## 2 Principle  
Coherent population trapping (CPT) is a quantum interference phenomenon where atoms are put in a coherent dark state. A characteristic of CPT is the disappearance of resonance fluorescence from a multi-level quantum system when a bichromatic optical field is applied to the system. The dark state is created when the optical fields couple two ground states to a common excited state in a \(\Lambda\)-type configuration, provided the frequency difference of the two fields exactly matches the frequency difference (\(\Delta\)) between the coupled states. Detailed theoretical treatment, experimental setup, and application of CPT can be found in excellent and comprehensive reviews by [PERSON] [16] and [PERSON] [17].  
One of the most important applications of CPT is in the field of magnetometry, where a sensitivity of the order of picoteslas has been reported [18]. This is mainly due to a very narrow resonance linewidth, which, depending on the experimental condition, can vary from a few Hz to several KHz. To create the dark state and thus observe a CPT signal that is responsive to changes in the B-field, usually two laser beams from two independent lasers are required [10; 14; 19; 20; 21]. Most commonly, one of the lasers, termed a coupling laser (\(\omega_{c}\)), is locked to a given Zeeman sub-level, while the other laser, termed probe (\(\omega_{p}\)), is scanned around another sub-level. This is unlike electromagnetically induced transparency (EIT) [22; 23], where intensity difference between the two beams plays a crucial role and therefore labeling the beams are critical. In CPT, due to near equal intensities between the two beams, the above name designations (namely probe and couple) are merely used for differentiating purposes. Another method for creating a dark state is to have a single laser plus a method for modulation, such as mechanically dithering the mirror of an external cavity diode laser (ECDL), direct modulation of the injection current, or external phase modulation. There are also reports where, instead of modulating the laser, the magnetic field that is applied to the vapor cell to lift the degeneracy in the atomic ensemble is modulated [12; 14; 24]. However, this approach is less relevant to our work, because we are interested in changes that are induced by a specific strength of the B-field, thus measuring this strength.  
Instead of the above modulation methods or the use of two independent lasers, we use a single laser and two AOMs. In this way, when a laser beam is incident on the AOM, a bichromatic beam is created at the exit due to diffraction of the optical wave as a result of variable refractive index, which is created by an acoustic wave in the AOM cell (Bragg cell).  
Application of a periodical waveform to the AOM driver creates a periodical frequency modulation for the first order diffracted beam. Therefore, by separating a beam into two components, where one of the beam is directly going to a reference vapor cell that contains rubidium (Rb) atoms while the other beam is put through an AOM then onto the interaction cell, a mutually coherent and bichromatic source is created. Thus, when the laser is tuned to a given Zeeman sub-level and the AOM is detuned periodically around another Zeeman sub-level, all the necessary conditions are created to observe a CPT signal.