How can a temperature of 100 pK be measured?

[martindrech]

I´ve read that a temperature of 100 pK has been reached by scientists. But how can they measure such a low temperature? And achieve that level of accuracy?

 

[M Quack]

Dunno, really, but you might find more information here:

http://ltl.tkk.fi/wiki/LTL/World_record_in_low_temperatures

 

[Redbelly98]

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.

Hope this helps you as well.

 

[M Quack]

Note that in this case they cooled down an old-fashioned lump of metal (Rhodium) using an old-fashioned dilfridge and (nuclear) adiabatic demagnetizatoin. Cooling atoms in an optical trap is a different ballgame.

The original publications seem to be these here (stupidly, there is no link to this on their web site!)

T. A. Knuuttila, J. T. Tuoriniemi, and K. Lefmann
Relaxation of Polarized Nuclei in Superconducting Rhodium
Phys. Rev. Lett. 85, 2573–2576 (2000)

http://prl.aps.org/abstract/PRL/v85/i12/p2573_1

J.T Tuoriniemi and T.A Knuuttila
Nuclear cooling and spin properties of rhodium down to picokelvin temperatures

Physica B: Condensed Matter
Volume 280, Issues 1–4, 11 May 2000, Pages 474–478

http://dx.doi.org/10.1016/S0921-4526(99)01839-6

From the second publication:

The first nuclear stage, a massive block of copper, acts as a thermal reservoir at about 100μK during initial polarization of the sample which is the second nuclear stage. The specimen is cooled further by adiabatically demagnetizing highly polarized spins at a rate which is fast in comparison to the spin-lattice relaxation time. Only the temperature of the nuclear spins is lowered, while the lattice and the conduction–electron system remains in thermal equilibrium with the first nuclear stage...

All thermodynamic quantities of the nuclear-spin system, polarization, entropy, temperature, etc., can easily be deduced from the NMR line at magnetic fields much higher than the internal fields representing the mutual interactions between the spins, i.e., in the ordinary paramagnetic state. The area of the absorption peak is proportional to the nuclear polarization, which can be used to calculate the other quantities of interest.

We made the polarization measurements on Rh at a frequency of 431 Hz, so that the resonance field was about 320 μT. The polarization scale was calibrated at relatively high temperatures between 0.3–1.5 mK, where the platinum-NMR thermometer on the copper-nuclear stage was still at very good thermal equilibrium with the sample. The highest polarization measured was p=0.86.

I just love the fact that they consider 0.3mK "relatively high temperature". Compared to 100pK it's almost tropical.

 

[Redbelly98]

Interesting, I did not realize condensed matter had been cooled down that low.