Molassa

Molassa stands where six laser beams cross, and atoms freeze in her palm. Then they got colder than the rules allowed.

The Physics

In optical molasses, red-detuned laser beams from every direction slow a moving atom like friction, while random recoils from spontaneous emission heat it. For a simple two-level atom the balance sets a minimum temperature, the Doppler limit, best reached at detuning δ = −Γ/2. For sodium's D2 line that limit is about 235 μK.

k_B T_D = ħ Γ / 2

• k_B: Boltzmann constant (J/K)
• T_D: Doppler limit temperature (K)
• ħ: reduced Planck constant (J·s)
• Γ: natural linewidth of the transition (1/s)

In 1988 Lett, Phillips and co-workers measured sodium atoms at 43 ± 20 μK, far below the limit; Chu, Cohen-Tannoudji and Phillips shared the 1997 Nobel Prize in Physics.

Honest caveat: The Doppler limit is right for two-level atoms. Real atoms have many sublevels, and polarisation-gradient (Sisyphus) cooling beats it.

On the shirt

Front: Molassa, the pixel-art character, with the title. Back: the Doppler limit, the sodium numbers, the 1988 measurement, the Sisyphus explanation and the 1997 Nobel. Printed on a unisex tee.