Physics 14—Laser Cooling Module
Final Exam Questions
The laser cooling portion of the final exam will be drawn from the following list of
questions:
1) The 2001 Nobel Prize in Physics was awarded to Eric Cornell and Carl Wieman
at JILA and the University of Colorado, and Wolfgang Ketterle at MIT, for their
work on creating and manipulating Bose-Einstein Condensate (BEC). Briefly
describe what a BEC (sometimes called an “atom laser”) is, and what special
properties it has that make its creation worthy of a Nobel Prize.
2) In the Ketterle lab at MIT, they start their experiments with an oven containing
sodium metal at a few hundred degrees Celsius (around 600 Kelvin), from which
atoms are emitted in an atomic beam with an average velocity of approximately
300 m/s. They end up with a BEC with an effective temperature of a few
nanokelvin (10-9 K). Give a short description (a couple of sentences) of the
following stages along the path to BEC, outlining how each step is accomplished.
a) Slowing the atomic beam from 300 m/s to ~1 m/s.
b) Trapping the slow atoms from the beam and cooling them further to
T~100 K.
c) Evaporative cooling of the trapped atoms from 100 K to the BEC
transition.
3) The principal laser cooling transition in Strontium (Sr) has a transition
wavelength of  = 461 nm, and an excited-state lifetime of  = 5.00 ns (1 ns = 10-9
s). The ground state of strontium is a “singlet state,” meaning it does not have
multiple sub-levels. Would you expect Sisyphus cooling to work for Sr? What
minimum temperature would you expect to reach in laser cooling of Sr?
4) Describe some of the advantages that an atomic clock based on laser cooled
atoms has over an atomic clock using atoms at higher temperatures.
5) Metastable xenon can be laser cooled using light with  = 882 nm, with  = 34 ns
in the excited state. What is the maximum acceleration of a beam of xenon atoms
(mass = 132 a.m.u.) due to light forces? Would you expect the acceleration of a
beam of metastable krypton atoms (mass = 83 a.m.u.,  = 810 nm,  = 28 ns) to be
larger or smaller? (Hint: You ought to be able to answer the second question
without doing any math.)
6) Bose-Einstein Condensation is created in dilute atomic samples through a process
known as “Evaporative Cooling.” Describe the process of evaporative cooling,
and explain why it is necessary to use evaporative cooling in BEC experiments.
7) Describe two different ways to slow a beam of atoms from 300 m/s to 1 m/s using
light forces.
8) Describe two different ways of measuring the temperature of a sample of lasercooled atoms.
9) Give a brief description of how “optical tweezers” work to trap small glass beads
suspended in water. Would you expect this technique to work for opaque objects
as well?
10) A physics student claims that he missed the final exam because the light pressure
force due to the bright sunlight outside his dorm prevented him from getting to
class. If the mass of the student is 75 kg, how many photons per second (assume
an average wavelength of 550 nm) would have to hit him in order to produce an
aceleration of 1 m/s2? How does this compare to the typical photon flux of 1021
photons/second hitting a 1m square on an average sunny day? Would this be a
good excuse to use in Physics 14?