Laser cooling of atoms
An extension of the photon-and-momentum topic. A photon carries momentum; laser beams set up against an atom's motion mean that with every absorbed photon the atom loses part of its momentum and slows down. Because temperature is the energy of motion, slowing atoms down means cooling them — all the way to microkelvins (Nobel 1997).
I will give you a target temperature — pick the number of absorbed photons to cool the atom to it. T ∝ v².
What just happened?
You illuminated the atom with photons from two opposite sides. Each time the atom absorbs a photon heading against its motion, the photon transfers its momentum to it — in the direction opposite to the atom's velocity. The result: the atom brakes.
Photon momentum and conservation of momentum:
— a photon of wavelength λ carries momentum p (h — the Planck constant). When the atom absorbs it, it takes over that momentum by conservation of momentum: the momentum of the atom+photon system must be conserved, so the atom loses exactly as much speed as the photon "pushed" it the other way.
Why must the beams come from two sides?
The lasers are red-detuned (using the Doppler effect) so that the atom mainly absorbs photons from the beam it is currently moving into — never from the beam that would speed it up. This is "optical molasses": from every side the atom is braked, no matter which way it moves.
Temperature is the speed of motion:
The temperature of a gas measures the average kinetic energy of its particles' motion. Since the photons systematically take momentum from the atoms and slow them down, they lower the temperature at the same time — this method cools atoms to microkelvins, fractions of a degree above absolute zero.
Why this matters for your exam:
Problems about photon momentum (p = h/λ) and conservation of momentum during the absorption/emission of light show up in the quantum physics unit. Laser cooling is a classic example of a macroscopic effect of photon momentum — it ties optics, mechanics and thermodynamics together in a single phenomenon that earned a Nobel Prize.
Remember: a photon has momentum p = h/λ; absorbing a photon head-on brakes the atom; a lower speed of motion = a lower temperature. More photons absorbed = a colder atom.
Theory & formulas
Photon momentum cools
A photon carries momentum . Laser beams aimed against the atom's motion mean that with every absorbed photon the atom loses momentum and slows down.
Temperature = motion
Slower atoms mean a lower temperature (the average thermal energy of motion is ). This is how atoms are cooled to microkelvins and held in an atom trap (Nobel 1997) — the basis of atomic clocks.
Symbols: — photon momentum, — Planck constant, — wavelength, — Boltzmann constant, — temperature.
Frequently asked questions
›How can light cool something?
The photons transfer momentum to the atom opposite to its motion — the atom slows down, and so it cools.
›What is an atom trap?
A system of laser beams and magnetic fields that holds the cooled atoms in one place.
›What is it for (atomic clocks)?
Slowly moving, cooled atoms allow an extraordinarily precise measurement of time.
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