Low-phase-noise laser system for the STIRAP transfer of Ultracold ⁶Li⁴⁰K molecules

Ultracold rovibrational ground state molecules have proved to be a versatile platform to perform quantum stimulation, quantum information, quantum chemistry and metrology. A stablished method to create them consists in performing stimulated Raman adiabatic passage (STIRAP) on Feshbach molecules. To perform STIRAP, we prepared a low phase-noise laser setup capable of maintaining coherence between the two Raman optical beams required, which are commonly referred as stokes and pump. First, we designed long (20cm) external-cavity-diode-lasers for both optical beams, this is known to reduce the achievable stabilized laser linewidth which is associated to the phase noise. Secondly, a dual wavelength optical high-finesse cavity (HFC) was constructed to stablish longer coherence lengths for the Raman lasers when both of them are stabilized to it. When frequency locking the lasers to the HFC, linewidths were measured at 300Hz for pump and 500Hz for stokes. The phase noise measurement was performed on each Raman laser, by using the transmission of the HFC as a noise filtered reference to produce a beatnote with the laser diode beam. The phase noise of the beat was measured using a balanced photodiode in order to reduce the amplitude noise. The HFC linewidth puts a lower limit on the frequency of the measurable phase noise (10kHz for pump and 40kHz for stokes) and the balanced photodiode floor noise puts the upper limit at 10MHz. The result of the phase noise measurement shows an improvement of one order of magnitude on the power ratio of sidebands over carrier frequency. The comparison was made to previously used short (2cm) external-cavity-diode-lasers. We calculated the expected STIRAP efficiency dependant on pulse duration and Rabi frequency of the lasers. With the improvements we could stablish a theoretical workable area where STIRAP efficiency surpassed the 90 percent mark.