The signal was obtained via optical rotation of linearly polarized light off-resonant to the D2 transition of 87 Rb and circularly polarized with a laser pump beam tuned to the D1 transition of 87 Rb in a cross-beam laser configuration . The laser beams cross in the center of a Rb vapor cell (500 torr N 2 buffer gas) with outer dimensions of 8 × 8 × 10 mm 3 and 1 mm-thickness walls (Twinleaf LCC). A photoelastic modulator (II/FS42A PEM, Acal BFi Germany GmbH) was used to modulate the signal at frequencies of 50 kHz and a lock-in amplifier (SRS 830, Standard Research Systems) demodulated the signal back to audio frequencies. The demodulated signal was then acquired with an acquisition card (NI-9239, National Instruments, Austin, USA) and magneticfield pulses and digital triggers were provided by output modules (NI-9263 and NI-9401, National Instruments, Austin,USA).