Experiments in Modern Physics P451: Optical Pumping of Rb Experiment
David Baxter
Indiana University, Bloomington, IN 47405
(Dated: January 9, 2014)
This experiment introduces you to the phenomenon of optical pumping (i.e. creating a nonequilibrium occupation of an excited state in an atom or molecule) and along the way you’ll be
introduced to a number of atomic physics phenomena as well. You will explore the use of atmoic
physics as a precise probe of magnetic fields, and gain experience with exploiting the unparalleled
precision with which frequency can be measured in order to calibrate the coils used to produce
magnetic fields in this appratus.
A.
Introduction
Quantum optics and the interaction of optical fields
with atoms plays a vital role in a very wide range of
contemporary physics research, ranging from quantum
computing and digital communications, to spin-polarized
targets for nuclear and particle physics, all the way to
dilute-atom realizations of Bose-Einstein condensation
and many-body phenomen. In this lab you will explore
a number of important elements of this interaction between atoms and photons. Some of the physics involvd is
subtle, and the connection between the physics you are
exploring and the measurements you perform are more
complicated than is the case in many of the other experiments in this course.
This experiment makes use of the apparatus built by
“Teach Spin” and most of your informaton should be obtained from reading the manuals provided by that company [1]. Here we will provide some guidlines to make
your reading of that manual more efficien as well as some
questions to help focus your efforts. There is much more
that can be done with this apparatus than you will likely
be able to accomplish in a three or four week period, so
you should discuss your options with your instructor. If
you want to do more than the most basic investigations,
it is vital that you spend some time reading sections 2C
and 2D of the manual before coming to the lab for your
first session with the equipment. If these sections introduce too many symbols and ideas that you have not seen
before, you should also look over sections 2A and 2B, but
sections A and B go into a lot more detail than you need
to appreciated the overall goals of the experiment, so it
may be more useful for you to hold off reading these sections until you have acquired some experience with the
apparauts.
FIG. 1. This is the front panel of the control box for the
Optical pumping apparatus.
cell, and an electronics unit to control all these components. In addition to the TeachSpin components, the
experiment also makes use of a Radio-Frequency (RF)
function generator, a frequency counter (in order to accurately measure the frequency of the applied RF field),
an oscilloscoope, and LabPro interface for observing and
recording the data. For initial alignment of the apparatus, it is most helpful to use the ’scope in XY mode (X
monitoring the seep field value, Y the detector output)
with a persistnace time of at least 5 sec. The main power
switch is on the back of the unit (left side as viewed from
the front panel) right beside the power cord. There is
another togle swith on the back of the unit (this time on
the right-hand side) that changes the unit from an internal supply to an external supply for the main field coils.
If the front pane dial appears to have no impact on the
main field, check the position of this switch (to the left,
again as viewed from the front panel, is the position for
the external supply, to the right is the position for the
internal supply).
Equipment
The Teach Spin apparatus consists of a Rb lamp as
its source of light, a temperature-controlled sample cell,
a detector that meaures the light passing through that
cell, several optical components to condition the light
both before and after it passes through the cell, various coils to control magnetic and RF fields around the
Alignment
The physical alignment of the apparatus has already
been performed (indicated by tape marks on the bench),
and so you should be primarily concerned with aligning the optics to produce a circularly polarized beam of
2
RF Field
Static Horiz.
field
¼-
plate
Filter
Lamp
Detector
LENS
Polarizer
LENS
FIG. 2. This figure shows a schematic diagram of the optical
component configuration needed to produce the signals this
lab investigates. Note that a second linear polarizer may be
placed between the sample cell and the detector in order to
check on the polarization of the incident light and/or adjust
the intensity at the detector.
sufficent intensity to assure that you can see the effects
outlined in the manual. You want to have a reasonably
uniform magnetic field at the sample cell position, so it is
important that you not have any significant pieces of iron
or other magnetic material on the bench or shelves near
the unit. Note that the apparatus is positioned with the
source on the right and the detector on the left (as viewed
from the operator’s position), as in figure 2 and unlike
the configuration shown in figure 4B-1 of the manual[1].
The detector gain and position of the two lenses should
be positioned adequately by the previous students on
this experiment, so you should start by confirming the
alignment of the polarizers as discussed on page 3-8 of
the TeachSpin manual[1]. Note that the detector output
is a differential signal betwen the light the dtector sees
and the setpoint selected by the 10-turn potentiometer
marked “Detector Offset”. For this alignment, you may
wish to have the detector on the x1 scale, but for the
investigations of optical pumping, you will want to be on
a much more sensitive scale, so you will need to adjust
the offset accordingly. This is a simple implementation of
a “differential measurement”, which allows you to have
great sensitivity over a wide dynamic range: the display
provides the difference between the output of the phtodiode and the voltage dialed in on the offset pot.
Similarly, you can change the “sweep magnetic field”
manually by varying the“start field” 10-turn pot. with
the sweep/reset switch in the reset position. After you
have achieved satisfactory circular polarization, increase
the detector sensitivity and search for the zero-field transition (discussed in section 2.E) by adjusting the veritcal
and sweep fields.
Questions to consider as you are performing this alignment:
1. What is circularly polarized light, and what aspect
of this experiment makes its use necessary?
2. How can you produce circularly polarized light with
the apparatus in front of you, and how can you
confirm that it is circularly polarized with the same
apparatus?
3. What would be the consequence of using elliptically
polarized light in this experiment?
4. Can you explain the qualitative impact of adjusting the vertical magnetic field on the shape of this
transition? What value of current through the vertical field coils produces the sharpest transition?
What is happening in the apparatus with this configuration of the veritcal and horizontal field coil
currents?
Experiments
1. Once you have the optics properly alligned, conduct
the low-field resonance experiment 4B, and determine the ratio of the Lande g-factors for the two
isotopes of Rb.
2. With the RF off, and the horizontal field set well
away from the zero-field resonance, conduct the experiments investigating Beer’s law as described in
section 4A.
3. Connect a low-frequency square wave to the RF
modulation input of the electronics box, and invstigate the transient effects as described in section
4D.
4. If you are feeling ambitious, investigate the
quadratic Zeeman effect as described in section 4C
of the manual.
[1] (OPRb) “Optical Pumping of Rubidium”, Teach Spin,
Buffalo, NY (2002)
[2] ”Optics”, E. Hecht,