How to Use Light to Cool and Trap Atoms, Part 2: Cooling Atoms with Light

By Jared Popowski

2019-02-16

This is Part 2 in our series on cooling and trapping atoms with light! Today we will focus on how atoms are cooled with light, but before we start I have a couple of things to point out. Last time we talked about the nature of light, and how it can sometimes act like a particle and sometimes like a wave. In atomic physics we often switch back and forth between the different interpretations of light, depending on what is most convenient for the current situation. So sometimes in this series I will talk about the light as being made of photons that collide with atoms, and sometimes I will describe it as a wave with a certain frequency, or number of cycles of the wave per unit time. Also, I will be talking specifically about one type of laser cooling called Doppler cooling. Singling out just this type of cooling to describe is justified in my mind for two reasons:

  1. It is the oldest and still the most commonly used method for cooling atoms with lasers.
  2. This would be a very long post if I tried to go into detail with multiple different types of laser cooling, but you should look into some of the other methods if you find this interesting!

On with the blog! Historically, there was a big push to reduce the speed of atoms because it would allow for more precise measurements of atomic spectra, and later because having colder atoms would greatly improve atomic clocks [1]. The method of Doppler cooling was proposed in 1975 independently by two groups, the first being David J. Wineland and Hans Georg Dehmelt, and the second being Theodor W. Hänsch and Arthur Leonard Schawlow [2,3]. It was first realized experimentally by Wineland, Drullinger, and Walls in 1978 [4].

In Doppler cooling, atoms are cooled with lasers by taking advantage of the Doppler effect. The Doppler effect will be familiar to anyone who has ever heard an ambulance or fire engine pass by while blaring their sirens: as it approaches, the sound of the siren is higher pitched than after it has passed by. This is because as the siren moves towards you, the distance between you and the source of the sound is decreasing over time. This decreases the distance between peaks of the resulting sound wave, thus making the frequency of the wave (i.e. the pitch you hear) higher. The opposite is true when the siren is moving away from you: the distance between you and the siren is increasing over time, making the distance between peaks of the sound waves larger and therefore the pitch of the siren lower. See Figure 1 below for a diagram of Doppler frequency shifts.

The distorted wavefronts produced by the Doppler effect. Figure 1: A diagram showing the Doppler Effect, which is when the frequency of a wave shifts when the source of the waves moves relative to the observer. The frequency of the wave is higher when the source is moving towards the observer, and lower when it is moving away from the observer. Image from Wikimedia Commons.

It turns out that the Doppler effect occurs not just for sound waves, but for all types of waves, including light waves. Atoms moving towards light with a certain frequency experience a Doppler shift in the frequency, making it appear to have a higher frequency to the atom, and vice versa. This is important because atoms are essentially transparent to most frequencies of light, except at a few narrow ranges of frequencies, called resonant frequencies. Each element has a different set of these resonant frequency bands, which make up the element’s so-called atomic spectra. If the light passing by an atom happens to fall within one of its resonant frequency bands, atoms will absorb the energy of the light and undergo the electronic transition that corresponds to this frequency, jumping from a lower energy level to a higher energy level. Importantly, since light has momentum as well as energy, the atom will also experience a “kick” backwards as the momentum of the light transfers to the atom, like what happens when billiard balls collide and transfer their momentum to each other. After a small amount of time, the atom will drop back down to the lower energy state by spontaneously emitting light of the same frequency in a random direction, experiencing another momentum kick in the opposite direction of the emitted light as it does so.

With all of these ideas, we can now explain the principle behind Doppler cooling! In Doppler cooling, we shine light that has a frequency slightly below a specific element’s resonant frequency on a large sample of these atoms. Since this light is slightly below resonance, it will not affect the atom unless the atom moves towards the light, in which case the frequency of the light is Doppler shifted up until it hits the atom’s resonant frequency. This causes the atom to absorb the light, where it experiences a momentum kick against its direction of motion, thus slowing down the atom. Although the atom will later re-emit a photon and feel another momentum kick in a random direction, this random momentum kick averages itself out over a large sample of atoms, and the net effect of this light will be to slow all of the atoms down. And that’s all there is to it, the atoms are cooled with light!

In the next and final blog post of this series, we will describe how principles that we have just learned about and a few new ones come together to allow for trapping of atoms with light! As always, feel free to comment any feedback you have about this post, or suggestions for future topics for this blog. Also, if you want to get an email every time I release a new blog, make sure to sign up for email updates below!

References

[1] Bardi, Jason Socrates. “Focus: Landmarks: Laser Cooling of Atoms.” Physics 21 (2008): 11. https://physics.aps.org/story/v21/st11

[2] Wineland, Dl, and Hans Dehmelt. “Proposed 10^14 delta upsilon less than upsilon laser fluorescence spectroscopy on t1+ mono-ion oscillator iii.” Bulletin of the American Physical Society. Vol. 20. No. 4. CIRCULATION FULFILLMENT DIV, 500 SUNNYSIDE BLVD, WOODBURY, NY 11797-2999: AMER INST PHYSICS, 1975.

[3] Hänsch, Theodor W., and Arthur L. Schawlow. “Cooling of gases by laser radiation.” Optics Communications 13.1 (1975): 68-69.

[4] Wineland, David J., Robert E. Drullinger, and Fred L. Walls. “Radiation-pressure cooling of bound resonant absorbers.” Physical Review Letters 40.25 (1978): 1639.