The momentum of photons present in a laser beam, when used to force an atom to decrease its velocity, causes a cooling effect to take place. This is the foundation of the laser cooling method.
Do you ever consider the innumerable uses of lasers in our technologically advanced world? From laser printing to bloodless surgeries, optical fiber communication to military warfare, detecting geological imbalances to drilling orifices in aerosol spray bottles… lasers have found their way into nearly all major fields. Among this vast range of applications, what is the most common property of lasers that have helped them rise to fame?
The heating effect of lasers, is likely your first guess.
Astonishingly, we have recently developed a deeper understanding of lasers, on a quantum level, which has helped researchers observe cooling effects in lasers that helped them achieve incredibly low temperatures of only 43 microKelvins. Considering the ambition of every scientist who wishes desperately to achieve the 0 Kelvin mark, these results are staggering.
Before going into the details of laser cooling, we need to understand the basics… what’s a laser?
Light Amplification by Stimulated Emission of Radiation
A laser is produced when certain types of glasses, gases or crystals are imbued with electric current or some other mode of energy. This excites the electrons of the atoms comprising that material. Electrons jump from a lower energy state to a higher energy state due to the absorption of the energy provided. Eventually, the electrons return to their original energy level, radiating out energy in the form of energy bundles called photons (light particles).
These photons, vibrating at the same frequency (coherently) produce the laser stream. When this stream hits a material, its atoms absorb the energy from the photons in the form of kinetic energy, thus creating vibrations on an atomic level. This causes the material molecules to randomly move and collide, producing friction and therefore causing a heating effect in the target material.
How Does Laser Cooling Work?
Although this may seem counterintuitive, researchers have applied their years of laser mastery to create an unimaginably sophisticated method of manipulating the heating nature of a laser to instead achieve the lowest possible temperatures that humans have ever encountered.
The trick to this seeming paradox lies in the light particles themselves. The basic principle governing laser cooling is that photons, in addition to carrying energy, E, also carry momentum, p (despite being massless particles), with p=E/c (c being the speed of light). This momentum, when transferred to an object, generates a force. If an atom moving in a particular direction absorbs an inbound photon from a laser beam propagating in the opposite direction, the velocity with which the atom was propagating will decrease.
Soon enough, the atom will emit that photon in some random direction, maintaining the average change in velocity due to this process at zero. Repeating this process of absorption and spontaneous re-emission over and over again will significantly lower the kinetic energy of an atom by reducing its velocity, thereby cooling it down.
The Doppler Shift
A necessary condition to achieve this cooling effect is that the photon absorption preferably takes place when the atoms are moving in a direction opposite to that of the photons from a laser beam.
The Doppler effect
This is helped along by a beneficial application of the Doppler effect. Just as you would observe a change in the pitch of a moving train whistle as the train approaches you and then passes by, similarly, a moving atom experiences a shift in the apparent frequency of incoming light (in this case, a laser) due to its relative motion to the source of light. Atoms moving towards the laser source observe the light at a slightly higher frequency; likewise, an atom moving away from a laser source observes the light at a slightly lower frequency. This is the Doppler shift effect in atoms.
How is the Doppler Shift used to our advantage?
By tuning the frequency of an emitted laser beam to a slightly lower range than the required amount for an atom at rest, it’s more likely that the light will be absorbed more efficiently if the atom is moving towards the laser, rather than moving away from it.
For an atom moving towards the laser beam, the Doppler shift increases the light frequency from the atom’s perspective, thus giving it a higher probability of absorbing the incoming photons in the opposite direction, and receiving a jolt of momentum, which will slow the atom. However, if the atom is traveling away from the beam, the Doppler shift reduces the light frequency, thus reducing the probability of photon absorption.
This method of laser cooling is called Doppler cooling.
Applications of laser cooling
- Laser cooling is primarily used for producing ultra-cold atoms for quantum physics experiments. These experiments ideally require “near absolute zero” temperatures in order to observe unique quantum effects, such as Bose-Einstein condensation. Although we haven’t reached this temperature yet, laser cooling is one of the best methods that has brought us close to what we consider “the lowest achievable temperature”.
- At lower temperatures, random motions of atoms decrease, so high-resolution spectroscopic measurements become more refined.
- Quantum optics and quantum computing are some of the most important applications of laser cooling.
- The Doppler shift of free-falling cooled atoms is used for highly precise measurements of gravitational fields.
- Lithography with cold atomic beams can form very accurately controlled structures. Supercooled atomic lithography forces the atomic beams (using lasers) to fabricate nanostructures on a planar surface.