Simultaneous two-photon resonant optical laser locking (STROLLing) in the hyperfine Paschen-Back regime

We demonstrate a technique to lock simultaneously two laser frequencies to each step of a two-photon transition in the presence of a magnetic field sufficiently large to gain access to the hyperfine Paschen-Back regime. A ladder configuration with the 5S_1/2, 5P_3/2, and 5D_5/2 terms in a thermal vapor of Rb87 atoms is used. The two lasers remain locked for more than 24 h. For the sum of the laser frequencies, which represents the stability of the two-photon lock, we measure a frequency instability of less than the Rb D₂ natural linewidth of 6 MHz for nearly all measured timescales.