Inside the Dancing MOT Demo
In collaboration with ColdQuanta, Vescent has a new MOT demo that we will unveil at DAMOP 2012. The demo has Rubidium (Rb) 87 magneto-optical trap (MOT) dancing to music. Check out the video below to see for yourself, or stop by our booth at DAMOP. If you’re at DAMOP, feel free to bring your own tunes and MP3 player to have the MOT dance to the beat of your own drummer!
How does it work?
We start with ColdQuanta’s Mini-MOT, which provides an ultra-high vacuum with large optical access. Inside the vacuum is a Rubidium dispenser which is turned on to provide a dilute vapor of Rb atoms inside the vacuum cell. Current is run through the MOT coils in an anti-Helmholtz configuration. In order to cool and trap Rb atoms, however, laser light is necessary.
I’ll skip the alignment and polarization as this is a standard 6-beam MOT using 3 retro-reflections. For a Rb 87 MOT, two frequencies of light need to be used: ‘cooling’ light tuned slightly to the red of the F=2, F’=3 cycling transition; and ‘repumping’ light tuned to the F=1 transitions. We use two lasers to produce these two frequencies. The first laser is our narrow line-width CECL Laser that is peak locked to a F=1 repump transition using our Laser Servo and Spectroscopy Module. The second laser is a DBR Laser used as a slave laser. Light from the master and the slave lasers are combined on our Heterodyne Module and sent via a fiber to the Offset Phase Lock Servo (OPLS). The OPLS converts the optical beat-note from the Heterodyne Module into a electrical beat note. The beat-note is an oscilating signal whose frequency corresponds to the frequency difference between the two lasers. The OPLS then locks this beat-note to a reference oscillator.
Tuning the reference oscillator gives us precise control of the frequency difference between the two lasers. Since the master laser is atomically referenced, now the slave laser is atomically referenced as well. Thanks to the Offset Phase Lock Servo, however, the slave laser is tunable as well by means of the reference frequency. In this way the frequency of the slave can be quickly and precisely adjusted without ever breaking lock.
For the demo we use a Voltage Controlled Oscillator (VCO) internal to the OPLS as the reference oscillator (alternatively an external oscillator source could be used as well). The OPLS locks the beat-note at 6,421 MHz when the VCO is tuned to 200.66 MHz (N is set to 32 — see OPLS page for more details). By slightly adjusting the VCO, we can modify the laser detuning and modulate the brightness and number of atoms captured in the MOT.
To make the MOT “dance,” we adjust the VCO by way of a rectified, low-passed input audio signal. The processed audio signal produces a voltage whose amplitude roughly corresponds to the volume of the bass. This signal goes through a variable attenuator (a ‘volume’ control) and the output goes into the back-panel OPLS port to connecting to the VCO. Now the VCO frequency is adjusted based on the volume of the bass of the audio signal. And like any good performer, the MOT dances to the bass, check it out: