In this section, we describe the apparatus and help you obtain an initial signal. In the image above, you see (from left to right) the main apparatus, the control console and digital scope, a function generator, a DC power supply, a couple digital multimeters, some compasses, and a computer.

The main apparatus consists of a Rb vapor cell in a temperature-controlled oven situated in the center of a pair of Helmholtz coils, all sitting atop an optical rail. Mounted to the optical rail, we have the following (from left to right):

• An amplified photodiode detector;
• a focusing lens;
• the vapor cell inside the Helmholtz coil assembly;
• a quarter-waveplate;
• a linear polarizer;
• a D1 transition filter;
• a collimating lens; and
• a rubidium lamp light source.

The light source uses a radio frequency (RF) oscillator to excite Rb atoms, thereby producing light at wavelengths needed to pump the Rb atoms in our vapor cell. Light from the source is collimated before passing through the filter which is tuned to only pass light from the D1 transition.

The filtered photons then pass through a linear polarizer and a quarter-waveplate whose optical axes are oriented such that light which emerges is right circularly-polarized. A quarter-waveplate will convert linearly polarized light into circularly polarized light. (If you send unpolarized light through a quarter-waveplate, you will get a mixture of left and right circularly-polarized light out. By placing a linear polarizer in front of the quarter waveplate, and setting the correct angle between the optical axes of the two, you can produce an output beam ranging from pure right circular polarization to pure left circular polarization (or anything in-between).) To produce a beam of right circularly-polarized photons, the linear polarizer should be set to an angle of 45° and the quarter waveplate should be set to 0° in their holders. If you do not remember how polarizers and quarter waveplates work, Google them to refresh your memory.

The vapor cell sits in the middle of a pair of orthogonal Helmholtz coils; one produces a vertical magnetic field when current is passed through it and the other produces a horizontal magnetic field when energized. These coils are used to produce the horizontal and vertical magnetic fields that define the net magnetic field in which the Rb atoms reside. If you do not recall how Helmholtz coils work, Wikipedia has a nice description.

The vertical coil frame contain $N = 20$ turns on each of the two coils. (Recall that a Helmholtz coil is actually a pair of coils, so the use of the word “coil” can get confusing.) The wire has been wound 5 across and 4 layers deep. According to the manufacturer, the coil radius is about $4.61 \pm 0.01$ inches. The spacing of the coils should match their radius.

With this information, you can calculate the magnitude of the magnetic field created by this coil if you know the current which is flowing through the wires. One of the functions of the TeachSpin control console is to control this current.

The horizontal coil frame actually contains two separate pairs of coils; the wire of the second coil is wound directly atop the first coil. This arrangement allows us to create two independently-controllable magnetic fields along the horizontal axis. We call one set of these coils the “Horizontal Sweep Coil” and the other the “Horizontal Field Coil.”

These coils contain $N = 154$ turns on each of the two coils of the pair. The wire has been wound 11 across and 14 layers deep. According to the manufacturer, the coil radius is about $6.22 \pm 0.01$ inches. The spacing of the coils should match their radius.

These coils will be used to create a steady magnetic field along the horizontal axis, similar to how the vertical Helmholtz coil is used. The TeachSpin control console can be used to set the current in this coil.

The top layer of wires which you can see make up the “Horizontal Sweep Coil”. This coil contains $N = 11$ turns on each of two coils of the pair. The wire has been wound 11 across in a single layer. According to the manufacturer the coil radius is about $6.46 \pm 0.01$ inches.

This Helmholtz coil is used to create a magnetic field which sweeps periodically over a range in a sawtooth pattern. This small time varying magnetic field component is central to how the TeachSpin apparatus achieves depumping (as you will soon see).

The TeachSpin control console is set up to allow you to measure the voltages and currents to the different coils so that you can then calculate the magnetic fields being generated.

For the Vertical Coil, there is a pair of test points across a 1$\Omega$ monitor resistor which is in series with the coil wire. Thus, by measuring the voltage drop across the 1$\Omega$ resistor, you can get the current flowing through the coil using Ohm's law.

For the Horizontal Sweep Coil, getting the current is a bit trickier. Since this voltage (and therefore the current) changes in time, we view it on the scope. However, the actual value of the voltage which is sent to the scope via the Recorder Output is amplified for viewing on a scope. Thus, the voltage you read on the scope is not the same as the voltage going to the sweep coils.

You can determine the conversion factor to go from scope voltage to coil current by either using the test points across the 1$\Omega$ monitor resistor on the front panel next to the sweep coil output jacks, or by reconnecting the coil wires at the output jacks to run through an ammeter to directly measure the current. In either case, you want to set the Sweep Range to zero, then set the Start voltage to several different values from its minimum to maximum. For each Start voltage setting, you can measure both the voltage of the signal at the scope and the actual current in the coils. This will give you all the information you need to determine the linear relationship between scope voltage and coil current.

The TeachSpin optical pumping control console is pictured above. Controls on the console are grouped based on function and consist of the following (from left to right):

• RF Amplifier - Not used (and therefore not highlighted in the photo).
• Cell Heater/Controller - Controls and indicates the temperature of the vapor cell. It automatically starts when the power switch is turned on and requires no user input. The temperature display will stabilize at 50${}^\circ$C in about 10 minutes.
• Magnetic Field Modulation - Not used (and therefore not highlighted in the photo.)
• Horizontal Magnetic Field Sweep - These controls are used to set up a time-varying current to the horizontal Helmholtz coils. Note that the BNC output labeled Recorder Output on the lower panel is connected to channel 1 of the scope and is used to monitor the current going to the coils.
  • The Range control allows you to set the range over which the current to these coils will sweep.
  • The Start control sets the starting current of the sweep. In other words, the Start control can be used to move the whole sweep Range up and down.
• Vertical Magnetic Field - This 10-turn potentiometer is used to set the current going to the vertical Helmholtz coils.
• Horizontal Magnetic Field - This 10-turn potentiometer sets a constant current to the horizontal Helmholtz coil. This current is in addition to any current being provided by the Horizontal Magnetic Field Sweep controls.
• Detector Amplifier - The output from the photodetector on the optical rail comes here. The user can amplify and add an offset voltage to the detector signal, and filter out high frequency noise. The final signal is then sent to channel 2 on the scope.