?

 

[shawl]

It requires ultra high vaccum.
And the cooling technology can be utilized is "Laser cooling" and "Magnetic refrigeration".

 

[Redbelly98]

How are ultra cold temperatures measured? For example, sometimes I see things measured in nanoKelvins. I'm thinking there has to be direct contact since the vacuum is already hotter than this, but how is it actually done?

It comes from measuring the velocity distribution of the atoms. A narrower distribution corresponds to colder temperatures.

It requires ultra high vaccum.
And the cooling technology can be utilized is "Laser cooling" and "Magnetic refrigeration".

That does not address the question, which is, how are these cold temperatures measured?

 

[widderjoos]

Thanks, but how do they measure the velocity distribution without significantly heating the system up?

 

[Redbelly98]

Thanks, but how do they measure the velocity distribution without significantly heating the system up?

I'm more familiar with measurements in the μK range that were common 20-or-so years ago. I'm not 100% sure if they still work in the nK range -- I believe they do, or some variation of them -- but here is a brief explanation:

After cooling and trapping atoms the optical trap is shut off, releasing the atoms that had been trapped. The collection of atoms then expands, owing to the different velocities of the atoms. Loosely speaking, the amount of expansion of this "cloud" of atoms is measured some time later. The expansion rate is a measure of the velocity distribution, from which the temperature can be inferred.

For more details, Bill Phillips has nicely described several methods, all relying on the expansion of atoms after shutting off the trap:
http://prl.aps.org/files/RevModPhys.70.721.pdf

One method described on p. 730 (p. 10 of the pdf file), in the paragraph that begins "Using the techniques for chirp cooling, ..."

A second method is described starting at the bottom of p. 731, in the paragraph that begins "In this time of flight (TOF) method,..."

Two more methods are described briefly, on p. 732, in the paragraph that begins "Another method was the 'fountain' technique..."

Hope that helps.

 

[widderjoos]

Thanks, this helped a lot